FAULTING PROCESS OF THE 1990 JUNE 20 IRAN EARTHQUAKE FROM BROAD-BAND RECORDS

We study the rupture process of the Iranian earthquake of 1990 June 20 from broad-band data. We redetermined its moment tensor and source mechanism from long-period surface waves. Then we studied the P and SH broad-band recordings from the GEOSCOPE and IRIS seismic networks. We inverted the body waves using a full waveform modelling. For the inversion we used the gradient method of Nabelek, mixed with a limited exploration of the parameter space. In order to control the resolution of inversion, we proceeded by a stepwise procedure increasing the model complexity with every new inversion. We used the reduction of cost functional as a criterion for the validity of the inverted model. We first inverted the body waves considering a single-point source model. We found that body waves arc dominated by a large energy release about 20 s from the onset of the signals. In the next step, fixing fault-plane solution, we inverted for the directivity of the source. We found clear evidence of propagation towards the east of the epicentre. We determined a rupture velocity of approximately 2.5 km s-1. The preceding source inversion reduced the variance of the residuals by about 30 per cent. Further improvement of the fit of the body-wave signals was obtained using an extended line source with constant fault-plane solution. We obtained a very good fit with an asymmetrical model. Initially rupture is bilateral but north-western propagation stops after 10 s. Later rupture continues unilaterally in the south-eastern direction with a rupture velocity of the order of 2.5 km s-1. Finally an inversion was attempted with a line source with varying fault mechanism. The final solution obtained by this procedure reduces the variance of the body-wave residuals by 60 per cent with respect to the variance of the observed body-wave signals. Variations in fault-plane solution along the fault arc considered to bc well resolved.