Stress Drop Estimates for Small to Moderate Earthquakes in Tohoku, Japan Considering Rupture Geometry, Speed, and Directivity

Abstract

AbstractThis study develops a spectral ratio method to estimate the earthquake stress drop considering the rupture geometry, speed, and directivity. This spectral ratio method is applied to Mw 3.3–5.9 earthquakes in Fukushima and Ibaraki, Japan. A rectangular source model with a square rupture front and uniform slip distribution is used to fit spectral ratios stacked by station and wave type for multiple empirical Green's functions, and estimate the finite source properties. Comparing the rupture areas obtained by the spectral ratio method with the slip inversion results shows that our results from the spectral ratio method are associated with a localized area of large slip rather than the overall rupture area. Hence, the rupture area obtained from this spectral ratio method is inappropriate to calculate the average static stress drop. This study proposes a procedure to estimate the average stress drop for the entire rupture area and one for the localized area with large slip, using a source model with a single asperity and heterogeneous stress drop distribution. As a result, we obtained average static stress drops of 0.42–40.2 MPa with a median of 3.7 MPa. The local stress drop is 5.7 times larger than the average static stress drop. This study suggests that a compact area compared to the overall rupture area mainly controls the shape of a broadband seismic spectrum. Thus, we should consider the source heterogeneity in addition to the rupture geometry, speed, and directivity when estimating the stress drops from seismic spectra.