The 1998 March 14 Fandoqa earthquake (Mw 6.6) in Kerman province, southeast Iran:: re-rupture of the 1981 Sirch earthquake fault, triggering of slip on adjacent thrusts and the active tectonics of the Gowk fault zone

The 1998 March 14 Fandoqa earthquake (M-s 6.6) was the penultimate in a series of five substantial earthquakes on the Gowk fault system of southeast Iran since 1981, all of which were associated with co-seismic surface ruptures. We use observations of surface faulting, analysis of P and SH body waves, SAR interferometry and geomorphology to investigate the ruptures in these earthquakes and how they are related both to each other and to the regional active tectonics. The 1998 Fandoqa earthquake produced 23 km of surface faulting with up to 3 m right-lateral strike-slip and 1 m vertical offsets. SAR interferometry and seismic waveforms show that the main rupture plane dipped west at similar to 50 degrees and had a normal component, although the surface ruptures were more complicated, being downthrown to both the east and the west on steep faults in nearsurface sediments. In addition, SAR interferometry shows that a nearby thrust with a similar strike but dipping at similar to6 degreesW moved about 8 cm in a time interval and in a position that makes it likely that its slip was triggered by the Fandoqa earthquake. The 1998 surface ruptures in the Gowk valley followed part of a much longer (similar to 80 km) set of co-seismic ruptures with smaller offsets that were observed after larger earthquakes in 1981 (M-w 6.6 and 7.1). The main ruptures in these 1981 earthquakes probably occurred on different, deeper parts of the same fault system, producing only minor reactivation of the shallower faults at the surface. Although the 1981-1998 earthquake sequence apparently ruptured parts of the same fault system repeatedly, these earthquakes had very different rupture characteristics: an important lesson for the interpretation of both palaeoseismological trenching investigations and historical accounts of earthquakes. The regional kinematics, which involve oblique right-lateral and convergent motion, are evidently achieved by a complex configuration of faults with normal, reverse and strike-slip components. Some of the complexity at the surface may be related to a ramp-and-flat fault geometry at depth, but could also be related to the large topographic contrast of similar to 2000 m across the fault system, which separates the high Kerman plateau from the low Dasht-e-Lut desert. Details of the fault geometry at depth remain speculative, but it must be unstable and evolve with time. It may be this requirement that causes the principal features of geological 'flower structures' to develop, such as series of subparallel faults which accommodate dip-slip components of motion.