Aftershock tectonics of the 1978 Tabas-e-Golshan (Iran) earthquake sequence: a documented active 'thin- and thick-skinned tectonic' case

Abstract

Summary. In this study a locally recorded aftershock sequence of the 1978 Tabas-e-Golshan earthquake (M, = 7.4) was accurately located. Out of 1560 located events, 329 best-located aftershocks passed a strict quality criterion. These well-located aftershocks, which have uncertainties in epicentre and in focal depth of about 1 and 2km respectively, together with the wellconstrained focal mechanisms, provided a detailed picture of active continental deformation during an aftershock sequence. Almost all aftershocks follow very closely the pattern of the earthquake faults at the surface and lie in the hanging-wall block of the active fault. The hypocentres occurred mainly at depths less than 23 km with a high concentration of seismic activity between 8-14 km depth. The aftershocks clearly demonstrate an active imbricate listric thrust system with fault planes flattening into a basement decollement zone, and the reactivation of different basement reverse faults in response to a considerable amount of shortening of the top sedimentary cover. The sense of motion was almost universally thrusting and the aftershocks shared the same tectonic causes as the main shock. The study indicates that the development of the young fold-thrust mountain belts necessarily involves basement shortening (thin- and thick-skinned tectonics) and that the ‘frontal reverse faults’ in young active fold-thrust mountain belts are the most seismically active faults. Geological and seismic data propose that the active frontal reverse fault systems are possibly reactivated old normal faults and may add support to the contention of reversal of fault motion during rethickening of continental crust. The active ‘thinand thick-skinned tectonics’ documented in this study may prevail in other young and active fold-thrust mountain belts which are characterized by a thick sequence of telescoped top sedimentary cover over a decollement detachment zone.