Spatial variation of stress drop, b-value and Z-value around the Bandar-Abbas earthquake

Abstract

The Southern Zagros fold-thrust belt is a seismically hazardous area with complex fault geometry, thick sedimentary cover, and extensive hydrocarbon infrastructure. This research examines the spatial variability of stress drop, b-value, and Z-value in the Southern Zagros during the time interval around the 2021 Mw 6.3 Bandar Abbas earthquake. Based on a seismic catalog of 10,616 earthquakes for the period 2006–2024, we examined these parameters over four time windows: 2.5 years prior to, 1 year prior to, 1 year after, and 2.5 years after the mainshock. Our combined results show a consistent seismic cycle pattern. In the pre-earthquake period, we found a notable stress accumulation (up to ~ 0.22 MPa), a clear reduction in b-value (down to ~ 0.7), and a clear pre-seismic seismic quiescence (rising Z-value) in the impending epicentral region. Such a combination marks a period of elevated differential stress and crustal preparation for failure. The mainshock ruptured this stress accumulation, leading to a dramatic post-seismic stress drop, a simultaneous rise in b-value, and an abrupt increase in Z-value in response to intensive aftershock activity. A modest recovery of stress and stabilization of both b- and Z-values over the subsequent years suggest the recommencement of tectonic loading by sustained Arabian-Eurasian convergence. The study highlights the collective diagnostic potential of stress drop, b-value, and Z-value analyses for the interpretation of stress evolution and for enhancing seismic hazard analyses in convergent tectonic environments.