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MEchanisms and PHysical processes governing Induced Seismicity: insights from Theoretical models and seismological Observations

Periodic Reporting for period 1 - MEPHISTO (MEchanisms and PHysical processes governing Induced Seismicity: insights from Theoretical models and seismological Observations)

Reporting period: 2018-11-01 to 2020-10-31

Over the last decade induced seismicity became an important topic of discussion both at scientific and societal level. Industrial activities altering the state of stress or the pore pressure of the shallow crust may, in fact, generate seismicity which can be felt by the population, reducing the public acceptance of such activities. The physical connection between operational parameters such as injected (or extracted) volume and the induced seismicity is complex and not yet fully understood. A better understanding of the influence of fluid injection/extraction parameters on the potential failure of pre-existing faults nearby the reservoir is furthermore a fundamental step towards the management of the hazard posed by induced earthquakes. The project MEPHISTO aims to a get insights into the physical processes governing induced seismicity, strengthening the link between seismological observations and geomechanical models. To pursue this scope the project is organized in two complementary activities. The first activity consists in the development/improvent of innovative seismological methods for microseimicity characterization, allowing to obtain larger and more reliable seismic catalogues. High-resolution seismicity locations allow to infer the geometry of active faults nearby the reservoir. These informations are fundamental for the both a-priori and near-real-time risk assessment operations. The second activity consists in the implementation and testing (with field data and in real-time) of risk mitigation tools based on seismicity modeling techniques. The comparison of the obeserved seismicity with the simulated one will be then used to calibrate the seismicity forecasting model, to investigate the influence of underground industrial activities on the generation of induced seismicity and most important to update the risk model in real time, in order to ensure the safety of the related underground industrial operations.
The main activities carried within the MEPHISTO project and summarized below, has been focused on the development of new methods for monitoring microseismicity and on the integration of seismicity modeling techniques within induced seismicity risk management frameworks. Within MEPHISTO we developed from scratch of a new location method for microseismic events using only one or two seismic stations. This new location method is based on distance-geometry concepts. A Distance Geometry Problem consists in the determination of the location of sets of points based only on the distances between some member pairs. Applied to seismology, our approach allows earthquake location using the inter-event distance between earthquakes pairs, which can be estimated using only one or two seismic stations.This method is extremely useful when working with very sparse networks, where only few stations are available. This is often the case of induced seismicity monitoring (especially for offshore applications).We first validate the method with synthetic data that resemble common cluster shapes, and then test the method with two seismic sequences in California: the August 2014 Mw 6.0 Napa earthquake and the July 2019 Mw 6.4 Ridgecrest earthquake sequence (Figure). We demonstrate that our approach provides robust and reliable results even for a single station. When using two seismic stations, the results capture the same structures recovered with high resolution Double Difference locations based on multiple stations. This approach has been also applied to the study of the induced seismicity sequence associated to natural gas storage operation in the offshore Spain. The induced seismicity sequence at the Castor platform highlights that microseismic monitoring of offshore industrial facilities is an extremely challenging problem.In this particular case the two seismic stations at about 20 km distance from the platform, allowed the detection of about 800 events. However, standard techniques produced poor quality location results that did not allow to identify the fault associated with the seismic sequence. By using at the new location method developed within MEPHISTO it was possible to locate with high-resolution a large number of weak events. The relocated seismicity shows a spatial pattern which allowed to identify the fault structure associated with the seismic sequence. The location method (published on the Geophysical Journal International) has been released as a computer program publicly available at https://github.com/wulwife. The second activity of MEPHISTO focused on the implementation and real-time testing of a workflow for induced seismicity risk assessment based on seismicity modeling. Within MEPHISTO we tested this risk mitigation workflow to an hydraulic stimulation experiment close Reykjavik, in Iceland. Specifically, we provide probabilistic estimates of peak ground acceleration, European Macroseismic Intensity, probability of light damage (damage risk), and individual risk, defined as the frequency of fatality over time span of a project. The application consists of 1) an a-priori risk assessment before the injection operations, which represents the basis for risk updating once the project has started and in situ real-time data become available and 2) a near-real time risk updating during the stimulation operations, which represents the first application of real-time evaluation of the seismic risk during fluid injection operations.
With the work produced within MEPHISTO we have achieved some important results that have major impact in induced seismicity monitoring. We have developed a new earthquake location method (with the code publicly available) that work with only one or two seismic stations producing high-resolution seismicity locations even with extremely poor monitoring conditions. This new method will have an extremely important implication for the monitoring of induced seismicity in presence of poor seismic monitoring networks. Furthermore with this new location method, it is possible to efficiently monitor offshore industrial facilities with seismic stations distributed on the coast without the need of the deployment of expensive of Ocean Bottom Seismometers. This new monitoring paradigm will allow to greatly enhance the performance of monitoring by keeping low the costs of the infrastructure. In addition this method can be used to re-analyse past datasets recorded by poor monitoring networks providing high-quality seismic catalogues that can be used to get new insights on the earthquake generation process, not only for induced earthquakes but also for the natural ones (e.e. study of offshore subduction zones). Within MEPHISTO we also developed a workflow for induced seismicity risk management that can be used for the early individuation of risk pitfalls and help to ensure the safety of geoenergy projects. During this project we also performed the first application of real-time evaluation of the seismic risk during fluid injection operations, showing that, using new generation tools, induced seismicity related risks can be efficiently managed. This dynamic risk management solution will help to ensure public acceptance and thus contribute to the continued successful the exploitation of georesources.
Ridgecrest Earthquake sequence blue dots locations using the new methods and only two stations