Physics-based modeling of earthquakes in slow deforming areas: a case study from the Eastern Betic Fault System (SE Spain)

The challenges in the characterization of slow-moving faults and the temporal limitations of the earthquake records in these regions complicate the seismic hazard assessment. The instrumental and historical seismic catalogs cover a short time period compared with the long recurrences between large destructive events in some faults. Paleoseismic evidence allows us to increase the time frame, but when field data is scarce, scattered or difficult to collect, numerical modeling provides us with an excellent tool to support the characterization of a fault system and its associated threat. Physics-based earthquake simulators overcome the limitations of actual earthquake catalogs and generate long-term synthetic seismicity. Recent numerical codes based on rate- and state-dependent friction allow the modeling of both the long-term seismic cycle deformation and the short-term rupture based on quasi-dynamic physical approximations. We use the RSQSim earthquake simulator to reproduce a 100 kyr synthetic catalog of earthquake ruptures based on a 3D fault model that contains the long-term slip rates, rakes and frictional properties of the main active sources of the Eastern Betic Fault System, a slow deforming area (< 1.5 mm/yr) at southeastern Spain with only one instrumental event greater than M_W 5.0: the 2011 Lorca earthquake (M_W 5.1). The resulting long-term earthquake statistics (more than 77.000 events) show that only about 10% of the simulated events have a magnitude greater than M_W 5.0, but all faults in the system are capable of generating M_W ≥ 6.0 earthquakes, supporting paleoseismic observations of surface ruptures and some historical events (I > VIII) that likely reached magnitudes greater than M_W 6.0 (e.g., 1522 Alhama de Almeria and 1829 Torrevieja earthquakes). Complex ruptures involving several fault segments and spatial-temporal clustering of events are physically compatible in this system, according to our simulations. The largest M_W > 6.5 events are as a result of complex ruptures between the major faults, with recurrence times of 1 kyr. The occurrence of larger earthquakes, even M_W ≥ 7.0 in the Alhama de Murcia and Carboneras faults, cannot be ruled out, contrasting with the low magnitudes of the instrumental earthquake catalog. Knowing the characteristics and behavior of these large seismic ruptures, with no instrumental data available, is crucial for the estimation of the maximum ground motion that could be reached in this region. With this contribution, we intend to discuss how physics-based models could contribute to this task for deterministic and probabilistic seismic hazard assessments (DSHA and PSHA). Funded by Project DT-GEO: A Digital Twin for GEOphysical extremes, project ID 101058129.