Origins of Amplitude Variations In Seismic Doublets: Source Or Attenuation Process?

Abstract

Accurate estimates of spectral ratios of two doublets (i.e. similar seismic events) occurring along the San Andreas Fault, in the Hollister region have been computed using a cross‐spectral method. Both events of each doublet occurred within a 24 hr interval. Although both waveforms of a doublet are remarkably similar, computed spectral ratios exhibit strong, but steady variations with frequency in the first arrivals. the attenuation process can explain neither the sign nor the order of magnitude of these variations. On the contrary, they are closely related to the source mechanism: we show that the slope computed from the linear and coherent part of the spectral ratio is an estimation of the variation in the apparent rupture duration between the two events. One doublet exhibits an azimuthal distribution of the spectral‐ratio variations that is interpreted as the result of a change in the position of the hypocentres, relative to the rupture area. For this doublet, location of hypocentres varies only from 10 to 30 per cent of the source length: this points out that small changes in rupture kinematics can lead to significant variations in the spectral ratio of earthquake doublets, i.e. in the amplitude of each event. For the other doublet, the variations are related to a change in source length. We also have computed spectral ratios all along the seismograms, using a moving window technique. When an azimuthal distribution of the spectral ratio is observed in the first arrivals, we notice a decrease of the spectral ratio all along the seismograms. the doublet with strong, but non‐azimuthal variations of the spectral ratio in the first arrivals, does not indicate such a decrease of the spectral ratio along the seismogram. This implies that the variations of the coda decay are related not only to temporal or spatial changes of attenuation but also to source parameters. These variations have the same order of magnitude as those provoked by a temporal change in coda Q of 10 per cent. They can occult time changes of spectral ratio, for instance in case the attenuation in the crust would vary before a strong earthquake. Copyright © 1993, Wiley Blackwell. All rights reserved