Seismotectonics of the Zagros continental collision zone and a comparison with the Himalayas

Accurate focal depths of 16 medium to large Zagros earthquakes were determined from identification of surface‐reflected phases by comparing observed, long‐period P waves with synthetic seismograms. All events are shallow (6–12 km) and show no tendency to deepen toward the Main Zagros Thrust. Geological and geophysical evidence suggests that most of these earthquakes probably occurred within the uppermost Precambrian basement. Because the focal mechanisms of these events indicate high‐angle, reverse faulting, the events do not occur on a single detachment surface but rather reflect horizontal shortening of the basement on numerous, short‐segmented (<100 km) and subparallel reverse faults. This geometry may explain the absence of great earthquakes ( M ≅ 8) in Zagros. Synchronous widespread deformation of both the sedimentary cover and basement is a unique feature for the Zagros region unlike other fold and thrust belts. In contrast to Zagros, medium to large, thrust‐type Himalayan earthquakes define a shallow, north dipping detachment that separates the underthrusting Indian plate from the Lesser Himalayan crustal block; the great Himalayan events ( M > 8) probably occurred along this detachment. A comparative study of the Zagros with the Himalayas suggests that the active tectonics of the Zagros continental collision zone and the Iranian blocks arc similar to those of the Himalayas and the Tibetan terranes in Oligocene time.