Showing papers in "Seismological Research Letters in 2020"

Geopsy: A User‐Friendly Open‐Source Tool Set for Ambient Vibration Processing

Abstract: Ambient vibrations are nowadays considerably used worldwide for numerous types of engineering applications and scientific research. Geopsy and its companion tools are part of that landscape. Since the first release of the program package in 2005, as outcome of the European Union project Site Effects aSsessment from AMbient noisE, Geopsy has become a mature multiplatform open-source package (released under GNU Public License version 3) that has already been recognized as a reference tool for analyzing ambient vibration data in the context of site characterization studies. The community of users has grown from a core group of researchers up to thousands of seismologists and engineers on every career level and on all continents. The versatility of geopsy allows for the processing of all kinds of data needed in site characterization studies, that is, from single station single trace to three-component array recordings. In all of the aforementioned cases, the steps from field acquisition to the production of publication-ready figures are covered and supported by user-friendly graphical user interfaces or corresponding command-line tools for the automation of the complete processing chain. To avoid black-box usage, a number of lower-level tools guarantee maximum flexibility in accessing and controlling processing results at any stage of the analysis.

Distributed Acoustic Sensing Turns Fiber‐Optic Cables into Sensitive Seismic Antennas

Abstract: Distributed acoustic sensing (DAS) is a new, relatively inexpensive technology that is rapidly demonstrating its promise for recording earthquake waves and other seismic signals in a wide range of research and public safety arenas. It should significantly augment present seismic networks. For several important applications, it should be superior. It employs ordinary fiber‐optic cables, but not as channels for data among separate sophisticated instruments. With DAS, the hair‐thin glass fibers themselves are the sensors. Internal natural flaws serve as seismic strainmeters, kinds of seismic detector. Unused or dark fibers are common in fiber cables widespread around the globe, or in dedicated cables designed for special application, are appropriate for DAS. They can sample passing seismic waves at locations every few meters or closer along paths stretching for tens of kilometers. DAS arrays should enrich the three major areas of local and regional seismology: earthquake monitoring, imaging of faults and many other geologic formations, and hazard assessment. Recent laboratory and field results from DAS tests underscore its broad bandwidth and high‐waveform fidelity. Thus, while still in its infancy, DAS already has shown itself as the working heart—or perhaps ear drums—of a valuable new seismic listening tool. My colleagues and I expect rapid growth of applications. We further expect it to spread into such frontiers as ocean‐bottom seismology, glacial and related cryoseismology, and seismology on other solar system bodies.

A High‐Resolution Seismic Catalog for the Initial 2019 Ridgecrest Earthquake Sequence: Foreshocks, Aftershocks, and Faulting Complexity

Abstract: I use template matching and precise relative relocation techniques to develop a high-resolution earthquake catalog for the initial portion of the 2019 Ridgecrest earthquake sequence, from 4 to 16 July, encompassing the foreshock sequence and the first 10+ days of aftershocks following the Mw 7.1 mainshock. Using 13,525 routinely cataloged events as waveform templates, I detect and precisely locate a total of 34,091 events. Precisely located earthquakes reveal numerous crosscutting fault structures with dominantly perpendicular southwest and northwest strikes. Foreshocks of the Mw 6.4 event appear to align on a northwest-striking fault. Aftershocks of the Mw 6.4 event suggest that it further ruptured this northwest-striking fault, as well as the southwest-striking fault where surface rupture was observed. Finally, aftershocks of the Mw 7.1 show a highly complex distribution, illuminating a primary northwest-striking fault zone consistent with surface rupture but also numerous crosscutting southwest-striking faults. Aftershock relocations suggest that the Mw 7.1 event ruptured adjacent to the previous northwest-striking rupture of the Mw 6.4, perhaps activating a subparallel structure southwest of the earlier rupture. Both the northwest and southeast rupture termini of the Mw 7.1 rupture exhibited multiple fault branching, with particularly high rates of aftershocks and multiple fault orientations in the dilatational quadrant northeast of the northwest rupture terminus.

Coseismic Displacements and Surface Fractures from Sentinel‐1 InSAR: 2019 Ridgecrest Earthquakes

