Excitation of short-period body-waves by great earthquakes

Abstract

Short-period seismograms from large earthquakes are generally complex and are hard to Fourier-analyze. An innovation of spectral analysis has been made for WWSSN short-period seismograms. The parameters necessary for the analysis are: dominant frequency ƒ 0; maximum amplitude A max; and a characteristic duration σ of complicated wave-packets. Spectrum of short-period seismograms, Y(ƒ), is shown to be approximated as Y(ƒ)=1.07 σ ƒ0 A max The spectrum, after correcting for the effects of geometrical spreading, unelastic attenuation instrumental response, and crustal magnifications, is used to retrieve a short-period seismic moment M 1 of earthquake sources. The method has been applied to more than 900 short-period seismograms from 79 large earthquakes throughout the world. These earthquakes cover the seismic moment, M 0, from 7.5 × 10 29 to 7.5 × 10 24 dyne.cm. M 1 would not be saturated in this range, although body-wave magnitude usually is. A relation of M 0 ∞ M 1 2 is obtained for the present data set. A relation between M 1 and redetermined body-wave magnitude m∗ b is also shown log M 1=1.24 m ∗ b+17.9 where m b ∗ is obtained from maximum amplitudes of P-wavetrains. The analysis method is based on an extension of algorithm of earthquake magnitude determination, however, it has a potential beyond the body-wave magnitude. As M 1 is related closely to the source excitation of short-period seismic waves, it is one of the key parameters in understanding the fracture mechanisms of irregular faults.