The 1978 Tabas, Iran, earthquake: An interpretation of the strong motion records

The 1978 Tabas, Iran, earthquake ( Ms = 7.4) was registered on 11 accelerograph stations, with source distances ranging from 3 to 350 km and corresponding recorded peak accelerations ranging from 0.95 down to 0.01 g, respectively. At least four subevents are recognized at Dayhook, the station nearest the epicenter at the southern limit of the fault. The rupture propagated to the northwest with a rupture velocity of 2.7 km/sec. The subevents occurred along the fault as the rupture proceeded. Curves which describe the distance dependence of peak values of acceleration, velocity, and band-passed acceleration are constructed for this earthquake. Although attenuation of seismic waves is similar in Iran and the Western United States, the prediction curves based on peak strong motion acceleration data from the Western United States systematically underestimate the peak values of the Tabas earthquake and the similar 1952 Kern County earthquake. Prediction curves based on peak velocity in the Western United States overestimate peak velocity for distances less than about 60 km. Site effects are recognized at some recording stations. The various estimates for stress operating during the earthquake range from 25 to 90 bars. The radiated energy of the main event estimated from accelerograms by two methods is about 2.3 × 1023 ergs, larger than the energy calculated by Gutenberg and Richter energy formula with Ms = 7.4. Each of the four subevents released approximately the same amount of energy, corresponding to Ms ≈ 7. Teleseismic energy estimates for this earthquake are one to two orders of magnitude smaller, indicating large systematic errors exist in one or both of the energy estimation procedures.