Abstract: Cite this article as Xu, X., D. T. Sandwell, and B. Smith-Konter (2020). Coseismic Displacements and Surface Fractures from Sentinel-1 InSAR: 2019 Ridgecrest Earthquakes, Seismol. Res. Lett. XX, 1–7, doi: 10.1785/ 0220190275. Interferometric Synthetic Aperture Radar is an important tool for imaging surface deformation from large continental earthquakes. Here, we present maps of coseismic displacement and strain from the 2019 Ridgecrest earthquakes usingmultiple Sentinel-1 images. We provide three types of interferometric products. (1) Standard interferograms from two look directions provide an overview of the deformation and can be used for modeling coseismic slip. (2) Phase gradient maps from stacks of coseismic interferograms provide high-resolution (∼30 m) images of strain concentration and surface fracturing that can be used to guide field surveys. (3) High-pass filtered, stacked, unwrapped phase is decomposed into east–west and up–down, south–north components and is used to determine the sense of fault slip. The resulting phase gradientmaps reveal over 300 surface fractures, including triggered slip on the Garlock fault. The east– west component of high-pass filtered phase reveals the polarity of the strike-slip offset (right lateral or left lateral) for many of the fractures. We find a small number of fractures that have slip polarity that is retrograde to the background tectonic stress. This is similar to observations of retrograde slip observed near the 1999 Mw 7.1 Hector Mine rupture, but the Ridgecrest observations are more completely imaged by the frequent and high-quality acquisitions from the twin Sentinel-1 spacecrafts. Determining whether the retrograde features are triggered slip on existing faults, or compliant fault deformation in response to stress changes from the Ridgecrest earthquakes, or new Coulombstyle failures, will require additional field work, modeling, and analysis. Introduction TheMw 7.1 Ridgecrest earthquake struck on 5 July 2019 at 8.19 p.m. local time at the China Lake Naval Air Center, 17 km northeast of the city of Ridgecrest, California (U.S. Geological Survey [USGS], 2019a). Thirty-six hours prior, on 4 July 2019, an Mw 6.4 foreshock occurred (10.33 a.m. local time), 11 km southwest of Searles Valley (USGS, 2019b). The two earthquakes ruptured two conjugate faults in the Airport Lake fault zone and Little Lake fault zone, oriented roughly northwest– southeast (right-lateral strike slip) and northeast–southwest (left-lateral strike slip), respectively. Field scientists reported 2–3 m of right-lateral offset along the southern section of the Mw 7.1 rupture. Twin Sentinel-1 satellites operated by the European Space Agency (ESA) were continuously collecting measurements over this region since 2015 (Torres et al., 2012). These satellites collect wide swath data (250 km) using a burst acquisition mode called terrain observation by progressive scan (TOPS). The twin satellites achieve complete coverage in a short-time interval of six days that is well suited for this earthquake sequence. The new wide swath mode requires along-track alignment of better than 1/200 of a pixel (<7 cm), which is possible using the very accurate orbital information provided by ESA (Sansosti et al., 2006; Xu et al., 2017); earthquake displacements greater than ∼7 cm in the along-track direction will cause phase discontinuities at burst boundaries that should be ignored in the interpretation of the maps in the following sections. Moreover, the Sentinel-1 coverage is excellent for these two earthquakes, providing critical high resolution spatially dense deformation observations of the largest earthquake to strike the Eastern California Shear Zone (ECSZ) in nearly 20 yr (Fig. 1). In this article, we focus on mapping coseismic displacement and strain with the objective of serving these products to the field mapping and modeling communities 1. Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, U.S.A.; 2. Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, Hawaii, U.S.A. *Corresponding author: xix016@ucsd.edu © Seismological Society of America Volume XX • Number XX • – 2020 • www.srl-online.org Seismological Research Letters 1 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190275/4921308/srl-2019275.1.pdf by UC San Diego Library user on 21 April 2020 Figure 1. (a) Overview map of the topography, faults, and Sentinel-1 frames surrounding the 2019 Ridgecrest earthquake sequence. Red and blue stars denote the epicenter of theMw 7.1 and 6.4 earthquakes, respectively. Black curves are faults mapped by U.S. Geological Survey (USGS). Red box indicates geographic location of the wrapped interferogram maps provided in (b) and (c). (b) Interferogram from the descending track 71 Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) data. Each fringe represents 2.8 cm of ground displacement away from the satellite. (c) Interferogram from the ascending track 64 Sentinel-1 InSAR data. The color version of this figure is available only in the electronic edition. 2 Seismological Research Letters www.srl-online.org • Volume XX • Number XX • – 2020 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190275/4921308/srl-2019275.1.pdf by UC San Diego Library user on 21 April 2020 (see Data and Resources). In addition, we highlight the ability to map small spatial scale (∼30 m) fractures having small offsets (>5 mm) using Sentinel-1 data. Data and Methods Here, we construct coseismic interferograms using two preearthquake acquisitions for each track and all data acquired within a month after the earthquakes (Table 1). Unfortunately, there are no acquisitions between the 36 hr that separated the two events. We produce three types of products using opensource Generic Mapping Tools Synthetic Aperture Radar (GMTSAR) (Sandwell et al., 2016) and Generic Mapping Tools (Wessel et al., 2013) software, with the phase unwrapped using the Statistical-cost, Network-flow Algorithm for Phase Unwrapping software (Chen and Zebker, 2002): 1. The standard interferograms shown in Figure 1 were produced using the nearest acquisitions spanning the earthquakes (Table 1). These were Gaussian filtered at 100 m half-wavelength and sampled at 50 m. Unwrapped and subsampled data, suitable for source modeling, are available on our website. Interestingly, the overall interferometric pattern from this sequence resembles the European Remote Sensing satellites interferogram for the 1999 Mw 7.1 Hector Mine earthquake (Sandwell et al., 2000; Fialko et al., 2002). Both events occurred in similar tectonic context in the ECSZ (Savage et al., 1990), yielding similar moment release and rupturing behavior along the E45S direction. 2. To extract information for smaller scale features, we produce phase-gradient maps directly from the real R x and imaginary I x parts of the full resolution interferograms (Sandwell and Price, 1998), in which the position vector x consists of the range r and azimuth a coordinates of the interferogram. Instead of computing the phase gradient from the phase φ x tan−1 I R , which as many 2π discontinuities, one can use the chain rule of differentiation to develop a formula for the phase gradient directly from R and I. The result is ∇φ x R∇I−I∇R R2 I2 , in which the gradient operator is ∇ ∂ ∂r ; ∂ ∂a , with r and a denoting the direction of gradient along range (look) and azimuth (flight). The numerical derivative filter must be designed to avoid aliasing short-wavelength noise at the Nyquist wavenumber to longer wavelengths, so we combined a central difference filter with a low-pass Gaussain filter having 0.5 gain at 30 m half-wavelength. Phase gradients are very small in the far field of the rupture, so we focus on the second subswath of each TOPS frame and process at full resolution (∼15 m). Unlike standard interferograms, these phase gradient maps can be directly stacked without phase unwrapping (Sandwell and Price, 1998). Thus, we applied the same algorithms to every interferogram (Table 1) and averaged them to produce the final phase gradient maps (Fig. 2). The phase gradient maps are essentially strain maps and thus highlight all types of small spatial scale deformation. There are two types of artifacts to consider when interpreting these maps. First, there are artificial linear phase discontinuities at the burst boundaries of the TOPSmode data. To overcome this, one can estimate the associated azimuthal motion by computing an earthquake source model and include this estimate at the Synthetic Aperture Radar (SAR) coregistration step. Second, the random patterns along the major rupture zones are areas of decorrelation due to extreme ground shaking or deformation rates beyond one radian per pixel. 3. To further define the deformation characteristics of each fracture, we unwrapped the full resolution interferogram following Xu et al. (2016), by imposing a coherence mask along the fault and allowing discontinuity in the map. We stacked the unwrapped phase and then highpass filtered using an 800 m Gaussian filter (Fig. 3). The stacking of unwrapped phase reduces the phase noise to ∼1 mm and also reduces atmospheric effect, especially the elevation-dependent component that possess resemblance to deformation pattern. These stacked phase maps are converted to line of sight (LoS) deformation and then decomposed into east–west motion (positive TABLE 1 Interferometric Pairs versus Perpendicular Baseline Direction Dates (yyyy/mm/dd) B⊥ (m) Descending average look vector: [0.633, −0.112, 0.765] 2019/06/22–2019/07/16 87.79 2019/06/22–2019/07/28 38.09 2019/07/04–2019/07/16 (Fig. 1b) 29.68 2019/07/04–2019/07/28 31.15 Ascending average look vector: [−0.636, −0.112, 0.763] 2019/06/28–2019/07/10 63.38 2019/06/28–2019/07/16 35.98 2019/06/28–2019/07/22 12.37 2019/06/28

A Worldwide and Unified Database of Surface Ruptures (SURE) for Fault Displacement Hazard Analyses

Abstract: Fault displacement hazard assessment is based on empirical relationshipsthat are established using historic earthquake fault ruptures.These relationships evaluate the likelihood of coseismicsurface slip considering on-fault and off-fault ruptures, for givenearthquake magnitude and distance to fault. Moreover, theyallow predicting the amount of fault slip at and close to theactive fault of concern. Applications of this approach includeland use planning, structural design of infrastructure, and criticalfacilities located on or close to an active fault.To date, the current equations are based on sparsely populateddatasets, including a limited number of pre-2000 events. In2015, an international effort started to constitute a worldwideand unified fault displacement database (SUrface Ruptures dueto Earthquakes [SURE]) to improve further hazard estimations.After two workshops, it was decided to unify the existingdatasets (field-based slip measurements) to incorporate recentand future cases, and to include new parameters relevant toproperly describe the rupture.This contribution presents the status of the SURE databaseand delineates some perspectives to improve the surface-faultingassessment. Original data have been compiled and adaptedto the structure. The database encompasses 45 earthquakesfrom magnitude 5–7.9, with more than 15,000 coseismic surfacedeformation observations (including slip measurements)and 56,000 of rupture segments. Twenty earthquake cases arefrom Japan, 15 from United States, two from Mexico, Italy,and New Zealand, one from Kyrgystan, Ecuador, Turkey,and Argentina. Twenty-four earthquakes are strike-slip faultingevents, 11 are normal or normal oblique, and 10 are reversefaulting.To pursue the momentum, the initial and common implementationeffort needs to be continued and coordinated, and themaintenance and longevity of the database must be guaranteed.This effort must remain based on a large and open communityof earthquake geologists to create a free and open accessdatabase.

Earthquake Early Warning ShakeAlert 2.0: Public Rollout

Abstract: The ShakeAlert earthquake early warning system is designed to automatically identify and characterize the initiation and rupture evolution of large earthquakes, estimate the intensity of ground shaking that will result, and deliver alerts to people and systems that may experience shaking, prior to the occurrence of shaking at their location. It is configured to issue alerts to locations within the West Coast of the United States. In 2018, ShakeAlert 2.0 went live in a regional public test in the first phase of a general public rollout. The ShakeAlert system is now providing alerts to more than 60 institutional partners in the three states of the western United States where most of the nation’s earthquake risk is concentrated: California, Oregon, and Washington. The ShakeAlert 2.0 product for public alerting is a message containing a polygon enclosing a region predicted to experience modified Mercalli intensity (MMI) threshold levels that depend on the delivery method. Wireless Emergency Alerts are delivered for M 5+ earthquakes with expected shaking of MMI≥IV⁠. For cell phone apps, the thresholds are M 4.5+ and MMI≥III⁠. A polygon format alert is the easiest description for selective rebroadcasting mechanisms (e.g., cell towers) and is a requirement for some mass notification systems such as the Federal Emergency Management Agency’s Integrated Public Alert and Warning System. ShakeAlert 2.0 was tested using historic waveform data consisting of 60 M 3.5+ and 25 M 5.0+ earthquakes, in addition to other anomalous waveforms such as calibration signals. For the historic event test, the average M 5+ false alert and missed event rates for ShakeAlert 2.0 are 8% and 16%. The M 3.5+ false alert and missed event rates are 10% and 36.7%. Real‐time performance metrics are also presented to assess how the system behaves in regions that are well‐instrumented, sparsely instrumented, and for offshore earthquakes.

