Variable fault geometry controls the cascading 2023 Herat, Afghanistan multiplet sequence
Lei Zhao, Edwin Nissen, Wenbin Xu, Mohammadreza Jamalreyhani, Eric Bergman, Dezheng Zhao, Lei Xie
Abstract
Earthquake multiplets are an important but poorly understood class of seismic sequence. On October 7th–15th 2023, a multiplet comprising five damaging, moment magnitude (Mw) 5.9–6.4 earthquakes struck northwestern Afghanistan, a region previously lacking in well-recorded seismicity. We mapped ground deformation with Interferometric Synthetic Aperture Radar (InSAR) and characterized the causative faulting using elastic dislocation modelling. Because of the tight clustering in time, only the fifth mainshock is ever imaged on its own, so we apply independent seismological constraints from epicentral relocations and moment tensor inversions to distinguish the contribution of each mainshock to the observed surface deformation. Our results support sequential rupture of five colinear, shallow (~5 km), north-dipping, blind reverse faults, with successive mainshocks stepping initially westwards and subsequently eastwards. Our modelling implies that gentle (10–15o) fault bends and/or step-overs may have halted rupture propagation in individual mainshock, acting to divide the sequence into its five distinct events. The epicentral region shows abundant geomorphic evidence for active shortening, including a ~70 km-long anticlinal ridge whose growth is likely driven by the underlying reverse faults. This sequence confirms that while to a first degree northwestern Afghanistan is part of stable Eurasia, slow internal deformation can nevertheless generate damaging earthquakes. InSAR observations of ground deformation and slip distribution suggest the Herat multiplet earthquakes sequentially ruptured five colinear shallow reverse faults, with relatively minor variations such as fault bends determining rupture propagation and segmentation.