THE 1990 RUDBAR-TAROM IRANIAN EARTHQUAKE SEQUENCE - EVIDENCE FOR SLIPPARTITIONING

The Rudbar-Tarom Iran earthquake (June 20, 1990, M(S) = 7.7) occurred in the high Alborz, mountains south of the Caspian Sea in northwest Ir an. Although the structural style of the region is dominated by revers e faulting and folding, the focal mechanisms of the events indicate pu re right-lateral strike slip motion. We have studied the source parame ters of the 11 largest aftershocks (M(S) > 4.6) using three different methodologies: (1)joint inversion of teleseismic and regional distance body waves; (2) single-station moment tenser inversion (SSMT); and (3 ) comparison of entire wave trains to those of master events. The afte rshocks show a clear bipolar distribution: thrusting earthquakes on NW -SE trending planes and strike slip mechanisms with fault geometry sim ilar to that of the mainshock. We interpret the faulting behavior of t he 1990 earthquake sequence to indicate slip partitioning. In northwes t Iran the Arabian and Eurasian plates are converging in a NE directio n and the Alborz mountains are undergoing shortening and left-lateral shear. This oblique motion results in dominantly thrusting earthquakes and infrequent strike slip earthquakes.