The 1998 March 14 Fandoqa earthquake (M-w 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 ser ies of five substantial earthquakes on the Gowk fault system of southeast I ran since 1981, all of which were associated with co-seismic surface ruptur es. We use observations of surface faulting, analysis of P and SH body wave s, SAR interferometry and geomorphology to investigate the ruptures in thes e earthquakes and how they are related both to each other and to the region al 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 t he surface ruptures were more complicated, being downthrown to both the eas t and the west on steep faults in nearsurface sediments. In addition, SAR i nterferometry 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 positi on that makes it likely that its slip was triggered by the Fandoqa earthqua ke. The 1998 surface ruptures in the Gowk valley followed part of a much lo nger (similar to 80 km) set of co-seismic ruptures with smaller offsets tha t were observed after larger earthquakes in 1981 (M-w 6.6 and 7.1). The mai n ruptures in these 1981 earthquakes probably occurred on different, deeper parts of the same fault system, producing only minor reactivation of the s hallower faults at the surface. Although the 1981-1998 earthquake sequence apparently ruptured parts of the same fault system repeatedly, these earthq uakes had very different rupture characteristics: an important lesson for t he 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 co mplex configuration of faults with normal, reverse and strike-slip componen ts. 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 topographi c contrast of similar to 2000 m across the fault system, which separates th e 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 w ith time. It may be this requirement that causes the principal features of geological 'flower structures' to develop, such as series of subparallel fa ults which accommodate dip-slip components of motion.