Subevent analysis for the Tabas earthquake of September 16, 1978, using near field accelerograms

We analyze the strong motion accelerograms of the large magnitude (m(b) = 6.4, M-s = 7.7, M-w = 7.4, M-o = 1.32 x 10(20)) Tabas earthquake of September 16, 1978. The earthquake occurred due to a unilaterally propagating complex rupture process on a thrust fault system. The imprints of rupture of four (and possibly five) asperities (sub-faults) have been identified on the accelerograms. Using derived 'SH-wave' records, the locations of these asperities, which lie on the main causative fault, and the relative times of their occurrence have been determined. A foreshock that occurred about 6 s prior to the main shock has also been identified and located. Further, using 'SH-wave' spectral data, we provide near field estimates of the strike, dip and slip of these asperities as well as their seismic moment, moment magnitude, fault area, released shear wave energy and average stress drop. The spectra of the sub-events conform to the omega(-2) Brune spectra. The total seismic energy of the sub-events is consistent with the radiated energy estimated from teleseismic observations. The fault plane solutions show generally WNW-ESE strike, which is sub-parallel to the regional strike of the geological fault system existing in the area. The stress drops are in the range of 7-30 bar, with the exception of the main sub-event that showed a stress drop of 100 bar. Overall the sub-events show self-similarity. The average shear wave quality factor Q(s), which in the near field is quite sensitive to the local site geology, has been found to lie between 600 and 1200. The decay of PGA with distance is found to be compatible with Niazi and Bozorgnia's [Niazi, M., Bozorgnia, Y. 1992. The 1990 Manjil, Iran earthquake: geology and seismological overview, PGA attenuation and observed damage. Bull. Seism. Soc. Am. 82, 774-799] attenuation relation for the region. (c) 2005 Elsevier B.V. All rights reserved.