Comparison between Distributed Acoustic Sensing and Geophones: Downhole Microseismic Monitoring of the FORGE Geothermal Experiment

Abstract: Cite this article as Lellouch, A., N. J. Lindsey, W. L. Ellsworth, and B. L. Biondi (2020). Comparison between Distributed Acoustic Sensing and Geophones: Downhole Microseismic Monitoring of the FORGE Geothermal Experiment, Seismol. Res. Lett. XX, 1–13, doi: 10.1785/0220200149. We compare the performance of a downhole distributed acoustic sensing (DAS) fiberoptic array with that of conventional geophones. The downhole collocated arrays are part of the Frontier Observatory for Research in Geothermal Energy (FORGE) geothermal experiment, in which stimulation of the rock volume in an enhanced geothermal system (EGS) causes microseismic events. The DAS acquisition system yields data sampled at every 1 m at 2000 samples per second for the entire length of the well, spanning to a depth of 985 m from the surface. Whereas single DAS channels are substantially noisier than geophones at the same location, their large number and spatial coherency allow for the application of effective array processing techniques. We follow a complete workflow for the fiber-optic array: velocity model building, event detection, event location, and magnitude estimation. Estimated velocity models agree well with sonic logging in a nearby well and map a granitic contact accurately. Detection performance is somewhat worse than geophones and yields magnitude completeness of −1:4 compared to −1:7 for geophones. Using a single vertical fiber array, we cannot retrieve the azimuth of the events relative to the well. However, we can very accurately estimate their depth and horizontal distance from the array. Magnitude estimation with DAS approaches geophone results to within a standard deviation ofM 0:115 and negligible mean difference. The DAS processing results outperform a regional and local surface array, consolidated with a shallow borehole sensor. Although downhole geophones in the FORGE experimental layout performed better, DAS holds several critical practical benefits that were not demonstrated. Thanks to its heat resistance, it can be deployed much closer to the reservoir; fibers can be deployed along cased active wells, eliminating the need for a dedicated monitoring well; the permanently installed fiber can be used for years or decades. Therefore, we argue that DAS holds vast potential for long-term monitoring of EGS projects.

Neural Network Applications in Earthquake Prediction (1994–2019): Meta‐Analytic and Statistical Insights on Their Limitations

Abstract: In the last few years, deep learning has solved seemingly intractable problems, boosting the hope to find approximate solutions to problems that now are considered unsolvable. Earthquake prediction, the Grail of Seismology, is, in this context of continuous exciting discoveries, an obvious choice for deep learning exploration. We review the entire literature of artificial neural network (ANN) applications for earthquake prediction (77 articles, 1994-2019 period) and find two emerging trends: an increasing interest in this domain, and a complexification of ANN models over time, towards deep learning. Despite apparent positive results observed in this corpus, we demonstrate that simpler models seem to offer similar predictive powers, if not better ones. Due to the structured, tabulated nature of earthquake catalogues, and the limited number of features so far considered, simpler and more transparent machine learning models seem preferable at the present stage of research. Those baseline models follow first physical principles and are consistent with the known empirical laws of Statistical Seismology, which have minimal abilities to predict large earthquakes.

Seismological Observatory Software: 30 Yr of SEISAN

Abstract: The SEISAN software package for processing of earthquake data has been in use for 30 yr. SEISAN is a collection of programs that help to carry out tasks from the basic processing at a seismological observatory to more advanced seismological research. During its history, the software has been adopted to different hardware and operating systems. However, the core of the software with a folder- and files-based database and event-based processing has remained stable. The main focus in the design and development of the software has been the efficiency in data processing for the user. The software comes with manual, tutorial, and training exercises. This together with regular training activities has made SEISAN a useful tool for many observatories around the world.

The New ShakeMap in Italy: Progress and Advances in the Last 10 Yr

Abstract: This work describes a procedure to configure U.S. Geological Survey (USGS)‐ShakeMap for a given region. The procedure is applied to Italy to update and improve the ShakeMap service provided by Istituto Nazionale di Geofisica e Vulcanologia (INGV). The new configuration features (1) the adoption of recently developed ground‐motion models (GMMs) and of an updated map of VS30 for the local site effects and (2) the adoption of the newly developed USGS‐ShakeMap version 4 (v.4) software (see Data and Resources). We have used the same subdivision in tectonic regimes adopted for the GMMs for the new Italian seismic hazard model (MPS19, Meletti et al., 2017) and selected the most appropriate GMMs after application of a ranking procedure consisting of statistical tests. A cross‐validation technique has been applied to test the goodness of the selected configuration and to compare the ShakeMaps obtained with the old (Michelini et al., 2008) and the new settings. Finally, the INGV ShakeMap workflow has been renovated to exploit the data and analysis chain implemented at INGV from real‐time data streams acquisition to analyst revised waveforms including additional data (e.g., revised location, fault geometry) that may become available days after the event occurrence.

Using Daily Observations from Planet Labs Satellite Imagery to Separate the Surface Deformation between the 4 July Mw 6.4 Foreshock and 5 July Mw 7.1 Mainshock during the 2019 Ridgecrest Earthquake Sequence

Abstract: On 4 July 2019, the Ridgecrest earthquake sequence began with a series of foreshocks including an Mw 6.4 event near Searles Valley, California. This was then followed 34 hr later by an Mw 7.1 mainshock located just 15 km to the north, with the earthquake sequence resulting in a complex array of intersecting faults. This earthquake sequence poses several interesting questions including, did the stress changes induced by the Mw 6.4 foreshock trigger the Mw 7.1 mainshock and what possible mechanism(s) could explain the occurrence of widespread secondary faulting surrounding both surface ruptures? However, most of the geodetic data (such as Interferometric Synthetic Aperture Radar, light detection and ranging, and optical satellite imagery) were acquired after both events had occurred making it difficult to discern which surface fractures happened when and their possible triggering mechanism. Here, we provide a dataset composed of high-resolution optical imagery, pixel-value difference maps, .kmz fracturing mapping, and horizontal deformation maps derived from subpixel image correlation, which can uniquely separate the surface fracturing and deformation between the foreshock and mainshock events that can help answer these questions. Separate imaging of the events is made possible by the daily acquisition of optical imagery by the Planet Labs cubesat constellation, which acquired data between the two earthquakes, in the morning of 4 and 5 July, at 11.13 a.m. and 05.12 p.m. PST, respectively, with the images acquired just 40 min after the foreshock and 56 min before the mainshock, respectively. Analysis from this optical imagery reveals the location of surface faulting that allows us to map their spatial extent and determine their timing. These data which we provide here can help guide and validate field survey observations to help understand which faults ruptured when, and constrain slip inversion models for more accurate estimates of stress changes induced by the foreshock imposed on the surrounding faults.

COVID-19 Societal Response Captured by Seismic Noise in China and Italy

Abstract: Seismic noise with frequencies above 1 Hz is often called "cultural noise" and is generally correlated quite well with human activities Recently, cities in mainland China and Italy imposed restrictions on travel and day-to-day activity in response to COVID-19, which gave us an unprecedented opportunity to study the relationship between seismic noise above 1 Hz and human activities Using seismic records from stations in China and Italy, we show that seismic noise above 1 Hz was primarily generated by the local transportation systems The lockdown of the cities and the imposition of travel restrictions led to an similar to 4-12 dB decrease in seismic noise power in mainland China Data also show that different Chinese cities experienced distinct periods of diminished cultural noise, related to differences in local response to the epidemic In contrast, there was only similar to 1-6 dB decrease of seismic noise power in Italy, after the country was put under a lockdown The noise data indicate that traffic flow did not decrease as much in Italy and show how different cities reacted distinctly to the lockdown conditions

The Community Code Verification Exercise for Simulating Sequences of Earthquakes and Aseismic Slip (SEAS)

Abstract: Numerical simulations of Sequences of Earthquakes and Aseismic Slip (SEAS) have made great progress over the past decades to address important questions in earthquake physics and fault mechanics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between aseismic fault slip, earthquake nucleation, and dynamic rupture; and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing capability and complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code-verification exercise for SEAS simulations, supported by the Southern California Earthquake Center (SCEC). Here we report the findings from our first two benchmark problems (BP1 and BP2), designed to test the capabilities of different computational methods in correctly solving a mathematically well-defined, basic problem in crustal faulting. These benchmarks are for a 2D antiplane problem, with a 1D planar vertical strike-slip fault obeying rate-and-state friction, embedded in a 2D homogeneous, linear elastic half-space. Sequences of quasi-dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of >70 simulation results from 11 groups using different numerical methods, uploaded to our online platform, show excellent agreements in long-term and coseismic evolution of fault properties. In BP1, we found that the truncated domain boundaries influence interseismic fault stressing, earthquake recurrence, and coseismic rupture process, and that agreement between models is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of long-term fault behavior depends on how well important physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in the generation of artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard, such as earthquake size distributions, moment release, and earthquake recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.

