Phenomenological Modeling of Coda-Wave Excitation

Abstract

As discussed in Section 2.4.1, the excitation of S-coda waves is one of the most compelling pieces of evidence supporting the existence of random heterogeneity in the lithosphere. Aki and Chouet [1975] summarized the characteristics of highfrequency S-coda waves of local earthquakes, like those shown in Figure 2.21, as follows: (1) the spectral contents of the later portions of the S-coda observed at different stations are nearly the same; (2) the total duration of a seismogram, defined as the length of time between the P-wave onset and the time when the coda amplitude equals the level of microseisms, is a reliable measure of earthquake magnitude; (3) bandpass-filtered S-coda traces of different local earthquakes recorded within a given region have a common envelope shape whose time dependence is independent of epicentral distance