Spectra of Great Earthquakes : Teleseismic Constraints on Rupture Process and Strong Motion

Short-period body waves recorded at teleseismic distances from great earth· quakes provide information about source rupture processes and strong motions. First, we examine mostly WWSSN records of 19 earthquakes of moment magni· tude Mw of 6.5 to 9.5. Four parameters are measured from the short-period Pwave train: the maximum amplitude; the period at maximum amplitude; the time between the first arrival and when the maximum amplitude is attained; and coda length. An extension, mb, of the teleseismic magnitude, mb, is defined using the maximum amplitude of the entire short-period P-wave rather than the amplitude achieved in the first few P·wave cycles. A least-squares fit to the data yields the following relationship between mb and Mw: mb = 0.53 Mw + 2.70 for Mw 6.5 to 9.5. The time from the first arrival until the maximum amplitude is achieved and the coda length are roughly proportional to Mw, but are further interpreted by a simple asperity model of the rupture process. These data support that short-period waves are, on average, generated preferentially in the same regions of the fault plane as long-period waves (with periods of 10 to 50 sec). We analyze the spectra of short· and intermediate-period teleseismic GDSN records for seven earthquakes with Mw's of 6.4 to 7.8 and hand-digitized shortperiod WWSSN records of the 1971 San Fernando earthquake. Significant differ· ences exist between the spectra of different events, due partly to variations in tectonic setting or seismic coupling. Using the digital data, we also investigate the relationship between time-domain amplitude and spectral amplitude for shortperiod P waves. From our empirical relation between spectral amplitude and time-domain amplitude, we estimate the spectral amplitudes implied by the mb data. We compare our results to the wand Gusev spectral models. Neither model can completely represent the data. Nevertheless, we consider the wmodel a useful reference model for comparing different events. The average source spectrum of six large events with Mw 7.4 to 7.8 does not have the spectral structure suggested by Gusev. An application to strong motion modeling is presented in which a 1971 San Fernando teleseismic short-period record is summed up to simulate teleseismic records produced by five great earthquakes. The summation procedure matches the moment of the event to be simulated, and includes rupture propagation, fault plane roughness, and randomness. The mb data provide an important constraint on the summation procedures. Thus constrained, this summation procedure can be more confidently used with near-field strong motion records as Green's functions to predict strong motions from great earthquakes.