The moment magnitude (Mw) 5.5 earthquake that struck South Korea in November 2017 was one of the largest and most damaging events in that country over the past century. Its proximity to an enhanced geothermal system site, where high-pressure hydraulic injection had been performed during the previous 2 years, raises the possibility that this earthquake was anthropogenic. We have combined seismological and geodetic analyses to characterize the mainshock and its largest aftershocks, constrain the geometry of this seismic sequence, and shed light on its causal factors. According to our analysis, it seems plausible that the occurrence of this earthquake was influenced by the aforementioned industrial activities. Finally, we found that the earthquake transferred static stress to larger nearby faults, potentially increasing the seismic hazard in the area.

/pdf/the-november-2017-mw-5-5-pohang-earthquake-a-possible-case-3oleaqfo4s.pdf

The November 2017 Mw 5.5 Pohang earthquake: A possible case of induced seismicity in South Korea

A review of the current status of induced seismicity monitoring for hydraulic fracturing in unconventional tight oil and gas reservoirs

\n Rapid association of seismic phases and event location are crucial for real‐time seismic monitoring. We propose a new method, named rapid earthquake association and location (REAL), for associating seismic phases and locating seismic events rapidly, simultaneously, and automatically. REAL combines the advantages of both pick‐based and waveform‐based detection and location methods. It associates arrivals of different seismic phases and locates seismic events primarily through counting the number of P and S picks and secondarily from travel‐time residuals. A group of picks are associated with a particular earthquake if there are enough picks within the theoretical travel‐time windows. The location is determined to be at the grid point with the most picks, and if multiple locations have the same maximum number of picks, the grid point among them with smallest travel‐time residuals. We refine seismic locations using a least‐squares location method (VELEST) and a high‐precision relative location method (hypoDD). REAL can be used for rapid seismic characterization due to its computational efficiency. As an example application, we apply REAL to earthquakes in the 2016 central Apennines, Italy, earthquake sequence occurring during a five‐day period in October 2016, midway in time between the two largest earthquakes. We associate and locate more than three times as many events (3341) as are in Italy's National Institute of Geophysics and Volcanology routine catalog (862). The spatial distribution of these relocated earthquakes shows a similar but more concentrated pattern relative to the cataloged events. Our study demonstrates that it is possible to characterize seismicity automatically and quickly using REAL and seismic picks.

Rapid Earthquake Association and Location

Source locations provide fundamental information on earthquakes and lay the foundation for seismic monitoring at all scales. Seismic source location as a classical inverse problem has experienced significant methodological progress during the past century. Unlike the conventional traveltime‐based location methods that mainly utilize kinematic information, a new category of waveform‐based methods, including partial waveform stacking, time reverse imaging, wavefront tomography, and full waveform inversion, adapted from migration or stacking techniques in exploration seismology has emerged. Waveform‐based methods have shown promising results in characterizing weak seismic events at multiple scales, especially for abundant microearthquakes induced by hydraulic fracturing in unconventional and geothermal reservoirs or foreshock and aftershock activity potentially preceding tectonic earthquakes. This review presents a comprehensive summary of the current status of waveform‐based location methods, through elaboration of the methodological principles, categorization, and connections, as well as illustration of the applications to natural and induced/triggered seismicity, ranging from laboratory acoustic emission to field hydraulic fracturing‐induced seismicity, regional tectonic, and volcanic earthquakes. Taking into account recent developments in instrumentation and the increasing availability of more powerful computational resources, we highlight recent accomplishments and prevailing challenges of different waveform‐based location methods and what they promise to offer in the near future. Plain language summary Earthquakes are a common physical phenomenon involving ground shaking and rupturing of the surface of the Earth. In addition to the well‐known catastrophic tectonic earthquakes, similar vibration sources also appear at various scales in engineering fields, such as acoustic emissions resulting from microcracks in building walls and bridges, rock bursts in mines, microseismic events generated by mining and fluid injection/extraction, and microseisms caused by crustal activity. Seismic information provides a powerful tool for geophysical and engineering surveys. The source location describes the spatial and temporal extent of an earthquake and lays the foundation for seismic monitoring. Seismic location methods have made significant progress over the last century. Specifically, a category of new waveform‐based location methods has emerged as a counterpart of conventional traveltime‐based inversion. These methods directly utilize the notion of a wavefield and, very similar to an optical lens, aim at spatially focusing a source's emitted energy. Waveform‐based methods have provided robust and effective source location results at various scales. We summarize the development history and current state of waveform‐based location methods and discuss the advantages and challenges through their applications for seismic source location at multiple scales.

Recent advances and challenges of waveform‐based seismic location methods at multiple scales

Rapid Characterization of the July 2019 Ridgecrest, California, Earthquake Sequence From Raw Seismic Data Using Machine-Learning Phase Picker

Pick- and waveform-based techniques for real-time detection of induced seismicity

