The vertical separation of mainshock rupture and microseismicity at Qeshm island in the Zagros fold-and-thrust belt, Iran

Abstract

We investigate the depth and geometry of faulting within a cluster of buried, reverse faulting earthquakes that struck Qeshm island, in the Zagros fold-and-thrust belt, over a four year period between November 2005 and July 2009. Of particular interest is our observation that there was a vertical separation between the largest two earthquakes ( M w 5.8 and 5.9), which ruptured the lower parts of a ∼ 10-km thick sedimentary cover, and microseismicity recorded by a local network after the first, M w 5.8 event, which was concentrated within the underlying basement at depths of 10–20 km. Although measured in different ways — the largest three earthquakes using radar interferometry, moderate-sized events with teleseismically-recorded, long-period waveforms, and the microseismicity using data from a local seismic network — we used consistent velocity and elastic parameters in all our modelling, and the observed vertical separation is robust and resolvable. We suggest that it reflects the influence of the Proterozoic Hormuz salt, a weak layer at the base of the sedimentary cover across which rupture failed to propagate. Because the full thickness of the seismogenic layer failed to rupture during the largest earthquakes in the sequence, the lower, unruptured part may constitute a continued seismic hazard to the region. Considering the rarity of earthquakes larger than M w 6.2 in the Zagros Simply Folded Belt, we suggest that the Hormuz salt forms an important, regional barrier to rupture, not just a local one. Finally, we note that buried faulting involved in the largest earthquakes is almost perpendicular to the trend of an anticline exposed at the surface immediately above them. This suggests that locally, faulting and folding are decoupled, probably along a weak layer of marls or evaporites in the middle part of the sedimentary cover.