Accelerograms of the 1978 Tabas, Iran, Earthquake

Three triaxial sets of accelerograms recorded in the near source region (within 50 km epicentral distance) of the September 16, 1978, Tabas earthquake (M s 7.4 -7.7) are of great engineering importance. The distances of the recording sites from the nearest approach of the rupture surface are approximately 3, 17, and 28 km for Tabas, Dayhook and Boshrooyeh stations, respectively. The measured horizontal peak ground accelerations of 0.94 and 0.88 g at Tabas are higher than previously estimated. The peak vertical ground acceleration recorded at this station is 0.74 g. The normalized response spectra at these three stations are consistent with the Newmark-Hall elastic design spectra, suggesting that the latter adequately represent the spectral amplification factors at frequencies above 1 Hz. The main shock accelerogram at Dayhook exhibits at least three distinct events as indication of a complex source behavior. The widened spacing of these events on the Dayhook records further confirms that the rupture front moved northwestward away from this station and towards Tabas. The measured S minus trigger times at Tabas, Dayhook and Boshrooyeh stations are in conflict with the teleseismically determined epicenter, requiring it to move approximately 30 km to the southwest to about 33° 17′N, 57° 09′ E.