Normal Fault Interactions in Seismic Cycles and the Impact of Fault Network Geometry

Abstract

Understanding the mechanisms behind the characteristics of earthquake cycles on normal faults is challenging due to their long recurrence times. Despite their moderate magnitude, normal faulting earthquakes can produce considerable damage. We investigate the effects of fault network geometry and spacing on the seismic cycle of two normal faults modeled with rate-and-state friction and elastic interactions. Our analysis examines how variable along-strike and across-strike distances between faults influence cycle periodicity, synchronicity, nucleation location, magnitude-frequency distribution, and rupture characteristics. To isolate network-geometry effects from dimensional and frictional effects, we model faults with a seismogenic width (W) over characteristic nucleation length (L_∞) ratio such that isolated faults produce periodic cycles with a characteristic magnitude (Mw) of 5.1. The cycle periodicity and Mw of earthquakes change depending on the spacing and geometry of the fault network. Faults become less periodic at short across-strike distances (≤0.2 km). Decreasing the across-strike spacing leads to variable hypocenter locations and the emergence of partial ruptures, producing magnitudes down to Mw 4.4 at spacings ≤0.2 km. Cycle periodicity and Mw remain unaffected by along-strike spacing. The long-term synchronization state of the faults' seismic cycle is influenced differently by across-strike and along-strike distances. Closely spaced faults (≤10 km) across-strike display variable degrees of synchronization, whereas faults arranged along-strike tend to evolve toward more synchronized states as along-strike separation decreases. Fault network geometry plays a prominent role, with across-strike distance having a larger effect on interevent time and rupture style variability than along-strike distance.