GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 3.3 ______ ___ GNGTS 2023 Distance Geometry Solvers for microseismic monitoring of offshore earthquakes: application to the Castor (Spain) induced seismicity sequence F. Grigoli 1 , S. Cesca 2 and W. Ellsworth 3 1 Department of Earth Sciences, University of Pisa, Italy 2 GFZ-Potsdam, Section 2.1 Earthquake and Volcano Physics, Germany 3 Department of Geophysics, Stanford University, United States Analyzing offshore seismic sequences is challenging both for the lack of optimal seismic monitoring networks and detailed enough velocity models. These conditions limit the application of standard data analysis procedures, leading to low accuracy results that may affect monitoring and early identification of the source of seismicity, particularly when potentially related to offshore industrial operations. Here we introduce location procedure for offshore seismic sequences capable of achieving high location accuracy with suboptimal monitoring conditions. This workflow combines array seismology techniques and a recently developed location technique based on Distance Geometry Solvers (DGS). The DGS approach takes inspiration from methods used in biochemistry to estimate the structures of proteins. It uses the inter-event distances between earthquake pairs measured at one or two seismic stations (Grigoli et al 2021). The application of this workflow improves the location of natural and induced offshore seismic sequences producing high-resolution locations of seismicity clusters with as few as two seismic stations. We apply the workflow to the September-October 2013 seismicity in the Gulf of Valencia (Spain) near the offshore Underground Gas Storage (UGS) project Castor. The Castor sequence has been established as one of the most important cases of induced seismicity in Europe and a rare example of seismicity induced by gas injection into a depleted oil field (Cesca et al 2021). Seismicity initiated on September 5th, 2013, 3 days after the start of gas injection operations in the UGS field. The close proximity of the hypocenters to the injection wells and the temporal correlation between injection operations and seismic activity, triggered the concern of the general public living close to the industrial facility. The seismic sequence continued for weeks, culminating with a Mw 4.3 earthquake on October 4th. Within this time frame about 800 events (magnitude range Ml 0.0-4.3) occurred. This anomalous level of activity with respect to known seismicity area provided key evidence for a casual relation between the injection operations and the earthquake sequence. The Castor project was permanently closed in 2019. With only two stations within 30 km of the