Seismic monitoring of the Basel Enhanced Geothermal System has been running for more than a decade. Yet the details of the long-term behavior of its induced seismicity remained unexplored because a seismic event catalog with consistent detection sensitivity and magnitudes did not exist. This knowledge is essential for developing guidelines and mitigation procedures on how to safely operate and terminate injection activities. Only few observational data exist that cover all phases of such projects in a consistent manner. Here we describe a method that overcomes these deficiencies based on sensitive matched filter detection and a machine learning approach to remove false detections. With an emphasis on consistency, we create a catalog that contains more than 280,000 events down to Mw − 1.5. The much higher temporal resolution allows us to analyze induced microearthquakes in great detail and to gain new insights. We resolved temporal variations of seismicity parameters and, in the post-operational phase, a preferential temporal clustering of events. We find a breakdown in the Gutenberg-Richter scaling during reservoir stimulation, which may have physical reasons or could be caused by a method-independent detection limit during high event rates. The scaling breakdown has implications for the design of Adaptive Traffic Light Systems and may limit the potential of real-time mitigation strategies in future Enhanced Geothermal System projects. Nevertheless, our catalog gives the opportunity to study the temporal evolution of the sequence in unprecedented detail, which will help to better understand the physical processes in a geothermal reservoir, potentially not only in the Basel case. Plain Language Summary Fluid injections into the deep underground, such as performed in geothermal projects, may cause earthquakes. These induced earthquakes provide important information about the involved physical processes but can sometimes be stronger than acceptable and hinder a project to continue. Avoiding unacceptable earthquakes requires a better understanding of the immediate and long-term seismic response of the underground to such operations. However, inconsistencies of existing earthquake catalogs and their generally low resolution restrict our ability to understand these processes. Such catalog restrictions also affected the case of the Basel deep geothermal project. As a result, specifically, the long-term behavior of its induced seismicity remained unexplored. To overcome these deficiencies, a consistent catalog with high resolution is needed. We reinvestigate the induced seismicity in Basel in detail over its whole life span (12 years). Using seismograms of known earthquakes, we search for similar earthquakes and detect an abundance of smaller ones that were previously unknown. To ensure catalog consistency, we further develop advanced techniques that provide robust magnitude estimates and maintain a high detection sensitivity. Like increasing the resolution of an image with a spyglass, the new catalog reveals previously unseen details of this particular sequence. In the injection period, for instance, we find deviations from the expected behavior of earthquakes and their magnitude distribution. These findings make it necessary to rethink earthquake mitigation strategies in geotechnical projects.

/pdf/a-consistent-high-resolution-catalog-of-induced-seismicity-444bi0zhqp.pdf

A Consistent High-Resolution Catalog of Induced Seismicity in Basel Based on Matched Filter Detection and Tailored Post-Processing

. Geothermal systems in the Hengill volcanic area, SW
Iceland, started to be exploited for electrical power and heat production
since the late 1960s. Today the two largest operating geothermal power
plants are located at Nesjavellir and Hellisheiði. This area is a
complex tectonic and geothermal site, located at the triple junction between
the Reykjanes Peninsula (RP), the Western Volcanic Zone (WVZ), and the South
Iceland Seismic Zone (SISZ). The region is seismically highly active with
several thousand earthquakes located yearly. The origin of such earthquakes
may be either natural or anthropogenic. The analysis of microseismicity can
provide useful information on natural active processes in tectonic,
geothermal and volcanic environments as well as on physical mechanisms
governing induced events. Here, we investigate the microseismicity occurring
in Hengill area, using a very dense broadband seismic monitoring network
deployed in Hellisheiði since November 2018, and apply sophisticated
full-waveform based method for detection and location. Improved locations
and first characterization indicate that it is possible to identify
different types of microseismic clusters, which are associated with either
production/injection or the tectonic setting of the geothermal area.

/pdf/full-waveform-based-methods-for-microseismic-monitoring-35hpayslbd.pdf

Full-Waveform based methods for Microseismic Monitoring Operations: an Application to Natural and Induced Seismicity in the Hengill Geothermal Area, Iceland

Advanced Real-time Monitoring for Natural and Induced seismic sequences

Microseismic monitoring is extensively employed in different underground industrial activities related to the production of energy, such as hydrocarbon production and natural gas storage, geothermal energy exploitation or mining. Since these activities can potentially generate induced seismicity, they require the deployment of dedicated microseismic monitoring networks. If well designed, these monitoring systems allow the detection of very weak earthquakes, even in the presence of strong seismic background noise. Improved detection performance, however, will generate extremely large seismic event catalogues and, for this reason, noise tolerant and fully automated data analysis procedures must be established. Furthermore, microseismic sequences are often characterized by multiple events with short inter-event times or overlapping events; in this case, correct phase identification and event association are challenging, and errors can lead to missed detections and/or reduced location resolution. To overcome these problems, various waveform-based methods for the simultaneous detection and location of microseismicity have been proposed during the last years. These methods exploit the coherence of the waveforms recorded at different stations and do not require any automated picking procedure. Although the adoption of such approaches led to recent promising results, an extensive comparison with sophisticated pick-based detection and location methods is still lacking. In this work we aim to fill this gap by a systematic comparison of the performance of one waveform-based method and two pick-based detection and location methods (SCAUTOLOC and SCANLOC) implemented in the SeisComP3 software package. SCANLOC is a relative new approach used to detect and to locate local and regional earthquakes. The method is based on a cluster search algorithm to associate detections to one or many potential earthquake sources. While the cluster search itself is based on P-phases only, in a second step S-phases are also associated and used for locating the earthquake. We compare the performance of LOKI and SCANLOC with a standard automated pick-detection and location procedure, using the SeisComP3 SCAUTOLOC module. We analyze the performance of the three methodologies for a synthetic dataset as well as for the first hour of continuous waveform data, including the Ml 3.5 St. Gallen earthquake, recorded by a dedicated microseismic network deployed in that area. For the second dataset we compare our detection and location results with a more complete catalogue based on waveform template matching and with a manual revised catalogue.

Luca Scarabello

Papers

Pick- and waveform-based techniques for real-time detection of induced seismicity

A Consistent High-Resolution Catalog of Induced Seismicity in Basel Based on Matched Filter Detection and Tailored Post-Processing

Full-Waveform based methods for Microseismic Monitoring Operations: an Application to Natural and Induced Seismicity in the Hengill Geothermal Area, Iceland

Advanced Real-time Monitoring for Natural and Induced seismic sequences

Picking vs Waveform based detection and location methods for induced seismicity monitoring