The Bayesian Earthquake Analysis Tool

Abstract: The authors thank Jiři Vackař, Romain Jolivet, one anynomous reviewer, and the editors for their comments that helped to improve the quality of this article. This research was supported by King Abdullah University of Science and Technology (KAUST), under Award Numbers BAS/1/1353-01-01 and BAS/1/1339-01-1. H. V.-B. was partially supported by Geo.X, the Research Network for Geosciences in Berlin and Potsdam under the Project Number SO_087_GeoX. Henriette Sudhaus, Andreas Steinberg, and Marius Paul Isken acknowledge founding by the German Research Foundation (DFG) through an Emmy-Noether Young Researcher Grant Number 276464525. Hannes Vasyura-Bathke owes the most gratitude to his belove wife Olha for her tireless support and tolerance during many evenings and nights spent writing the code and this article.

Real-Time High-Rate GNSS Displacements: Performance Demonstration During the 2019 Ridgecrest, CA Earthquakes

Abstract: Traditional real-time seismology has relied on inertial sensors to characterize ground motions and earthquake sources, particularly for hazards applications such as warning systems. In the past decade, a revolution in high-rate, real-time Global Navigation Satellite System (GNSS) displacement have provided a new source of data to augment traditional measurement devices. The Ridgecrest, California earthquake sequence in 2019 provided one of the most complete recordings of real-time GNSS displacements to date, helping to aid in an initial source characterization over the first few days. In this manuscript, we analyze and make available the archived real-time displacement streams and compare their performance to post-processed results, which we also provide. We find good agreement for all stations showing a noticeable signal. This demonstrates that simple modeling in real-time, such as peak ground displacement scaling, would be practically identical to post-processed results. Similarly, we find good agreement across the full spectral range, from the coseismic offsets (~0 Hz) to the Nyquist frequency. We also find low latency between the measurement acquisition at the field site and the position calculation at the datacenter. In aggregate, the performance during the Ridgecrest earthquakes is strong evidence of the viability and usefulness of real-time GNSS as a monitoring tool.

Surface Deformation Related to the 2019 Mw 7.1 and 6.4 Ridgecrest Earthquakes in California from GPS, SAR Interferometry, and SAR Pixel Offsets

Abstract: We analyzed Synthetic Aperture Radar (SAR) images from Copernicus Sentinel‐1A and 1B satellites operated by the European Space Agency and the Advanced Land Observation Satellite‐2 (ALOS‐2) satellite operated by the Japan Aerospace Exploration Agency and Global Navigation Satellite System (GNSS) data from the Network of the Americas for the 4 July 2019 M_w 6.4 and 5 July (local; 6 July UTC) M_w 7.1 Ridgecrest earthquakes. We integrated geodetic measurements for the 3D vector field of coseismic surface deformation for the two events, using SAR data from Sentinel‐1 and ALOS‐2 satellites. We combined less precise large‐scale displacements from SAR images by pixel offset tracking or matching, including the along‐track component, with the more precise SAR interferometry (Interferometric Synthetic Aperture Radar [InSAR]) measurements in the radar line of sight (LoS) direction and intermediate‐precision along‐track InSAR to estimate all three components of the surface displacement for the two events together. We also estimated the coseismic deformation for the two earthquakes from time‐series processing of continuous Global Navigation Satellite System data stations in the area. InSAR coherence and coherence change maps the surface disruptions due to fault ruptures reaching the surface. Large slip in the M_w 6.4 earthquake was on a NE‐striking fault that intersects with the NW‐striking fault that was the main rupture in the M_w 7.1 earthquake. The main fault bifurcates towards the southeast ending 3 km from the Garlock Fault. The Garlock fault had triggered slip of about 20 mm in the radar LoS along a short section directly south of the main rupture. About 3 km northwest of the M_w 7.1 epicenter, the surface fault separates into two strands that form a pull‐apart with about 1 m of down‐drop. Further northwest is a wide zone of complex deformation.

The Alaska Amphibious Community Seismic Experiment

Abstract: The Alaska Amphibious Community Seismic Experiment (AACSE) is a shoreline-crossing passive- and active-source seismic experiment that took place from May 2018 through August 2019 along an ∼700 km long section of the Aleutian subduction zone spanning Kodiak Island and the Alaska Peninsula. The experiment featured 105 broadband seismometers; 30 were deployed onshore, and 75 were deployed offshore in Ocean Bottom Seismometer (OBS) packages. Additional strong-motion instruments were also deployed at six onshore seismic sites. Offshore OBS stretched from the outer rise across the trench to the shelf. OBSs in shallow water (<262 m depth) were deployed with a trawl-resistant shield, and deeper OBSs were unshielded. Additionally, a number of OBS-mounted strong-motion instruments, differential and absolute pressure gauges, hydrophones, and temperature and salinity sensors were deployed. OBSs were deployed on two cruises of the R/V Sikuliaq in May and July 2018 and retrieved on two cruises aboard the R/V Sikuliaq and R/V Langseth in August–September 2019. A complementary 398-instrument nodal seismometer array was deployed on Kodiak Island for four weeks in May–June 2019, and an active-source seismic survey on the R/V Langseth was arranged in June 2019 to shoot into the AACSE broadband network and the nodes. Additional underway data from cruises include seafloor bathymetry and sub-bottom profiles, with extra data collected near the rupture zone of the 2018 Mw 7.9 offshore-Kodiak earthquake. The AACSE network was deployed simultaneously with the EarthScope Transportable Array (TA) in Alaska, effectively densifying and extending the TA offshore in the region of the Alaska Peninsula. AACSE is a community experiment, and all data were made available publicly as soon as feasible in appropriate repositories.

Contemporary Earthquake Hazards in the West‐Northwest Himalaya: A Statistical Perspective through Natural Times

Abstract: Himalayan earthquakes have deep societal and economic impact. In this article, we implement a surrogate method of nowcasting (Rundle et al., 2016) to determine the current state of seismic hazard from large earthquakes in a dozen populous cities from India and Pakistan that belong to the west-northwest part of Himalayan orogeny. For this, we (1) perform statistical inference of natural times, intersperse counts of small-magnitude events between pairs of succeeding large events, based on a set of eight probability distributions; (2) compute earthquake potential score (EPS) of 14 cities from the best-fit cumulative distribution of natural times; and (3) carry out a sensitivity testing of parameters—threshold magnitude and area of city region. Formulation of natural time (Varostos et al., 2005) based on frequency–magnitude power-law statistics essentially avoids the daunting need of seismicity declustering in hazard estimation. A retrospective analysis of natural time counts corresponding to M≥6 events for the Indian cities provides an EPS (%) as New Delhi (56), Chandigarh (86), Dehradun (83), Jammu (99), Ludhiana (89), Moradabad (84), and Shimla (87), whereas the cities in Pakistan observe an EPS (%) as Islamabad (99), Faisalabad (88), Gujranwala (99), Lahore (89), Multan (98), Peshawar (38), and Rawalpindi (99). The estimated nowcast values that range from 38% to as high as 99% lead to a rapid yet useful ranking of cities in terms of their present progression to the regional earthquake cycle of magnitude ≥6.0 events. The analysis inevitably encourages scientists and engineers from governments and industry to join hands for better policymaking toward land-use planning, insurance, and disaster preparation in the west-northwest part of active Himalayan belt.

Rose Parade Seismology: Signatures of Floats and Bands on Optical Fiber

Abstract: The 2020 Rose Parade in Pasadena, California, was recorded by the Pasadena distributed acoustic sensing array, which utilizes the underground telecom fiber optic cables as sensors. The floats and bands generate remarkable broadband seismic signatures that can be captured at meters’ resolution.

