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Co-seismic and aseismic deformation on faults in eastern Iran

Eric Fielding(1) and James Jackson(2)

(1) Jet Propulsion Lab/Caltech, MS 300-233, Pasadena California 91109, United States
(2) COMET and University of Cambridge, Madingley Rise, Cambridge CB3 0EZ, United Kingdom

Abstract

Eastern Iran is a wide zone of active deformation related to the collision between the Arabian Plate and the Eurasian Plate. The overall shear across the zone is accommodated on both strike-slip and thrust faults in complex geometries. The complexity of the faults makes seismic hazard evaluation more difficult than in simpler fault systems. Several large earthquakes have struck eastern Iran while the ERS-1 and ERS-2 satellites have been in operation, with the 1997 Mw=7.2 Zirkuh earthquake the largest and most destructive. The arid climate facilitates C-band SAR interferometry, with interferograms intervals up to 6 years remaining coherent. We used InSAR analysis of ERS data to measure slip on faults during earthquakes and slip that likely happened without generating a significant seismic event. The 1997 May 10 Zirkuh earthquake ruptured 125 km of the Abiz fault system and killed more than 1500 people despite the low population density of the area near the border of Afghanistan. The combination of interferograms from ascending and descending orbits shows that the main earthquake ruptured several strike-slip fault segments and at least one thrust fault segment. This is consistent with seismic body-wave analysis finding four sub-events with the last sub- event having a thrust mechanism. Two Mw=5.4 to 5.7 aftershocks of the Zirkuh earthquake were also located on the interferograms. Accurate locations for these smaller events can help improve the seismic propagation models of the region.

Another large earthquake, the 1998 Fandoqa event, struck about 400 km to the southwest of the Zirkuh earthquake. The 1998 March 14 Mw=6.6 earthquake ruptured 23 km of the Gowk Fault. InSAR and seismic body-wave analysis both indicate oblique- normal slip on a shallow fault. Surface ruptures in 1998 occurred in part of the same length of the Gowk fault that previously ruptured in 1981 during a Mw=7.1 earthquake. The 1981 event likely concentrated slip on a deeper fault than the 1998 event. In addition to the co-seismic deformation of the Fandoqa earthquake, a second deformation event was mapped by InSAR from 10 to 30 km to the southeast. This deformation is best explained by slip on a very shallow and shallowly dipping thrust fault that we call the Shahdad thrust. The best-fitting slip patch is roughly 22 x 28 km with 70 mm of displacement, but no seismic waves were detected consistent with this event. This deformation was probably aseismic.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry