Tectonics of the Central Zagros (Iran) deduced from microearthquake seismicity

Microearthquakes in Central Zagros, recorded for 7 weeks in 1997, lie in a ∼6–8 km zone that is likely located beneath 11 km of sediments. They are not located on an active décollement between the sediments and the crystalline crust, but rather define a pattern of NNW–SSE trending lineaments parallel to the fold axes observed at the surface. The spacing between the seismic lineaments is ∼15–20 km and therefore different from that between the folds (∼10–15 km), which suggests that there is not a simple relationship between the two. Focal mechanisms and precise relative locations are consistent with NW–SE striking reverse faulting connected by NNW–SSE striking right lateral strike-slip faults. The dip of the reverse faults is not certain but is likely NE for the northernmost faults. The strain pattern deduced from the P-axes is remarkably similar to the shortening deduced from GPS-based geodesy suggesting that microearthquakes are the response of the prefractured brittle crust to strain rather than localized on single active faults.