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Showing papers in "Pure and Applied Geophysics in 2016"


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the link between coupling and seismicity over the Chilean subduction zone that ruptured three times in the last 5 years with major earthquakes (Maule Mw 8.8 in 2010, Iquique Mw8.1 in 2014 and Illapel Mw 6.4 in 2015).
Abstract: The recent expansion of dense GPS networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults. The interseismic coupling coefficient is related to the ratio between slipping velocity on the fault during the interseismic period and the long-term plates velocity, but the interpretation of coupling in terms of mechanical behavior of the fault is still unclear. Here, we investigate the link between coupling and seismicity over the Chilean subduction zone that ruptured three times in the last 5 years with major earthquakes (Maule Mw 8.8 in 2010, Iquique Mw 8.1 in 2014 and Illapel Mw 8.4 in 2015). We combine recent GPS data acquired over the margin (38°–18° S) with older data to get the first nearly continuous picture of the interseismic coupling variations on the subduction interface. Here, we show that at least six low coupling zones (LCZ), areas where coupling is low relatively to the neighboring highly coupled segments can be identified. We also find that for the three most recent Mw > 8 events, coseismic asperities correlate well with highly coupled segments, while LCZs behaved as barriers and stopped the ruptures. The relation between coupling and background seismicity in the interseismic period before the events is less clear. However, we note that swarm sequences are prone to occur in intermediate coupling areas at the transition between LCZ and neighboring segments, and that the background seismicity tends to concentrate on the downdip part of the seismogenic locked zone. Thus, highly coupled segments usually exhibit low background seismicity. In this overall context, the Metropolitan segment that partly ruptured during the 2015 Illapel earthquake appears as an outlier since both coupling and background seismicity were high before the rupture, raising the issue of the remaining seismic hazard in this very densely populated area.

155 citations


Journal ArticleDOI
TL;DR: In this article, three tsunami survey teams covered approximately 700 km of the Pacific coast, recording 83 tsunami flow depth and runup measurements, and the maximum runup was found to be 10.8 m at only one small bay, in front of the inferred tsunami source area.
Abstract: On September 16, 2015 a magnitude Mw 8.3 earthquake took place off the coast of the Coquimbo Region, Chile. Three tsunami survey teams covered approximately 700 km of the Pacific coast. The teams surveyed the area, recording 83 tsunami flow depth and runup measurements. The maximum runup was found to be 10.8 m at only one small bay, in front of the inferred tsunami source area. However, it was observed that runup in other locations rarely exceed 6 m. Tsunami runup was larger than those of the 2014 Pisagua event, despite the similar earthquake magnitude. Moreover, tsunami arrival times were found to be shorter than those of previous tsunamis along the Chilean subduction zone. Numerical simulations of the tsunami event showed a good agreement with field data, highlighting that tsunami arrival time and the spatial variation of the tsunami amplitudes were strongly influenced by the bathymetry, coastal morphology and the slip distribution of the causative earthquake.

80 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used short-period teleseismic P wave back-projections to characterize the source process of a great M w 8.3 interplate thrust earthquake in Chile, which produced a 4.7m local tsunami.
Abstract: On 16 September 2015, a great (M w 8.3) interplate thrust earthquake ruptured offshore Illapel, Chile, producing a 4.7-m local tsunami. The last major rupture in the region was a 1943M S 7.9 event. Seismic methods for rapidly characterizing the source process, of value for tsunami warning, were applied. The source moment tensor could be obtained robustly by W-phase inversion both within minutes (Chilean researchers had a good solution using regional data within 5 min) and within an hour using broadband seismic data. Short-period teleseismic P wave back-projections indicate northward rupture expansion from the hypocenter at a modest rupture expansion velocity of 1.5–2.0 km/s. Finite-fault inversions of teleseismic P and SH waves using that range of rupture velocities and a range of dips from 16°, consistent with the local slab geometry and some moment tensor solutions, to 22°, consistent with long-period moment tensor inversions, indicate a 180- to 240-km bilateral along-strike rupture zone with larger slip northwest to north of the epicenter (with peak slip of 7–10 m). Using a shallower fault model dip shifts slip seaward toward the trench, while a steeper dip moves it closer to the coastline. Slip separates into two patches as assumed rupture velocity increases. In all cases, localized ~5 m slip extends down-dip below the coast north of the epicenter. The seismic moment estimates for the range of faulting parameters considered vary from 3.7 × 1021Nm(dip 16°) to 2.7 × 1021 Nm (dip 22°), the static stress drop estimates range from 2.6 to 3.5 MPa, and the radiated seismic energy, up to 1 Hz, is about 2.2–3.15 × 1016 J.

72 citations


Journal ArticleDOI
TL;DR: The Environmental Seismic Intensity Scale (ESI) as discussed by the authors is a new scale developed and tested by an interdisciplinary group of scientists in the frame of the International Union for Quaternary Research (INQUA) activities, to the widest community of earth scientists and engineers dealing with seismic hazard assessment.
Abstract: The main objective of this paper was to introduce the Environmental Seismic Intensity scale (ESI), a new scale developed and tested by an interdisciplinary group of scientists (geologists, geophysicists and seismologists) in the frame of the International Union for Quaternary Research (INQUA) activities, to the widest community of earth scientists and engineers dealing with seismic hazard assessment. This scale defines earthquake intensity by taking into consideration the occurrence, size and areal distribution of earthquake environmental effects (EEE), including surface faulting, tectonic uplift and subsidence, landslides, rock falls, liquefaction, ground collapse and tsunami waves. Indeed, EEEs can significantly improve the evaluation of seismic intensity, which still remains a critical parameter for a realistic seismic hazard assessment, allowing to compare historical and modern earthquakes. Moreover, as shown by recent moderate to large earthquakes, geological effects often cause severe damage”; therefore, their consideration in the earthquake risk scenario is crucial for all stakeholders, especially urban planners, geotechnical and structural engineers, hazard analysts, civil protection agencies and insurance companies. The paper describes background and construction principles of the scale and presents some case studies in different continents and tectonic settings to illustrate its relevant benefits. ESI is normally used together with traditional intensity scales, which, unfortunately, tend to saturate in the highest degrees. In this case and in unpopulated areas, ESI offers a unique way for assessing a reliable earthquake intensity. Finally, yet importantly, the ESI scale also provides a very convenient guideline for the survey of EEEs in earthquake-stricken areas, ensuring they are catalogued in a complete and homogeneous manner.

