How Fluid Pressures and Viscous Rheologies Affect Slip Patterns on Low‐Angle Normal Faults: The Example of the Altotiberina Fault, Northern Apennines, Italy

Abstract

Low‐angle normal faults are a mechanical anomaly, hosting normal‐sense slip despite having a dip shallower than 30°, in contravention of Andersonian prediction. Slip on such faults has been attributed to overpressurized fluids or low‐friction minerals. An example of such a fault is the Altotiberina Fault (ATF) in central Italy. The ATF hanging wall is cut by numerous seismically active, high‐angle normal faults called epidetachments. Active slip along the ATF manifests in myriad small (ML < 2) earthquakes concentrated at depths >5 km, as well as infrequent, damaging earthquakes that may be shallower. In the area have been documented overpressurized, deeply sourced CO2 and a complex mechanical stratigraphy with viscous lithologies. To better understand the relationship between fluid pressure, rheology, and earthquake patterns, we integrate these processes using the Mineralization, Earthquake, and Fluid‐flow Integrated SimulaTOr (MEFISTO). This numerical model simulates earthquakes with simultaneous changes in fluid pressure, porosity, permeability, and fault strength. We investigated the combined effects of epidetachments, which can serve as either pressure‐limiting hydraulic valves or fluid‐flow barriers, and pressure‐solution creep along the ATF fault‐zone. Model results reproduce salient features of the observed seismicity pattern. Upward‐propagating fluid‐pressure waves generate transient clusters of seismic activity. However, large, shallow–and therefore potentially damaging–earthquakes occur that are often not directly triggered by fluid‐pressure waves, limiting the waves' utility as hazard predictors.