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

Nissen, Edwin and Jackson, James and Jahani, Salman and Tatar, Mohammad (2014) Zagros “phantom earthquakes” reassessed — the interplay of seismicity and deep salt flow in the Simply Folded Belt? Journal of Geophysical Research: Solid Earth, 119 (4). pp. 3561-3583. ISSN 2156-2202 DOI https://doi.org/10.1002/2013JB010796

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Abstract

Unravelling the contributions of mainshock slip, aftershocks, aseismic after- slip and postseismic relaxation to the deformation observed in earthquake sequences height- ens our understanding of crustal rheology, triggering phenomena and seismic hazard. Here, we revisit two recent earthquakes in the Zagros mountains (Iran) which exhibited un- usual and contentious after-effects. The M w ∼6 earthquakes at Qeshm (2005) and Fin (2006) are both associated with large InSAR signals, consistent with slip on steep re- verse 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 ver- tical separation between mainshock and aftershocks, reflecting a complex stress state near the basement–cover interface; or (2) the aftershocks delimit the mainshock slip and the InSAR signals were caused by shallow, up-dip afterslip (“phantom earthquakes”) with very similar magnitudes, mechanisms and geographical positions as the original earth- quakes. Here, we show that mainshock centroid depths obtained from body-waveform modelling — 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 Forma- tion, an Infracambrian sequence of intercalated evaporitic and non-evaporitic sediments. These aftershocks may reflect the breaking up of harder Hormuz sediments and adja- cent strata as the salt flows in response to mainshock strain at the base of the cover. This work bolsters recent suggestions that most large earthquakes in the Zagros are con- tained within carbonate rocks in the mid–lower sedimentary cover and that the crystalline basement shortens mostly aseismically.

Item Type:	Article
Additional Information:	The paper is a green open access publication and the copy available is post-print (ie final draft post-refereeing).
Uncontrolled Keywords:	2014AREP; IA67;
Subjects:	02 - Geodynamics, Geophysics and Tectonics
Divisions:	02 - Geodynamics, Geophysics and Tectonics 08 - Green Open Access
Journal or Publication Title:	Journal of Geophysical Research: Solid Earth
Volume:	119
Page Range:	pp. 3561-3583
Identification Number:	https://doi.org/10.1002/2013JB010796
Depositing User:	Sarah Humbert
Date Deposited:	11 Apr 2014 16:53
Last Modified:	02 Nov 2014 00:04
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3028

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