Documentation of Surface Fault Rupture and Ground‐Deformation Features Produced by the 4 and 5 July 2019 Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence

Abstract: Cite this article asaa Ponti, D. J., J. L. Blair, C. M. Rosa, K. Thomas, A. J. Pickering, S. Akciz, S. Angster, J.P. Avouac, J. Bachhuber, S. Bacon, et al. (2020). Documentation of Surface Fault Rupture and Ground-Deformation Features Produced by the 4 and 5 July 2019 Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence, Seismol. Res. Lett. XX, 1–18, doi: 10.1785/0220190322. The Mw 6.4 and Mw 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground-deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented bymapping of fault rupture and ground-deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes aswell as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts. Introduction The July 2019 Ridgecrest earthquake sequence included two large earthquakes that each produced extensive surface faulting and shaking-related ground deformation in Indian Wells Valley and Searles Valley, within the eastern California shear zone of southern California. Much of this deformation occurred within the boundary of the Naval Air Weapons Station China Lake (NAWSCL) where civilian access is restricted (Fig. 1). In the weeks following the earthquakes, surface faulting and ground deformation features were documented in the field bymore than six dozen geologists and other scientists from the California Geological Survey (CGS), U.S. Geological Survey (USGS), National Aeronautics and Space Administration (NASA), U.S. Navy, and numerous universities and commercial firms. These researchers worked together in the field with the coordination and support of the California Earthquake Clearinghouse (Earthquake Engineering Research Institute [EERI], 2020) and the U.S. Navy, whose personnel were also instrumental in arranging access and for providing support for field activities within the boundary of the NAWSCL. To support emergency response efforts and planning for field investigations and instrument deployments, data collected in the field were rapidly synthesized by USGS and CGS office teams (Pickering et al., 2019), and provisional maps of observation sites and fault rupture were updated on a regular basis and disseminated through the California Earthquake Clearinghouse, Full author list and affiliations appear at the end of this article. *Corresponding author: dponti@usgs.gov © Seismological Society of America Volume XX • Number XX • – 2020 • www.srl-online.org Seismological Research Letters 1 Data Mine Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190322/5101761/srl-2019322.1.pdf by UC San Diego Library user on 12 August 2020 the Earthquake Engineering Research Institute's Virtual Clearinghouse website (see Data and Resources) and the Southern California Earthquake Center’s (SCEC) earthquake response blog (see Data and Resources). Field documentation of earthquake effects, including the length and continuity of surface faulting, amount and sense of fault slip, and the occurrence and nature of shaking-induced ground failure, is fundamental to evaluating earthquake processes and for assessing earthquake hazards. In the case of the Ridgecrest earthquake sequence, postearthquake geologic activities included an initial reconnaissance phase during the first few days after the events, followed by a data collection phase that extended into December 2019. The initial reconnaissance phase focused on quickly determining overall fault-rupture location and extent, maximum coseismic fault slip, possible postseismic creep, and areas and extent of ground failure due to liquefaction and slope failure. This information provided critical situational awareness for emergency response and recovery efforts, for guiding seismic and geodetic instrument deployments, and for acquiring airborne light detection and ranging (lidar) and other imagery (Hudnut et al., 2020) to support follow-on research. The data collection phase included detailed observations and mapping of faulting and grounddeformation features. Detailed field measurements of slip vectors derived from offset features or observed slickenlines on fault planes provide insight into rupture dynamics (e.g., Haddon et al., 2019) and fault kinematics. Slip distributions along strike and data on rupture extent (Olson et al., 2019; DuRoss et al., 2020) offer important constraints for finite fault models, dynamic rupture simulations, and strong ground motion modeling (e.g., Hough et al., 2019; Pollitz et al., 2019; Thomas et al., 2019; Lozos and Harris, 2020) and facilitate the comparison of field-based and remotely sensed observations (Gold et al., 2019). Maps of fault rupture provide insights into cross-fault interactions (e.g., Hudnut et al., 2019) and how faults may, or may not, be linked. Maps detailing rupture complexity and the association of coseismic rupture with existing tectonic geomorphology provide important insights into long-term fault activity and for refining fault hazard zones (e.g., Dawson et al., 2019; Thompson Jobe et al., 2019). Reconnaissance teams also identified and documented localized slip on both previously mapped and unmapped faults (Bilham and Costello, 2019; Hernandez and Dawson, 2019), further highlighting other potentially active fault zones within the region. In addition to surface faulting, both earthquakes produced shaking-related ground deformation associated with liquefaction and slope failure; these features were also documented in detail throughout the epicentral area. Recent technological advances, such as data collection software that can run on smartphones and mobile tablets, allow for the simultaneous capture of notes, photographs, video and audio clips, and Global Positioning System (GPS) or Global Navigation Satellite Systems (GNSS) coordinate information. As a result, it is now possible to rather easily collect, compile, disseminate, and most importantly preserve large volumes of original field data in formats that can be readily accessed and searched. Similar to the postearthquake effort following the 2014 South Napa earthquake (Ponti et al., 2019a,b), CGS and USGS collected and compiled field observations and data from all contributing researchers involved in the Ridgecrest postearthquake field-response effort. This article describes what data were captured following the earthquakes and how they were compiled. In a companion USGS data release (Ponti et al., 2020), we present comprehensive documentation of observable faulting and surface deformation produced by the Ridgecrest earthquake sequence. These data include: (1) site-specific observations with field descriptions, displacement measurements, and photographs, and (2) maps of surface rupture that integrate the field observations with field-based mapping, airborne imagery, lidar, and satellite-based geodetic imaging products. We anticipate these datasets to be updated and added to incrementally as information is obtained, compiled and reviewed to produce an extensive curated collection of field observations and a comprehensive large-scale regional map showing all documented zones of surface faulting and ground deformation produced by the Ridgecrest earthquake sequence. Data Collection The first of two major earthquakes in the Ridgecrest sequence was an Mw 6.4 that occurred at 10:33 a.m. Pacific Daylight Time (PDT) on 4 July 2019 with an epicenter located approximately 17 km northeast of the town of Ridgecrest, California (Fig. 1). Early aftershock locations and initial media reports of damage to Highway 178 indicated a unilateral rupture to the southwest of the epicenter with left-lateral slip along a northeast–southwest-striking fault zone (Stewart et al., 2019). By the evening of 4 July, field reconnaissance teams had arrived in the area and documented approximately 40–50 cm of left-lateral offset distributed across a 165 m wide zone crossing Highway 178. The following day, both ground and helicopter reconnaissance had identified most of the extent of northeast– southwest rupture. That evening, at 8:19 p.m. PDT, an Mw 7.1 earthquake nucleated at a depth of 8 km about 11.5 km northwest of the Mw 6.4 epicenter and ruptured bilaterally along a northwest–southeast-striking dextral fault zone. Fault rupture from this second event intersected the northeast portion of the Mw 6.4 rupture and, as documented later that night, ruptured Highway 178 approximately 5 km east of the Mw 6.4 offsets (Fig. 1). Field work to document both rupture zones and shaking-induced ground failure intensified over the next several weeks. Because access to the NAWSCL was generally limited to researchers from CGS and USGS, nongovernmen

Along‐Arc Heterogeneity in Local Seismicity across the Lesser Antilles Subduction Zone from a Dense Ocean‐Bottom Seismometer Network

Abstract: The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65 km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.

The Oklahoma Geological Survey Statewide Seismic Network

Abstract: The Oklahoma Geological Survey (OGS) monitors seismicity throughout the state of Oklahoma utilizing permanent and temporary seismometers installed by OGS and other agencies, while producing a real-time earthquake catalog. The OGS seismic network was recently added to the Advanced National Seismic System (ANSS) as a self-supporting regional seismic network, and earthquake locations and magnitudes are automatically reported through U.S. Geological Survey and are part of the ANSS Comprehensive Earthquake Catalog. In Oklahoma, before 2009, background seismicity rates were about 2 M 3.0+ earthquakes per year, which increased to 579 and 903 M 3.0+ earthquakes in 2014 and 2015, respectively. After seismicity peaked, the rate fell to 624, 304, and 194 M 3.0+ earthquakes in 2016, 2017, and 2018, respectively. The catalog is complete down to M 2.2 from mid-2014 to present, despite the significant workload for a primarily state-funded regional network. That astonishing uptick in seismicity has been largely attributed to wastewater injection practices. The OGS provides the Oklahoma Corporation Commission, the agency responsible for regulating oil and gas activities within the state, with technical guidance and earthquake products that inform their “traffic-light” mitigation protocol and other mitigating actions. We have initiated a citizen-scientist-driven, educational seismometer program by installing Raspberry Shake geophones throughout the state at local schools, museums, libraries, and state parks. The seismic hazard of the state portends a continued need for expansion and densification of seismic monitoring throughout Oklahoma.