69 citations


Journal ArticleDOI
TL;DR: In this article, the authors present an approach to define a probability distribution based on subdividing the fault geometry into many subfaults and prescribing a desired covariance matrix relating slip on one sub-fault to slip on any other subfault.
Abstract: To perform probabilistic tsunami hazard assessment for subduction zone earthquakes, it is necessary to start with a catalog of possible future events along with the annual probability of occurrence, or a probability distribution of such events that can be easily sampled. For near-field events, the distribution of slip on the fault can have a significant effect on the resulting tsunami. We present an approach to defining a probability distribution based on subdividing the fault geometry into many subfaults and prescribing a desired covariance matrix relating slip on one subfault to slip on any other subfault. The eigenvalues and eigenvectors of this matrix are then used to define a Karhunen-Loeve expansion for random slip patterns. This is similar to a spectral representation of random slip based on Fourier series but conforms to a general fault geometry. We show that only a few terms in this series are needed to represent the features of the slip distribution that are most important in tsunami generation, first with a simple one-dimensional example where slip varies only in the down-dip direction and then on a portion of the Cascadia Subduction Zone.

61 citations


Journal ArticleDOI
TL;DR: In this paper, the first time-dependent slip distributions and Coulomb failure stress changes for the six major slow slip events (SSEs) that occurred below Mexico between late 2005 and mid-2011 are estimated from all continuous GPS data in central and southern Mexico, which better resolves slow slip in space and time than was previously possible in this region.
Abstract: To further our understanding of the seismically hazardous Mexico subduction zone, we estimate the first time-dependent slip distributions and Coulomb failure stress changes for the six major slow slip events (SSEs) that occurred below Mexico between late 2005 and mid-2011. Slip dist ributions are the first to be estimated from all continuous GPS data in central and southern Mexico, which better resolves slow slip in space and time than was previously possible in this region. Below Oaxaca, slip during previously un-modeled SSEs in 2008/9 and 2010/11 extended farther to the west than previous SSEs. This constitutes the first evidence that slow slip accounts for deep slip within a previously noted gap between the Oaxaca and Guerrero SSE source regions. The slip that we estimate for the two SSEs that originated below Guerrero between 2005 and 2011 agrees with slip estimated in previous, mostly static-offset SSE modeling studies; however, we show that both SSEs migrated eastward toward the Oaxaca SSE source region. In accord with previous work, we find that slow slip below Guerrero intrudes up-dip into the potentially seismogenic region, presumably accounting for some of the missing slip within the well-described Guerrero seismic gap. In contrast, slow slip below Oaxaca between 2005 and 2011 occurred mostly down-dip from the seismogenic regions defined by the rupture zones of large thrust earthquakes in 1968 and 1978 and released all of the slip deficit that accumulated in the down-dip region during this period.

52 citations


Journal ArticleDOI
TL;DR: In this article, an electrical resistivity tomography (ERT) survey was conducted to examine the spatial distribution and shape of underground cavities in the karst area in Jebel Sabah anticline and Ain Sallem-Zahret Medien syncline.
Abstract: The Amdoun region is characterized by a high degree of karstification due to the climate impact (±1500 mm year−1) and the development of fracture network. Survey using electrical resistivity tomography (ERT) is deployed to provide a cost-effective characterization of the subsurface karst environments. A total of seven ERT profiles with lengths of 315 m were evaluated at the Beja governorate (NW Tunisia). The area represents a small syncline of Boudabbous limestone rocks (Lower Eocene), which is covered by a thin layer of clay. In this study, an ERT survey was conducted to examine the spatial distribution and shape of underground cavities in the karst area in Jebel Sabah anticline and Ain Sallem-Zahret Medien syncline. In this study, geological, hydro-geological and electrical resistivity tomography (ERT) methods were applied to determine the geometry of the perched aquifer in the Amdoun region (NW Tunisia). The area is characterized by fractured and karstic limestone aquifer of Late Cretaceous (Abiod Fm.) and Lower Eocene (Boudabbous Fm.). The aquifers have a karstic functioning and drain aquifers of economical interest, despite some wells exploiting them. Seven resistivity profiles were conducted along the survey area at three sites. The orientation, extension and the degree of inclination of those profiles are shown in the location map. The correct resistivity data were interpreted using Earth Imager 2D software. The results of the interpreted geo-electrical sections showed that the resistivity of the carbonate aquifer varied between 2.5 to over 5794 Ωm. The thickness of the perched aquifer ranged from 15 to 50 m, while its depth from the surface lies between 10 and 60 m. The ERT not only provided precise near surface information, but was also very useful for establishing the 3D geometry and the position of several potential cavities and karts. The results show the presence of small to large isolated cavities at various depths. The low resistivity of cavities in the Boudabbous Formation has been explained by the groundwater saturation. The ERT technique could be effectively used for 3D detection of underground limestone cavities.

