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.