NoisePy: A New High‐Performance Python Tool for Ambient‐Noise Seismology

Abstract: The fast-growing interests in high spatial resolution of seismic imaging and high temporal resolution of seismic monitoring pose great challenges for fast, efficient, and stable data processing in ambient-noise seismology. This coincides with the explosion of available seismic data in the last few years. However, the current computational landscape of ambient seismic field seismology remains highly heterogeneous, with individual researchers building their own homegrown codes. Here, we present NoisePy—a new high-performance python tool designed specifically for large-scale ambient-noise seismology. NoisePy provides most of the processing techniques for the ambient field data and the correlations found in the literature, along with parallel download routines, dispersion analysis, and monitoring functions. NoisePy takes advantage of adaptable seismic data format, a parallel input and output enabled HDF5 data format designed for seismology, for a structured organization of the cross-correlation data. The parallel computing of NoisePy is performed using Message Passing Interface and shows a strong scaling with the number of cores, which is well suited for embarrassingly parallel problems. NoisePy also uses a small memory overhead and stable memory usage. Benchmark comparisons with the latest version of MSNoise demonstrate about four-time improvement in compute time of the cross correlations, which is the slowest step of ambient-noise seismology. NoisePy is suitable for ambient-noise seismology of various data sizes, and it has been tested successfully at handling data of size ranging from a few GBs to several tens of TBs.

The 2018 Mw 6.8 Zakynthos, Greece, Earthquake: Dominant Strike‐Slip Faulting near Subducting Slab

Abstract: Cite this article as Sokos, E., F. Gallovič, C. P. Evangelidis, A. Serpetsidaki, V. Plicka, J. Kostelecký, and J. Zahradník (2020). The 2018 Mw 6.8 Zakynthos, Greece, Earthquake: Dominant Strike-Slip Faulting near Subducting Slab, Seismol. Res. Lett. 91, 721–732, doi: 10.1785/0220190169. Supplemental Material With different styles of faulting, the eastern Ionian Sea is an ideal natural laboratory to investigate interactions between adjacent faults during strong earthquakes. The 2018 Mw 6.8 Zakynthos earthquake, well recorded by broadband and strong-motion networks, provides an opportunity to resolve such faulting complexity. Here, we focus on waveform inversion and backprojection of strong-motion data, partly checked by coseismic Global Navigation Satellite System data. We show that the region is under subhorizontal southwest–northeast compression, enabling mixed thrust faulting and strike-slip (SS) faulting. The 2018 mainshock consisted of two fault segments: a lowdip thrust, and a dominant, moderate-dip, right-lateral SS, both in the crust. Slip vectors, oriented to southwest, are consistent with platemotion. The sequence can be explained in terms of trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. The 2018 event, and anMw 6.6 event of 1997, occurred near three localized swarms of 2016 and 2017. Future numerical models of the slab deformation and ocean-bottom seismometer observations may illuminate possible relations among earthquakes, swarms, and fluid paths in the region. Introduction Multiple faults acting during an earthquake have been generally well known, but on global scale less observations have been available for near-simultaneous ruptures of different faulting mechanisms, specifically in subduction zones. For example, Lay et al. (2013) reported a doublet of twoMw ∼ 7 events below Japan trench, where a thrust faulting (TF) along the subduction interface was followed after 14 s by a shallower normal faulting in the overriding plate. A rare evidence of a thrust event on a plate interface, which triggered a normal-faulting event in the overriding plate was provided for an Mw ∼ 7 earthquake in the Chile subduction zone (Hicks and Rietbrock, 2015). Particularly challenging in terms of strain partitioning are the regions where subduction terminates, and plate motions continue along transform faults. A good example is the 2016 Mw 7.8 earthquake in New Zealand, in which a dominant strike-slip (SS) faulting occurred in the upper plate and possibly triggered minor slip on the underlying subduction thrust (Mouslopoulou et al., 2019; Ulrich et al., 2019). Resolving fault complexity for strong earthquakes (Mw 6.0–6.9) in the shallowest parts of the subduction termination zones is even more challenging, and this article focuses on such a task in western Greece. The major ongoing convergent tectonic process in western Greece is subduction, imaged by seismic tomography and other structural studies (Spakman et al., 1993; Laigle et al., 2004; Suckale et al., 2009; Sachpazi et al., 2016; Halpaap et al., 2019). The active overriding of the Aegean plate over the subducting African plate, derived from Global Navigation Satellite System (GNSS) data, is oriented toward southwest, approximately perpendicular to the trench, being consistent with slip vectors of many earthquakes (Kiratzi and Louvari, 2003; Hollenstein et al., 2006; Shaw and Jackson, 2010). Zakynthos (or Zante) Island is situated at a subduction-termination zone, a part of the Ionian Islands–Akarnania Block (IAB) (Pérouse et al., 2017). At the northwest edge, this block is separated from Apulian–Ionian microplate by the right-lateral Cephalonia transform fault. The southwest boundary of IAB is the Hellenic subduction backstop front. The northeast boundary of IAB is a mixture of SS and extensional structures (e.g., the Corinth Gulf). The southeast boundary of IAB has not been well known until the 2008Mw 6.3 Movri Mountain earthquake 1. Department of Geology, Seismological Laboratory, University of Patras, Patras, Greece; 2. Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic; 3. National Observatory of Athens, Institute of Geodynamics, Athens, Greece; 4. Faculty of Mining and Geology, University of Ostrava, Ostrava, Czech Republic *Corresponding author: esokos@upatras.gr © Seismological Society of America Volume 91 • Number 2A • March 2020 • www.srl-online.org Seismological Research Letters 721 Downloaded from http://pubs.geoscienceworld.org/ssa/srl/article-pdf/91/2A/721/4956466/srl-2019169.1.pdf by Charles University user on 21 December 2020 (Gallovič et al., 2009; Konstantinou et al., 2009; Serpetsidaki et al., 2014), which proved activity of a blind right-lateral transform fault crossing the western Peloponnese and possibly continuing further toward the southwest into the Ionian Sea. The interior of IAB, south of Zakynthos, is a zone of “diffuse deformation” (Pérouse et al., 2017). Although being continuously seismically active, and being obviously related to shallow subduction process, its faulting style has not been understood yet. In this respect, the recent 2018 Mw 6.8 event plays an important role for seismotectonic interpretations in western Greece. Lessons learned here may apply also for other subduction-termination zones (Mouslopoulou et al., 2019), where the deformation partitioning between the slab and upper plate may take a variety of forms. Knowledge of the crustal structure of the studied region has been significantly improved by a mixed onshore and offshore (ocean-bottom) temporary seismic network (Papoulia et al., 2014). The latter study illuminated the spatial variation of the Moho depth, and provided a layered velocity model consistently used throughout this article. A weakly northeastdipping (∼5°) seismic reflector has been detected at the depth of ∼10–15 km, supposedly mapping the top of the subducting plate in the area west and southwest of Zakynthos (Clément et al., 2000; Laigle et al., 2004), situated ∼10 km above Moho (Halpaap et al., 2019). Lacking more detailed information, in the following parts of this article we use the wellmapped Moho depth of Papoulia et al. (2014) and plot the slab top 10 km above the Moho. It provides an approximate (schematic) location of the slab top, at the depths 10–20 km, close to the above-cited reflector depths. In the instrumental era, the region near Zakynthos experienced three Mw 6–7 earthquakes, roughly every 20 yr (1959, 1976, 1997, and 2018), consistently with a high seismic coupling (Laigle et al., 2002; Chousianitis et al., 2015). No historicalM > 7 event has been documented. On 25 October 2018, at 22:54 UTC, an Mw 6.8 earthquake occurred southwest of Zakynthos. It caused limited damage on the island and no casualties (Institute of Engineering Seismology and Earthquake Engineering [ITSAK], 2018). The event was observed globally, and its broad characteristics were soon outlined as follows. The Global Centroid Moment Tensor (Global CMT) project suggested a centroid depth of 12 km, scalar moment M0 2:3 × 1019 N · m, and strike/dip/rake angles of 13°/24°/165°, an oblique-TF mechanism. The Global CMT solution comprised a significant non-double-couple (non-DC) component, namely a compressional compensated linear vector dipole, CLVD −44%. The European data centers reported centroid depths <20 km, with focal mechanisms ranging from the mixed SS and thrust type to an almost pure SS, often with a notable non-DC component. For example, the National Observatory of Athens (NOA) published the moment tensor (MT) with CLVD of −61%. Interestingly, an Mw 4.8 foreshock was a pure low-dip thrusting mechanism (strike/dip/rake ∼300°=10°=100°), similar to four major Mw 5+ aftershocks; however, smaller aftershocks were of both types, thrust and SS. Our preliminary analysis, reported to European Mediterranean Seismological Centre two weeks after the event, speculated about a segmented fault (Zahradník et al., 2018). The goal of this article is to improve understanding of the complex faulting style taking place near Zakynthos, complementing our previous earthquake studies of the Ionian Sea islands of Lefkada and Cephalonia (Sokos et al., 2015, 2016). To this goal, we analyze source process of the 2018 mainshock and aftershocks using regional broadband, accelerometric, and GNSS data, considering also the 2011–2018 seismicity of the region. We interpret the mainshock in terms of a segmented source model, possibly related to trench-orthogonal fractures in the subducting plate and reactivated faults in the upper plate. Source Modeling Point-source models of mainshock and aftershocks The mainshock nucleated ∼45 km southwest of Zakynthos, close to a local bathymetric low (the sea depth of 4 km; see Fig. 1). Exact hypocenter position is unknown because a small foreshock of an unknown position and magnitude preceded the mainshock by a few seconds, thus complicating the arrivaltime picking. For the same reason, the first-motion polarities are problematic. We made a probabilistic location (Lomax et al., 2001), see Text S1, and Figures S1 and S2 (available in the supplemental material to this article), pointing to a shallow depth, and hereafter we use the epicenter (latitude/longitude 37.27°/20.43°) corresponding to the arbitrarily fixed source depth of 5 km, with origin time of 22:54:47.5 UTC. None of the following modeling methodologies relies on the particular hypocenter position. A significant Mw 4.8 foreshock occurred 32 min before the mainshock (Fig. S1). The mainshock was followed by a standard exponentially decaying aftershock sequence that we first located with Hypoinverse (Klein, 2002), and then relocated with hypoDD code (Waldhauser, 2001). Their median formal errors are ∼1 km, and a few hundred meters, respectively. The sequence included one Mw 5.1 event 15 min after mainshock, and four other events of Mw 5+ in the first week. After ∼80 days, anot

Co‐ and Early Postseismic Deformation Due to the 2019 Ridgecrest Earthquake Sequence Constrained by Sentinel‐1 and COSMO‐SkyMed SAR Data

Abstract: Cite this article as Wang, K., and R. Bürgmann (2020). Coand Early Postseismic Deformation Due to the 2019 Ridgecrest Earthquake Sequence Constrained by Sentinel-1 and COSMOSkyMed SAR Data, Seismol. Res. Lett. XX, 1–12, doi: 10.1785/0220190299. The 2019 Ridgecrest earthquake sequence ruptured a series of conjugate faults in the broad eastern California shear zone, north of the Mojave Desert in southern California. The average spacing between Global Navigation Satellite System (GNSS) stations around the earthquakes is 20–30 km, insufficient to constrain the rupture details of the earthquakes. Here, we use Sentinel-1 and COSMO-SkyMed (CSK) Synthetic Aperture Radar data to derive the high-resolution coseismic and early postseismic surface deformation related to the Ridgecrest earthquake sequence. Line of sight (LoS) Interferometric Synthetic Aperture Radar displacements derived from both Sentinel-1 and CSK data are in good agreement with GNSS measurements. The maximum coseismic displacement occurs near the Mw 7.1 epicenter, with an estimated fault offset of ∼4:5 m on a northwest-striking rupture. Pixel tracking analysis of CSK data also reveals a sharp surface offset of ∼ 1 m on a second northwest-striking fault strand on which the Mw 6.4 foreshock likely nucleated, which is located ∼ 2–3 km east of the major rupture. The lack of clear surface displacement across this fault segment during theMw 6.4 event suggests this fault might have ruptured twice, with more pronounced and shallow slip during the Mw 7.1 mainshock. Both Sentinel-1 and CSK data reveal clear postseismic deformation following the 2019 Ridgecrest earthquake sequence. Cumulative postseismic deformation near the Mw 7.1 epicenter ∼2 months after the mainshock reaches ∼5 cm along the satellites’ LoSs. The observed postseismic deformation near the fault is indicative of both afterslip and poroelastic rebound. We provide data derived in this study in various data formats, which will be useful for the broad community studying this earthquake sequence. Introduction On 4 July 2019, anMw 6.4 earthquake struck the Searles Valley, near the town of Ridgecrest in southern California. TheMw 6.4 earthquake was followed by an Mw 7.1 event that took place about 34 hr later. Reports from field surveys and satellite imagery, including Interferometric Synthetic Aperture Radar (InSAR) and optical images, indicate a complex network of faults activated by the 2019 Ridgecrest earthquakes (Ross et al., 2019). To understand the rupture process, the interactions between the two large events, and the associated stress changes on the regional fault system, detailed fault-slip models are necessary. High precision and resolution geodetic data are essential for this effort. As part of the Plate Boundary Observation (PBO) project, thousands of continuous Global Navigation Satellite System (GNSS) stations are deployed across much of the western United States, providing near-real-time 3D measurements of surface movement across the Pacific-North American plate boundary at an accuracy of subcentimeter level. Despite their high accuracy and temporal resolution, GNSS measurements generally have poor spatial coverage and low-spatial resolution, due to the overall high cost of installation and maintenance. For PBO, the current average spacing between GNSS stations is 20–30 km in California, which is comparable to the rupture length of the 2019 Ridgecrest earthquake. In addition, temporally sparse campaign GNSS measurements help fill in some of the gaps in the coverage (e.g., Floyd et al., 2019). Measurements of surface deformation at such a spatial resolution are useful to characterize the overall rupture process, but they are 1. Department of Earth and Planetary Sciences, University of California, Berkeley, California, U.S.A. *Corresponding author: kwang@seismo.berkeley.edu © Seismological Society of America Volume XX • Number XX • – 2020 • www.srl-online.org Seismological Research Letters 1 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190299/4939897/srl-2019299.1.pdf by University of California Berkeley Library user on 07 May 2020 insufficient to reliably constrain the details of the rupture sequence. For example, they cannot account for the subsurface slip distribution or slip on secondary fault strands, which are important for understanding the stress interactions between the Mw 6.4 foreshock and the Mw 7.1 mainshock and their effects on nearby faults. Spaceborne Synthetic Aperture Radar (SAR) measures the phase and amplitude of reflecting targets at a spatial resolution of a few meters, over a wide area. It is therefore ideal to study the ground deformation related to tectonic and nontectonic processes, including earthquakes, volcanoes, landslides, and land subsidence (Bürgmann et al., 2000). Here, we document the coand early postseismic surface deformation, due to the 2019 Ridgecrest earthquake sequence, derived from Sentinel-1 and COSMO-SkyMed (CSK) SAR data. For data from both sensors, we derive the coseismic line of sight (LoS) displacements with conventional InSAR methods. We correct for various potential error sources (e.g., unwrapping errors and elevationdependent atmospheric artifacts), and then we compare the results with GNSS observations to ensure that there are no systematic errors in our InSAR data. For the CSK data, we also derive the coseismic displacement along the azimuth direction of the satellite paths, using a pixel tracking technique (e.g., Fialko et al., 2001). We provide the data products in various formats, including the high-resolution grid data in Network Common Data Form (NetCDF) and Geographic Tagged Image File Format (GeoTiff), as well as deliberately downsampled ASCII files that can be directly used for coseismic slip modeling and other analyses (see Data and Resources). This article is complementary to several other contributions in this volume that focus on space geodetic observations. Specifically, Xu and Sandwell (2019) use Sentinel-1 InSAR to map the overall deformation field and focus on phase gradients associated with the smallscale fractures that are associated with 2019 Ridgecrest earthquake sequence. Fielding et al. (2019) report the surface deformation of the earthquake sequence with additional Advanced Land Observation Satellite-2 data (ALOS-2). Milliner and Donnellan (2019) present a unique dataset of surface deformation due to both the Mw 6.4 and the Mw 7.1 events using PlanetLab optical imagery. Floyd et al. (2019) present GNSS measurements collected immediately after theMw 6.4 foreshock on 4 July, which will be essential to constrain the rupture process of the Mw 6.4 foreshock. In our article, in addition to the coseismic Sentinel-1 observations that are also considered in Xu and Sandwell (2019) and Fielding et al. (2019), we also include the LoS displacement and azimuthal offset measurements that were derived from the X-band CSK data, as well as early postseismic deformation measurements derived from both Sentinel-1 and CSK data. The high-resolution X-band CSK data allow us to derive detailed maps of surface deformation due to the Ridgecrest earthquake sequence along the satellite’s flying direction, complementing the InSAR phase measurements along the LoS. Data and Methods C-band Sentinel-1 The epicentral area of the 2019 Ridgecrest earthquake is covered by two Sentinel-1 satellite tracks; these would be ascending track A064 and descending track D071 (Fig. 1). Both tracks had the last pre-earthquake image acquisitions hours before the 4 July 2019 Mw 6.4 earthquake (∼18 : 51 PDT 3 July for ascending track A064 and ∼06 : 52 PDT 4 July for descending track D071). The first postseismic acquisitions along the ascending track T064 and the descending track T071 were on UTC 10 July (∼18 : 51 on 9 July PDT), and 16 July (∼06 : 52 16 July PDT), respectively. Unfortunately, no images were acquired in between theMw 6.4 foreshock and theMw 7.1 mainshock. Subsequent acquisitions were made at six-day intervals, and we include observations up through 2 September 2019 in our data compilation that documents early postseismic deformation. Orbital control of the Sentinel-1 satellites is so good that the geometric baseline between any two paths is generally smaller than 200 m, well below the critical value for C-band interferometry. To reduce the contribution from potential postseismic deformation, we form the coseismic interferograms using images acquired closest to the earthquake. We process the data using GMTSAR (Sandwell et al., 2011). The images of each track are aligned to a single master with geometrical alignment (Xu et al., 2017). We use the 1 arcsec (∼30 m ground resolution) Shuttle Radar Topography Mission digital elevation model for the image alignment and removal of topographic phase. We filter the real and imaginary parts of the original interferometric phase using a low-pass Gaussian filter with a 0.5 gain at a wavelength of 90 m. We unwrap the phase using Statistical-Cost Network-Flow Algorithm for Phase Unwrapping (SNAPHU) (Chen and Zebker, 2001), after masking out the pixels with a correlation that is smaller than 0.1. Because of the lack of thick vegetation and the arid to semiarid climate in southern California, the 2019 Ridgecrest earthquake area is characterized by generally high-radar coherence. Except for places very close to the rupture, where ground shaking is so strong that the reflection properties of the radar backscatters have been altered, the C-band Sentinel-1 phases remain well-correlated throughout the region, allowing for robust phase unwrapping. We visually check the unwrapped interferograms to ensure that there are no obvious phase jumps across neighboring patches of high-phase correlation. Because of the intrinsic ambiguity of phase unwrapping, the unwrapped interferograms may contain a constant shift. Because the Sentinel-1 scenes cover a larger area that contains the 2019 Ridgecr