51 citations


Journal ArticleDOI
TL;DR: In this paper, the authors revisited the BIGSEES analysis for Romania and compared the results with the results obtained in the recently completed SHARE research project, and pointed out the need for further analyses and thorough discussions related to the two seismic hazard models, especially in the light of a possible future harmonized hazard map for Europe.
Abstract: The probabilistic seismic hazard analysis for Romania is revisited within the framework of the BIGSEES national research project ( http://infp.infp.ro/bigsees/default.htm ) financed by the Romanian Ministry of Education and Scientific Research in the period 2012–2016. The scope of this project is to provide a refined description of the seismic action for Romanian sites according to the requirements of Eurocode 8. To this aim, the seismicity of all the sources influencing the Romanian territory is updated based on new data acquired in recent years. The ground-motion models used in the analysis, as well as their corresponding weights, are selected based on the results from several recent papers also published within the framework of the BIGSEES project. The seismic hazard analysis for Romania performed in this study are based on the traditional Cornell-McGuire approach. Finally, the results are discussed and compared with the values obtained in the recently completed SHARE research project. The BIGSEES and SHARE results are not directly comparable since the considered soil conditions are different—actual soil classes for BIGSEES and rock for SHARE. Nevertheless, the analyses of the seismic hazard results for 200 sites in Romania reveal considerable differences between the seismic hazard levels obtained in the present study and the SHARE results and point out the need for further analyses and thorough discussions related to the two seismic hazard models, especially in the light of a possible future harmonized hazard map for Europe.

50 citations


Journal ArticleDOI
TL;DR: A field campaign designed to study fog processes in complex terrain was a component of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program as discussed by the authors, which was conducted in the Wasatch Mountains during January 5-February 15, 2015.
Abstract: A field campaign design to study fog processes in complex terrain was a component of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. The experiment was conducted in the Wasatch Mountains during January 5–February 15, 2015. Fog and in particular, Ice fog (IF), defined as fog composed of only ice crystals, was studied during a part of the campaign, and this component of the program was dubbed MATERHORN-Fog. Ice fog often occurs in mountainous regions due do rapid cooling, such as radiative cooling, advective cooling, and cooling associated with mountain circulations (e.g., slope and valley winds). A variety of major instrument platforms were deployed, which included meteorological towers, a SODAR, a LiDAR, ceilometers, and a tethersonde profiler. In addition, in situ measurements took place at several locations surrounding Salt Lake City and Heber City. During the campaign, ice fog occurred at temperatures below −5 °C down to −13 °C and lasted for several hours until radiative heating became significant. The visibility (Vis) during ice fog events ranged from 100 m up to 10 km. At the Heber City site an array of sensors for measuring microphysical, radiative, and dynamical characteristics of IF events were deployed. Some local effects such as upslope advection were observed to affect the IF conditions. As expected during these events, ice water content (IWC) varied from 0.01 up to 0.2 g m−3, with radiative cooling fluxes as strong as 200 W m−2; turbulent heat and moisture fluxes were significantly lower during fog events than those of fog dissipation. At times, the measured ice crystal number concentration was as high as 100 cm−3 during periods of saturation with respect to ice. N i was not a constant as usually assumed in forecasting simulations, but rather changed with increasing IWC. Measurement based statistics suggested that the occurrence of IF events in the region was up to 30 % during the study period in the winter of 2015. Temperature profiles suggested that an inversion layer contributed significantly to IF formation at Heber. Ice fog forecasts via Weather Research and Forecasting (WRF) model indicated the limitations of IF predictability. Results suggest that IF predictions need to be improved based on ice microphysical parameterizations and ice nucleation processes.

49 citations


Journal ArticleDOI
TL;DR: In this article, the Earth's emitted radiation in the Thermal InfraRed spectral region (TIR) is considered for its possible correlation with M ≥ 4 earthquakes occurred in Greece in between 2004 and 2013.
Abstract: Real-time integration of multi-parametric observations is expected to accelerate the process toward improved, and operationally more effective, systems for time-Dependent Assessment of Seismic Hazard (t-DASH) and earthquake short-term (from days to weeks) forecast. However, a very preliminary step in this direction is the identification of those parameters (chemical, physical, biological, etc.) whose anomalous variations can be, to some extent, associated with the complex process of preparation for major earthquakes. In this paper one of these parameters (the Earth’s emitted radiation in the Thermal InfraRed spectral region) is considered for its possible correlation with M ≥ 4 earthquakes occurred in Greece in between 2004 and 2013. The Robust Satellite Technique (RST) data analysis approach and Robust Estimator of TIR Anomalies (RETIRA) index were used to preliminarily define, and then to identify, significant sequences of TIR anomalies (SSTAs) in 10 years (2004–2013) of daily TIR images acquired by the Spinning Enhanced Visible and Infrared Imager on board the Meteosat Second Generation satellite. Taking into account the physical models proposed for justifying the existence of a correlation among TIR anomalies and earthquake occurrences, specific validation rules (in line with the ones used by the Collaboratory for the Study of Earthquake Predictability—CSEP—Project) have been defined to drive a retrospective correlation analysis process. The analysis shows that more than 93 % of all identified SSTAs occur in the prefixed space–time window around (M ≥ 4) earthquake's time and location of occurrence with a false positive rate smaller than 7 %. Molchan error diagram analysis shows that such a correlation is far to be achievable by chance notwithstanding the huge amount of missed events due to frequent space/time data gaps produced by the presence of clouds over the scene. Achieved results, and particularly the very low rate of false positives registered on a so long testing period, seems already sufficient (at least) to qualify TIR anomalies (identified by RST approach and RETIRA index) among the parameters to be considered in the framework of a multi-parametric approach to t-DASH.

