At depth, many fold-and-thrust belts are composed of a gently dipping, basal thrust fault and steeply dipping, shallower splay faults that terminate beneath folds at the surface. Movement on these buried faults is difficult to observe, but synthetic aperture radar (SAR) interferometry has imaged slip on at least 600 km2 of the Shahdad basal-thrust and splay-fault network in southeast Iran. Approximately 70 mm of thrust motion on the 8°-dipping Shahdad basal thrust occurred 8–30 km to the east of the 14 March 1998 Fandoqa earthquake (Mw = 6.6) that involved 1.6 m of oblique (strike slip and normal slip) displacement on a steeply dipping fault. That earthquake transferred stress to the Shahdad basal thrust and associated splays, triggering slip either immediately or in the following six months. Modeling shows that, to produce the observed surface deformation, the Shahdad faults must have accommodated a nearly complete release of the shear stress increase. This could be due to either low friction on the faults or deformation throughout a wedge of material that is everywhere close to failure. The anomalously small magnitude of displacement on the Shahdad basal thrust and splay faults compared to the area of slip suggests a slip mechanism that is likely aseismic.

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