PyKonal: A Python Package for Solving the Eikonal Equation in Spherical and Cartesian Coordinates Using the Fast Marching Method

Abstract: This article introduces PyKonal: a new open-source Python package for computing travel times and tracing ray paths in 2D or 3D heterogeneous media using the fast marching method for solving the eikonal equation in spherical and Cartesian coordinates. Compiled with the Cython compiler framework, PyKonal offers a Python application program interface (API) with execution speeds comparable to C or Fortran codes. Designed to be accurate, stable, fast, general, extensible, and easy to use, PyKonal offers low- and high-level API functions for full control and convenience, respectively. A scale-independent implementation allows problems to be solved at micro, local, regional, and global scales, and precision can be improved over existing open-source codes by combining different coordinate systems. The resulting code makes state-of-the-art computational capabilities accessible to novice programmers and is efficient enough for modern research problems in seismology.

The 30 November 2018 Mw 7.1 Anchorage Earthquake

Abstract: The Mw 7.1 47 km deep earthquake that occurred on 30 November 2018 had deep societal impacts across southcentral Alaska and exhibited phenomena of broad scientific interest. We document observations that point to future directions of research and hazard mitigation. The rupture mechanism, aftershocks, and deformation of the mainshock are consistent with extension inside the Pacific plate near the down‐dip limit of flat‐slab subduction. Peak ground motions >25%g were observed across more than 8000 km2, though the most violent near‐fault shaking was avoided because the hypocenter was nearly 50 km below the surface. The ground motions show substantial variation, highlighting the influence of regional geology and near‐surface soil conditions. Aftershock activity was vigorous with roughly 300 felt events in the first six months, including two dozen aftershocks exceeding M 4.5. Broad subsidence of up to 5 cm across the region is consistent with the rupture mechanism. The passage of seismic waves and possibly the coseismic subsidence mobilized ground waters, resulting in temporary increases in stream flow. Although there were many failures of natural slopes and soils, the shaking was insufficient to reactivate many of the failures observed during the 1964 M 9.2 earthquake. This is explained by the much shorter duration of shaking as well as the lower amplitude long‐period motions in 2018. The majority of observed soil failures were in anthropogenically placed fill soils. Structural damage is attributed to both the failure of these emplaced soils as well as to the ground motion, which shows some spatial correlation to damage. However, the paucity of instrumental ground‐motion recordings outside of downtown Anchorage makes these comparisons challenging. The earthquake demonstrated the challenge of issuing tsunami warnings in complex coastal geographies and highlights the need for a targeted tsunami hazard evaluation of the region. The event also demonstrates the challenge of estimating the probabilistic hazard posed by intraslab earthquakes.

A Shallow Shock: The 25 February 2019 ML 4.9 Earthquake in the Weiyuan Shale Gas Field in Sichuan, China

Abstract: Earthquakes rarely occur at extremely shallow depths, for example, less than 2 km. Even for induced earthquakes that are typically shallower than tectonic events, only very small ones have been reported in such depths. The ML 4.9 earthquake (Mw 4.3) that struck the Rongxian County, Sichuan, China on 25 February 2019 was an extremely shallow event. Seismological and geodetic data constrained the mainshock depth at ∼1 km with a thrust-faulting mechanism, consistent with the Molin fault orienting northwest. Two foreshocks with magnitudes larger than 4 occurred on an unmapped fault striking northeast, right next to an injection well where hydraulic fracturing (HF) was conducted. The focal depths of the two foreshocks were at ∼2.7 km, coinciding with the depth of HF. Coulomb failure stresses of the two foreshocks on the Molin fault was ∼3 kPa, smaller than typical static triggering threshold (10 kPa), and thus their triggering effects were mild. As the fault was hydraulically sealed from HF, we suggested that the ML 4.9 earthquake was possibly triggered by nearby HF activities through poroelastic stress transfer. Such findings held significant implications for shale gas development by considering seismic hazard associated with shallow faults.

Earthquake Rupture Scaling Relations for Mainland China

Abstract: Magnitude‐rupture scaling relations describe how the length, width, and area of fault rupture vary with earthquake magnitude. These parameters are required in seismic hazard models to fit the models’ earthquakes onto faults and to define the site‐rupture distances needed in ground‐motion prediction equations. We collected the magnitude and rupture parameters of 91 earthquakes in Mainland China and nearby regions to study magnitude‐rupture scaling relations. We find no systematic deviations for the subsurface rupture length (RLD) obtained from different methods versus earthquake magnitude. We performed regressions of RLD versus magnitude and versus rupture width using general orthogonal regression. Then, we derived the relations between rupture area and magnitude. Our relations are not statistically different from the results derived by others using global datasets, if the parameters of the five pre‐1900 great earthquakes in eastern China are not used. However, if the five earthquakes are used, the magnitude‐rupture length scaling relation for large strike‐slip earthquakes in eastern China gives shorter rupture lengths than earthquakes in western China and other plate boundary regions in the world.

The 2018 Geothermal Reservoir Stimulation in Espoo/Helsinki, Southern Finland: Seismic Network Anatomy and Data Features

Abstract: A seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time- and frequency-dependent variations of the signal-to-noise ratio of earthquake records from a 260-meter-deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identified by the company. The fault-plane solutions of 14 selected ML 0.6–1.8 events indicate a dominant reverse-faulting style, and the associated SH radiation patterns appear to control the first-order features of the macroseismic report distribution. Beamforming of earthquake data from six arrays suggests heterogeneous medium properties, in particular between the injection site and two arrays to the west and southwest. Ambient-noise cross-correlation functions reconstruct regional surface-wave propagation and path-dependent body-wave propagation. A 1D inversion of the weakly dispersive surface waves reveals average shear-wave velocities around 3.3 km/s below 20 m depth. Consistent features observed in relative velocity change time series and in temporal variations of a proxy for wavefield partitioning likely reflect the medium response to the stimulation. The resolution properties of the obtained data can inform future monitoring strategies and network designs around natural laboratories.