47 citations


Journal ArticleDOI
TL;DR: The results of the survey in coastal towns with evident damage and isolated sites where the tsunami signature remained almost intact are summarized in this paper, where the type of damage in specific areas of biological interest and in coastal cities such as Concon, Tongoy and Coquimbo is also reported.
Abstract: The September 16th 2015 Illapel M8.3 earthquake, Chile, generated a tsunami that affected a sparsely populated region, causing 15 casualties and destroying 1069 houses (USGS 2015). A maximum surface elevation of +4.5 m was observed in Coquimbo’s tide gauge while in other sites of the tide network, the tsunami did not exceed +2.0 m. A post-tsunami survey team comprised by local researchers was deployed from September 17th to November 14th 2015. The survey covered approximately 80 sites along 500 km of the primary impact zone, from the northernmost site where damage was reported, Bahia Carrizalillo (29.11°S; 71.46°W), southward to El Yali National Reserve (33.75°S; 71.73°W) beyond which no tsunami damage occurred. The results of the survey in coastal towns with evident damage and isolated sites where the tsunami signature remained almost intact are summarized in this paper. A large amount of quantitative material is presented; including (1) inundation lines in five coastal sites, (2) 157 profiles including wave runup and flow depths and (3) 47 interviews to eyewitness, generally 2–3 per site. About two-thirds of the data were collected in isolated areas to guarantee spatial homogeneity along the impact zone. The type of damage in specific areas of biological interest and in coastal cities such as Concon, Tongoy and Coquimbo is also reported. A maximum runup of 13.6 m was recorded in La Cebada (30.97°S; 71.65°W). The information presented herein provides spatial completeness in places that may have not been surveyed by other teams, and redundancy in areas surveyed by others.

Journal ArticleDOI
TL;DR: In this article, the authors performed a probabilistic seismic hazard assessment of the region bounded by latitudes 20°30°N and longitudes 87°98°E using an improved General Orthogonal Regression methodology.
Abstract: Northeast India bounded by latitudes 20°–30°N and longitudes 87°–98°E is one of the most seismically active areas in the world. This region has experienced several moderate-to-large-sized earthquakes, including the 12 June, 1897 Shillong earthquake (M w 8.1) and the 15 August, 1950 Assam earthquake (M w 8.7) which caused loss of human lives and significant damages to buildings highlighting the importance of seismic hazard assessment for the region. Probabilistic seismic hazard assessment of the region has been carried out using a unified moment magnitude catalog prepared by an improved General Orthogonal Regression methodology (Geophys J Int, 190:1091–1096, 2012; Probabilistic seismic hazard assessment of Northeast India region, Ph.D. Thesis, Department of Earthquake Engineering, IIT Roorkee, Roorkee, 2013) with events compiled from various databases (ISC, NEIC,GCMT, IMD) and other available catalogs. The study area has been subdivided into nine seismogenic source zones to account for local variation in tectonics and seismicity characteristics. The seismicity parameters are estimated for each of these source zones, which are input variables into seismic hazard estimation of a region. The seismic hazard analysis of the study region has been performed by dividing the area into grids of size 0.1° × 0.1°. Peak ground acceleration (PGA) and spectral acceleration (S a) values (for periods of 0.2 and 1 s) have been evaluated at bedrock level corresponding to probability of exceedance (PE) of 50, 20, 10, 2 and 0.5 % in 50 years. These exceedance values correspond to return periods of 100, 225, 475, 2475, and 10,000 years, respectively. The seismic hazard maps have been prepared at the bedrock level, and it is observed that the seismic hazard estimates show a significant local variation in contrast to the uniform hazard value suggested by the Indian standard seismic code [Indian standard, criteria for earthquake-resistant design of structures, fifth edition, Part-I. Bureau of Indian Standards, New Delhi, 2002]. Not only holistic treatment of earthquake catalog and seismogenic zones has been performed, but also higher resolution in spatial distribution could be achieved. The COV maps have been provided with the strong ground-motion maps under various conditions to show the confidence in the results obtained. Results obtained in the present study would be helpful for risk assessment and other disaster mitigation-related studies.

Journal ArticleDOI
TL;DR: In this article, the authors defined two mode types based on the characteristics of the vertical eigendisplacements calculated by generalized reflection and transmission coefficient method, which is commonly seen in near-surface earth models.
Abstract: Identifying correct modes of surface waves and picking accurate phase velocities are critical for obtaining an accurate S-wave velocity in MASW method. In most cases, inversion is easily conducted by picking the dispersion curves corresponding to different surface-wave modes individually. Neighboring surface-wave modes, however, will nearly meet (kiss) at some frequencies for some models. Around the frequencies, they have very close roots and energy peak shifts from one mode to another. At current dispersion image resolution, it is difficult to distinguish different modes when mode-kissing occurs, which is commonly seen in near-surface earth models. It will cause mode misidentification, and as a result, lead to a larger overestimation of S-wave velocity and error on depth. We newly defined two mode types based on the characteristics of the vertical eigendisplacements calculated by generalized reflection and transmission coefficient method. Rayleigh-wave mode near the kissing points (osculation points) change its type, that is to say, one Rayleigh-wave mode will contain different mode types. This mode type conversion will cause the mode-kissing phenomenon in dispersion images. Numerical tests indicate that the mode-kissing phenomenon is model dependent and that the existence of strong S-wave velocity contrasts increases the possibility of mode-kissing. The real-world data shows mode misidentification caused by mode-kissing phenomenon will result in higher S-wave velocity of bedrock. It reminds us to pay attention to this phenomenon when some of the underground information is known.

