Source parameters of 1978 Tabas and 1979 Qainat, Iran, earthquakes from long-period surface waves

Abstract

The teleseismic records of three strong earthquakes which inflicted heavy damage and loss of many thousands of lives in northeastern Iran during 1978 and 1979 are analyzed. The observed R_2 and R_3 phases of the 16 September 1978 Tabas earthquake across the IDA network are inverted to derive the gross source parameters of the main shock. The synthetic seismograms computed for the inferred source model are compared with observations. The agreement is generally good at most of the stations. However, unexpected asymmetrical observations at ESK, CMO, KIP, and PFO stations cannot be explained by directivity alone. Possible factors responsible for these observations may include regional heterogeneities or the departure of fault plane from simple planar geometry. The resulting focal mechanism agrees with nearly pure reverse mechanism with a NW-SE strike. The seismic moment estimate for this earthquake is about 1.5 × 10^(27) dyne-cm (M_w = 7.4). The seismic moment of the same earthquake obtained from long-period P-wave observations at six WWSSN stations is smaller than the above estimate by a factor of 2. Starting with the geologic field data of the Qainat earthquakes of 14 and 27 November 1979, the R_2 and G_2 phases on the ultra-long-period instruments at Pasadena and Berkeley are analyzed to estimate their seismic source moments. The estimated values are 1.0 × 10^(26) dyne-cm for the 14 November and 8.0 × 10^(26) dyne-cm for the 27 November 1979 events. Their corresponding moment magnitudes are 6.6 and 7.2, respectively.

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