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 (8) probably occurred along this detachment. A comparative study of the Zagros continental collision zone and the Iranian blocks are similar to those of the Himalayas and the Tibetan terranes in Oligocene time.