Journal ArticleDOI
TL;DR: In this article, the authors constructed a seismic source model for the 2015 MW 8.3 Illapel, Chile earthquake, which was carried out with the kinematic waveform inversion method adopting a novel inversion formulation that takes into account the uncertainty in the Green's function, together with the hybrid backprojection method enabling them to track the spatiotemporal distribution of high-frequency (0.3-2.0 Hz) sources at high resolution by using globally observed teleseismic P-waveforms.
Abstract: We constructed a seismic source model for the 2015 MW 8.3 Illapel, Chile earthquake, which was carried out with the kinematic waveform inversion method adopting a novel inversion formulation that takes into account the uncertainty in the Green’s function, together with the hybrid backprojection method enabling us to track the spatiotemporal distribution of high-frequency (0.3–2.0 Hz) sources at high resolution by using globally observed teleseismic P-waveforms. A maximum slip amounted to 10.4 m in the shallow part of the seismic source region centered 72 km northwest of the epicenter and generated a following tsunami inundated along the coast. In a gross sense, the rupture front propagated almost unilaterally to northward from the hypocenter at <2 km/s, however, in detail the spatiotemporal slip distribution also showed a complex rupture propagation pattern: two up-dip rupture propagation episodes, and a secondary rupture episode may have been triggered by the strong high-frequency radiation event at the down-dip edge of the seismic source region. High-frequency sources tends to be distributed at deeper parts of the slip area, a pattern also documented in other subduction zone megathrust earthquakes that may reflect the heterogeneous distribution of fracture energy or stress drop along the fault. The weak excitation of high-frequency radiation at the termination of rupture may represent the gradual deceleration of rupture velocity at the transition zone of frictional property or stress state between the megathrust rupture zone and the swarm area.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the feasibility of the injection-extraction strategy and computed the spatiotemporal change in Coulomb stress on basement normal faults, including the change in poroelastic stresses and pore-pressure.
Abstract: Large-scale carbon dioxide (CO2) injection into geological formations increases pore pressure, potentially inducing seismicity on critically stressed faults by reducing the effective normal stress. In addition, poroelastic expansion of the reservoir alters stresses, both within and around the formation, which may trigger earthquakes without direct pore-pressure diffusion. One possible solution to mitigate injection-induced earthquakes is to simultaneously extract pre-existing pore fluids from the target reservoir. To examine the feasibility of the injection–extraction strategy, we compute the spatiotemporal change in Coulomb stress on basement normal faults, including: (1) the change in poroelastic stresses $$\Delta \tau _s+f\Delta \sigma _n$$ , where $$\Delta \tau _s$$ and $$\Delta \sigma _n$$ are changes in shear and normal stress. respectively, and (2) the change in pore-pressure $$f\Delta p$$ . Using the model of (J. Geophys. Res. Solid Earth 99(B2):2601–2618, 1994), we estimate the seismicity rate on basement fault zones. Fluid extraction reduces direct pore-pressure diffusion into conductive faults, generally reducing the risk of induced seismicity. Limited diffusion into/from sealing faults results in negligible pore pressure changes within them. However, fluid extraction can cause enhanced seismicity rates on deep normal faults near the injector as well as shallow normal faults near the producer by poroelastic stressing. Changes in seismicity rate driven by poroelastic response to fluid injection–extraction depends on fault geometry, well operations, and the background stressing rate.

Journal ArticleDOI
TL;DR: In this article, a linear least squares approach is proposed to interpret magnetic anomalies of the buried structures by using a new magnetic anomaly formula, which depends on solving different sets of algebraic linear equations in order to invert the depth (z), amplitude coefficient (K), and magnetization angle (θ) of buried structures using magnetic data.
Abstract: A new linear least-squares approach is proposed to interpret magnetic anomalies of the buried structures by using a new magnetic anomaly formula. This approach depends on solving different sets of algebraic linear equations in order to invert the depth (z), amplitude coefficient (K), and magnetization angle (θ) of buried structures using magnetic data. The utility and validity of the new proposed approach has been demonstrated through various reliable synthetic data sets with and without noise. In addition, the method has been applied to field data sets from USA and India. The best-fitted anomaly has been delineated by estimating the root-mean squared (rms). Judging satisfaction of this approach is done by comparing the obtained results with other available geological or geophysical information.

Journal ArticleDOI
TL;DR: In this article, the authors present a real-time assessment of the 2015 Chile tsunami using the Short-term Inundation Forecasting for Tsunamis system, and post-event analyses with local community models in Chile.
Abstract: The magnitude 8.3 earthquake in central Chile on 16 September 2015 and the resulting tsunami severely affected the region, with 15 deaths (ONEMI in Monitoreo por sismo de mayor intensidad. (In Spanish) [Available at: http://www.onemi.cl/alerta/se-declara-alerta-roja-por-sismo-de-mayor-intensidad-y-alarma-de-tsunami/], 2015), over one million evacuated, and flooding in nearby coastal cities. We present our real-time assessment of the 2015 Chile tsunami using the Short-term Inundation Forecasting for Tsunamis system, and post-event analyses with local community models in Chile. We evaluate three real-time tsunami sources, which were inverted at the time that the first quarter-, half-, and full-wave passed the first tsunameter (DART 32402, located approximately 580 km north–northwest of the epicenter), respectively. Measurement comparisons from 26 deep-ocean tsunameters and 38 coastal tide stations show that good model accuracies are achieved for all three sources, particularly for the local sites that recorded the most destructive waves. The study highlights the forecast speed, time and accuracy dependence, and their implications for the local forecast capability. Our analyses suggest that the tsunami’s main origination area is about 100–200 km long and 100 km wide, to the north of the earthquake epicenter along the trench and the total estimated tsunami wave energy is 7.9 × 1013 J (with 13 % uncertainty). The study provides important guidelines for the earliest reliable estimate of tsunami energy and local forecasts. They can be obtained with the first quarter-wave of tsunameter recording. These results are also confirmed by a forecast analysis of the 2011 Japan tsunami. Furthermore, we find that the first half-wave tsunameter data are sufficient to accurately forecast the 2015 Chile tsunami, due to the specific orientation between the nearest tsunameter and the source. The study also suggests expanding the operational use of the local community models in real time, and demonstrates the applicability of the model results for ‘‘all-clear’’ evaluations, search and rescue operations, and near-real-time mitigation planning in both near and far fields.

