Small Earthquakes Deviate from the Omega-Square Model as Revealed by Multiple Spectral Ratio Analysis

We examine the omega-square model by estimating the source spectra of 196 M-w 3.2-4.0 shallow earthquakes in the northern Ibaraki prefecture and the Fukushima prefecture Hamadori area of Japan using multiple spectral ratio analysis. Spectral ratios of the seismograms of target events to those of multiple nearby small events (empirical Green's function [EGF] events) allow for negation of path and site effects and are fitted by the omega-square model. The residual spectra for different EGF events exhibit similar curves, indicating how the source spectra of the target events deviate from the omega-square model. Many earthquakes deviate remarkably, whereas other events fit well. The possible implications of these findings are incoherencies in the rupture process, the double-corner-frequency model, and a high-frequency falloff exponent value slightly higher than 2, although this is still an open question. This study will help to improve the source spectrum model, enhancing not only source studies, but strong motion assessments and other seismological studies.