Zagros “phantom earthquakes” reassessed—The interplay of seismicity and deep salt flow in the Simply Folded Belt?

Abstract

Unraveling the contributions of main shock slip, aftershocks, aseismic afterslip, and postseismic relaxation to the deformation observed in earthquake sequences heightens our understanding of crustal rheology, triggering phenomena, and seismic hazard. Here, we revisit two recent earthquakes in the Zagros mountains (Iran) which exhibited unusual and contentious aftereffects. The Mw ~6 earthquakes at Qeshm (2005) and Fin (2006) are both associated with large interferometric synthetic aperture radar (InSAR) signals, consistent with slip on steep reverse faults in carbonate rocks of the middle sedimentary cover, but small aftershocks detected with local seismic networks were concentrated at significantly greater depths. This discrepancy can be interpreted in one of two ways: either (1) there is a genuine vertical separation between main shock and aftershocks, reflecting a complex stress state near the basement‐cover interface, or (2) the aftershocks delimit the main shock slip and the InSAR signals were caused by shallow, updip afterslip (phantom earthquakes) with very similar magnitudes, mechanisms, and geographical positions as the original earthquakes. Here, we show that main shock centroid depths obtained from body waveform modeling—which in this instance is the only method that can reveal for certain the depth at which seismic slip was centered—strongly support the first interpretation. At Qeshm, microseismic aftershock depths are centered at the level of the Hormuz Formation, an Infracambrian sequence of intercalated evaporitic and nonevaporitic sediments. These aftershocks may reflect the breaking up of harder Hormuz sediments and adjacent strata as the salt flows in response to main shock strain at the base of the cover. This work bolsters recent suggestions that most large earthquakes in the Zagros are contained within carbonate rocks in the midlower sedimentary cover and that the crystalline basement shortens mostly aseismically.