Journal ArticleDOI
TL;DR: In this paper, the authors examined foreshock occurrence patterns for isolated $$M \ge 5$$ earthquakes in southern California from 1981 to 2011 and in northern California from 1984 to 2009, and found that foreshocks are largely controlled by the regional tectonic stress field and fault zone properties.
Abstract: We analyze foreshock activity in California and compare observations with simulated catalogs based on a branching aftershock-triggering model. We first examine foreshock occurrence patterns for isolated $$M \ge 5$$ earthquakes in southern California from 1981 to 2011 and in northern California from 1984 to 2009. Among the 64 $$M \ge 5$$ mainshocks, excluding 3 swarms and 3 doubles, 53 % of the rest are preceded by at least one foreshock within 30 days and 5 km. Foreshock occurrence appears correlated with mainshock faulting type and depth. Foreshock area is correlated with the magnitude of the largest foreshock and the number of foreshocks, however, it is not correlated with mainshock magnitude. We then examine the occurrence pattern of all seismicity clusters without a minimum magnitude requirement, and the possibility that they are “foreshocks” of larger mainshocks. Only about 30 % of the small clusters lead to a larger cluster. About 66 % of the larger clusters have foreshock activities, and the spatial distribution pattern is similar to $$M \ge 5$$ mainshocks, with lower occurrence rates in the Transverse Range and central California and higher occurrence rates in the Eastern California Shear Zone and the Bay Area. These results suggest that foreshock occurrence is largely controlled by the regional tectonic stress field and fault zone properties. In special cases, foreshock occurrence may be useful for short-term forecasting; however, foreshock properties are not reliably predictive of the magnitude of the eventual “mainshock”. Comparison with simulated catalogs suggest that the “swarmy” features and foreshock occurrence rate in the observed catalogs are not well reproduced from common statistical models of earthquake triggering.

Journal ArticleDOI
TL;DR: In this article, the main basic data acquired along the Chile Triple Junction (CTJ) area (45°-48°S) where an active spreading center is presently subducting beneath the Andean continental margin.
Abstract: This paper aggregates the main basic data acquired along the Chile Triple Junction (CTJ) area (45°–48°S), where an active spreading center is presently subducting beneath the Andean continental margin. Updated sea-floor kinematics associated with a comprehensive review of geologic, geochemical, and geophysical data provide new constraints on the geodynamics of this puzzling area. We discuss: (1) the emplacement mode for the Pleistocene Taitao Ridge and the Pliocene Taitao Peninsula ophiolite bodies. (2) The occurrence of these ophiolitic complexes in association with five adakite-like plutonic and volcanic centers of similar ages at the same restricted locations. (3) The inferences from the cooccurrence of these sub-coeval rocks originating from the same subducting oceanic lithosphere evolving through drastically different temperature–pressure (P–T) path: low-grade greenschist facies overprint and amphibolite-eclogite transition, respectively. (4) The evidences that document ridge-jump events and associated microplate individualization during subduction of the SCR1 and SCR-1 segments: the Chonos and Cabo Elena microplates, respectively. The ridge-jump process associated with the occurrence of several closely spaced transform faults entering subduction is controlling slab fragmentation, ophiolite emplacement, and adakite-like production and location in the CTJ area. Kinematic inconsistencies in the development of the Patagonia slab window document an 11- km westward jump for the SCR-1 spreading segment at*6.5-to-6.8 Ma. The SCR-1 spreading center is relocated beneath the North Patagonia Icefield (NPI). We argue that the deep-seated difference in the dynamically sustained origin of the high reliefs of the North and South Patagonia Icefield (NPI and SPI) is asthenospheric convection and slab melting, respectively. The Chile Triple Junction area provides the basic constraints to define the basic signatures for spreading-ridge subduction beneath an Andean-type margin

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TL;DR: In this article, two independent approaches have been proposed to determine tsunami source energy: one inverted from the Deep-ocean Assessment and Reporting of Tsunamis (DART) data during the tsunami propagation, and the other derived from the land-based coastal global positioning system (GPS) during tsunami generation.
Abstract: Early tsunami warning critically hinges on rapid determination of the tsunami hazard potential in real-time, before waves inundate critical coastlines. Tsunami energy can quickly characterize the destructive potential of generated waves. Traditional seismic analysis is inadequate to accurately predict a tsunami’s energy. Recently, two independent approaches have been proposed to determine tsunami source energy: one inverted from the Deep-ocean Assessment and Reporting of Tsunamis (DART) data during the tsunami propagation, and the other derived from the land-based coastal global positioning system (GPS) during tsunami generation. Here, we focus on assessing these two approaches with data from the March 11, 2011 Japanese tsunami. While the GPS approach takes into consideration the dynamic earthquake process, the DART inversion approach provides the actual tsunami energy estimation of the propagating tsunami waves; both approaches lead to consistent energy scales for previously studied tsunamis. Encouraged by these promising results, we examined a real-time approach to determine tsunami source energy by combining these two methods: first, determine the tsunami source from the globally expanding GPS network immediately after an earthquake for near-field early warnings; and then to refine the tsunami energy estimate from nearby DART measurements for improving forecast accuracy and early cancelations. The combination of these two real-time networks may offer an appealing opportunity for: early determination of the tsunami threat for the purpose of saving more lives, and early cancelation of tsunami warnings to avoid unnecessary false alarms.

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TL;DR: In this article, a gray-level co-occurrence matrix (GLCM) algorithm was used to compute second-order statistical measures of textural characteristics, such as contrast, energy, entropy, and homogeneity.
Abstract: We evaluate the applicability and the effectiveness of texture attribute analysis of 2-D and 3-D GPR datasets obtained in different archaeological environments. Textural attributes are successfully used in seismic stratigraphic studies for hydrocarbon exploration to improve the interpretation of complex subsurface structures. We use a gray-level co-occurrence matrix (GLCM) algorithm to compute second-order statistical measures of textural characteristics, such as contrast, energy, entropy, and homogeneity. Textural attributes provide specific information about the data, and can highlight characteristics as uniformity or complexity, which complement the interpretation of amplitude data and integrate the features extracted from conventional attributes. The results from three archaeological case studies demonstrate that the proposed texture analysis can enhance understanding of GPR data by providing clearer images of distribution, volume, and shape of potential archaeological targets and related stratigraphic units, particularly in combination with the conventional GPR attributes. Such strategy improves the interpretability of GPR data, and can be very helpful for archaeological excavation planning and, more generally, for buried cultural heritage assessment.

