Physics

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 waysthe largest three earthquakes using radar interferometry, moderate-sized events with teleseismically-recorded, longperiod waveforms, and the microseismicity using data from a local seismic networkwe 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. Crown extent to which the weaker units, especially the Proterozoic Hormuz salt at the base of the cover, separate deformation in underlying and overlying layers. A recent cluster of eleven, M w 5-6 earthquakes at Qeshm island (SE Zagros) provide an opportunity to investigate these problems. In this paper we establish the vertical distribution of faulting that occurred during these earthquakes, using detailed observations from radar interferometry (InSAR), locally-recorded seismic data, and teleseismically-recorded body-waves. By combining these different methodologies we are able to locate the causative faulting more precisely than would be possible using any single method on its own.

From 2005 December 2 to 2006 February 26, a dense seismological network of 17 stations was installed in the epicentral region of the 2005 November 27 Qeshm sequence. The epicentral distribution of aftershocks, including an ENE-WSW trend of seismicity, is terminated on both sides by NW-SE alignments of events. The depth distribution of events (i.e., 8-20 km depth) is diffuse but in the eastern part of the aftershock zone reveals an alignment of seismicity dipping ∼40 • toward the NW. Focal mechanisms of the aftershocks are strike-slip, different from that of the mainshock, which had a reverse mechanism. The scenario of a mainshock at shallow depth which is followed by a separated, deeper aftershock sequence implies different mechanisms of deformation in the sedimentary layer and the basement at the western edge of the Hormuz Strait. The epicentral distribution of aftershocks and their focal mechanisms suggest that shortening in the basement, due to convergence between Arabia and Eurasia, is accommodated in this region mainly by strike-slip motions. (F. Yaminifard). the main-shock? What is the depth range of the aftershocks in this region of the Zagros? What is the reason for the existence of such different focal mechanisms for the mainshock and the largest aftershock? How is deformation distributed in the southeastern-most part of the Zagros? And, is high-resolution analysis using local data consistent with teleseismic and radar interferometry studies? 0264-3707/$ -see front matter