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TL;DR: In this article, the authors consider the quality of the catalogue and produce a revised catalogue for South Iceland, the area with the highest seismic risk in Iceland, and explore the effects of filtering events using some common recommendations based on network geometry and station spacing and, as an alternative, filtering based on multivariate analysis that identifies outliers in the hypocentre error distribution.
Abstract: In 1991, a new seismic monitoring network named SIL was started in Iceland with a digital seismic system and automatic operation. The system is equipped with software that reports the automatic location and magnitude of earthquakes, usually within 1–2 min of their occurrence. Normally, automatic locations are manually checked and re-estimated with corrected phase picks, but locations are subject to random errors and systematic biases. In this article, we consider the quality of the catalogue and produce a revised catalogue for South Iceland, the area with the highest seismic risk in Iceland. We explore the effects of filtering events using some common recommendations based on network geometry and station spacing and, as an alternative, filtering based on a multivariate analysis that identifies outliers in the hypocentre error distribution. We identify and remove quarry blasts, and we re-estimate the magnitude of many events. This revised catalogue which we consider to be filtered, cleaned, and corrected should be valuable for building future seismicity models and for assessing seismic hazard and risk. We present a comparative seismicity analysis using the original and revised catalogues: we report characteristics of South Iceland seismicity in terms of b value and magnitude of completeness. Our work demonstrates the importance of carefully checking an earthquake catalogue before proceeding with seismicity analysis.

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TL;DR: In this article, the authors presented a new method for calculating probabilities for large events in systems such as earthquakes, typhoons, market crashes, electricity grid blackouts, floods, droughts, wars and conflicts, and landslides.
Abstract: Large devastating events in systems such as earthquakes, typhoons, market crashes, electricity grid blackouts, floods, droughts, wars and conflicts, and landslides can be unexpected and devastating. Events in many of these systems display frequency-size statistics that are power laws. Previously, we presented a new method for calculating probabilities for large events in systems such as these. This method counts the number of small events since the last large event and then converts this count into a probability by using a Weibull probability law. We applied this method to the calculation of large earthquake probabilities in California-Nevada, USA. In that study, we considered a fixed geographic region and assumed that all earthquakes within that region, large magnitudes as well as small, were perfectly correlated. In the present article, we extend this model to systems in which the events have a finite correlation length. We modify our previous results by employing the correlation function for near mean field systems having long-range interactions, an example of which is earthquakes and elastic interactions. We then construct an application of the method and show examples of computed earthquake probabilities.

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TL;DR: In this paper, the authors used Hidden Markov Models (HMMs) and Symbolic Aggregate ApproXimation (SAX) to estimate the root mean square (RMS) of the seismic signal of Mt. Etna.
Abstract: From January 2011 to December 2015, Mt. Etna was mainly characterized by a cyclic eruptive behavior with more than 40 lava fountains from New South-East Crater. Using the RMS (Root Mean Square) of the seismic signal recorded by stations close to the summit area, an automatic recognition of the different states of volcanic activity (QUIET, PRE-FOUNTAIN, FOUNTAIN, POST-FOUNTAIN) has been applied for monitoring purposes. Since values of the RMS time series calculated on the seismic signal are generated from a stochastic process, we can try to model the system generating its sampled values, assumed to be a Markov process, using Hidden Markov Models (HMMs). HMMs analysis seeks to recover the sequence of hidden states from the observations. In our framework, observations are characters generated by the Symbolic Aggregate approXimation (SAX) technique, which maps RMS time series values with symbols of a pre-defined alphabet. The main advantages of the proposed framework, based on HMMs and SAX, with respect to other automatic systems applied on seismic signals at Mt. Etna, are the use of multiple stations and static thresholds to well characterize the volcano states. Its application on a wide seismic dataset of Etna volcano shows the possibility to guess the volcano states. The experimental results show that, in most of the cases, we detected lava fountains in advance.

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TL;DR: In this article, the authors investigate the characteristics of the tsunamis triggered by the Mw8.3 Illapel, Mw 8.2 Iquique and Mw7.8 Maule Chile earthquakes and find that these three earthquakes present faults with important extensions beneath the continent which result in the generation of tsunamus with short wavelengths, relative to the fault widths involved, and with reduced initial potential energy.
Abstract: Tsunamis occur quite frequently following large magnitude earthquakes along the Chilean coast. Most of these earthquakes occur along the Peru–Chile Trench, one of the most seismically active subduction zones of the world. This study aims to understand better the characteristics of the tsunamis triggered along the Peru–Chile Trench. We investigate the tsunamis induced by the Mw8.3 Illapel, the Mw8.2 Iquique and the Mw8.8 Maule Chilean earthquakes that happened on September 16th, 2015, April 1st, 2014 and February 27th, 2010, respectively. The study involves the relation between the co-seismic deformation and the tsunami generation, the near-field tsunami propagation, and the spectral analysis of the recorded tsunami signals in the near-field. We compare the tsunami characteristics to highlight the possible similarities between the three events and, therefore, attempt to distinguish the specific characteristics of the tsunamis occurring along the Peru–Chile Trench. We find that these three earthquakes present faults with important extensions beneath the continent which result in the generation of tsunamis with short wavelengths, relative to the fault widths involved, and with reduced initial potential energy. In addition, the presence of the Chilean continental margin, that includes the shelf of shallow bathymetry and the continental slope, constrains the tsunami propagation and the coastal impact. All these factors contribute to a concentrated local impact but can, on the other hand, reduce the far-field tsunami effects from earthquakes along Peru–Chile Trench.

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TL;DR: In this paper, the authors analyzed the efficiency of the ratio between particle velocity and shear wave velocity as a strain proxy for evaluating the nonlinear seismic response of sediments and derived the in situ stress-strain relationships from accelerometric vertical array recordings at the TST site in Volvi (Thessaloniki, Greece).
Abstract: In this study, we analyze the efficiency of the ratio between particle velocity and shear wave velocity as a strain proxy for evaluating the nonlinear seismic response of sediments. The in situ stress–strain relationships are derived from accelerometric vertical array recordings at the TST site in Volvi (Thessaloniki, Greece). First, the shear wave velocity between two successive sensors was computed by seismic interferometry and strain was computed as the velocity ratio or the relative displacement between sensors. The shear-wave velocity profile and in situ shear modulus degradation curve with strain were compared with previous studies performed at the TST site. Finally, the stress–strain relationships were derived from data recorded at the surface by extending the strain proxy and stress values to the ratio between peak ground velocity and the Vs30 parameter used for site classification, i.e. without requiring the accelerometric vertical array. Our model captures the in situ nonlinear response of the site, without consideration of azimuth or distance of the earthquakes. In conclusion, the acceleration (stress) values, based on the accelerometric response spectra instead of peak ground acceleration compared with the deformation (strain) proxy, provide an effective model of the in situ nonlinear response, providing information that can be integrated into ground motion prediction equations.

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TL;DR: In this article, meteorotsunamis were particularly numerous on 25 and 26 June in the Adriatic Sea, where at least six harbours and bays were stricken by powerful waves: strongest events occurred in Vela Luka (Korcula Island), a known meteotsunami hot-spot, where waves reached height of 3 m, and in Rijeka dubrovacka Bay, where strong ~5 m/s currents accompanied ~2.5 m high waves.
Abstract: A series of meteotsunamis hit a few locations in the Mediterranean and Black Seas during 22–27 June 2014. Meteotsunamis were particularly numerous on 25 and 26 June in the Adriatic Sea, where at least six harbours and bays were stricken by powerful waves: strongest events occurred in Vela Luka (Korcula Island), a known meteotsunami hot-spot, where waves reached height of ~3 m, and in Rijeka dubrovacka Bay, where strong ~5 m/s currents accompanied ~2.5 m high waves. Intensification of high-frequency sea level activity was observed at both the eastern and western Adriatic tide gauge stations, with maximum recorded wave heights reaching ~68 cm (Ortona, Italy). A series of individual air pressure disturbances characterized by pronounced rates of air pressure change (up to 2.4 hPa/5 min), limited spatial extent (~50 km) and high temporal variability, propagated over the Adriatic on 2 days in question. Numerical hydrodynamic model SCHISM forced by measured and idealised air pressure disturbances was utilised to reproduce the observed Adriatic sea level response. Several important conclusions were reached: (1) meteotsunamis occurring at various parts of the coast were generated by different atmospheric air pressure disturbances; (2) topographic influence can be removed from sea level spectra by computing spectral signal-to-background ratios; the result, being related to the external forcing, resembles atmospheric pressure spectra; (3) sea response is strongly dependant on details of atmospheric forcing; and (4) over complex bathymetries, like the middle and south Adriatic ones, numerous effects, including Proudman resonance, edge waves, strong topographical enhancement and refractions on the islands placed on the pathway of atmospheric disturbances should be taken into account to fully understand meteotsunami generation and dynamics. An in-depth numerical study is planned to supplement the latter conclusion and to quantify contribution of each process.

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TL;DR: In this article, a field campaign to study cold season fog in complex terrain was conducted as a component of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program from 07 January to 01 February 2015 in Salt Lake City and Heber City, Utah, United States.
Abstract: A field campaign to study cold season fog in complex terrain was conducted as a component of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program from 07 January to 01 February 2015 in Salt Lake City and Heber City, Utah, United States. To support the field campaign, an advanced research version of the Weather Research and Forecasting (WRF) model was used to produce real-time forecasts and model evaluation. This paper summarizes the model performance and preliminary evaluation of the model against the observations. Results indicate that accurately forecasting fog is challenging for the WRF model, which produces large errors in the near-surface variables, such as relative humidity, temperature, and wind fields in the model forecasts. Specifically, compared with observations, the WRF model overpredicted fog events with extended duration in Salt Lake City because it produced higher moisture, lower wind speeds, and colder temperatures near the surface. In contrast, the WRF model missed all fog events in Heber City, as it reproduced lower moisture, higher wind speeds, and warmer temperatures against observations at the near-surface level. The inability of the model to produce proper levels of near-surface atmospheric conditions under fog conditions reflects uncertainties in model physical parameterizations, such as the surface layer, boundary layer, and microphysical schemes.

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TL;DR: In this article, the authors compare two alternative models for the Arabian plate and surrounding area, the Airy model gives very significant regional isostatic anomalies, which cannot be explained by the upper crust structure or disturbances of the isostatics equilibrium.
Abstract: The isostatic modeling represents one of the most useful “geological” reduction methods of the gravity field. With the isostatic correction, it is possible to remove a significant part of the effect of deep density heterogeneity, which dominates in the Bouguer gravity anomalies. Although there exist several isostatic compensation schemes, it is usually supposed that a choice of the model is not an important factor to first order, since the total weight of compensating masses remains the same. We compare two alternative models for the Arabian plate and surrounding area. The Airy model gives very significant regional isostatic anomalies, which cannot be explained by the upper crust structure or disturbances of the isostatic equilibrium. Also, the predicted “isostatic” Moho is very different from existing seismic observations. The second isostatic model includes the Moho, which is based on seismic determinations. Additional compensation is provided by density variations within the lithosphere (chiefly in the upper mantle). According to this model, the upper mantle under the Arabian Shield is less dense than under the Platform. In the Arabian platform, the maximum density coincides with the Rub’ al Khali, one of the richest oil basin in the world. This finding agrees with previous studies, showing that such basins are often underlain by dense mantle, possibly related to an eclogite layer that has caused their subsidence. The mantle density variations might be also a result of variations of the lithosphere thickness. With the combined isostatic model, it is possible to minimize regional anomalies over the Arabian plate. The residual local anomalies correspond well to tectonic structure of the plate. Still very significant anomalies, showing isostatic disturbances of the lithosphere, are associated with the Zagros fold belt, the collision zone of the Arabian and Eurasian plates.