Simultaneous estimation of stress drop and rupture directivity of small earthquakes in Japan: prevalence of asymmetrical rupture leads to systematic estimation error of stress drop

Abstract:

Stress drops of small earthquakes have been estimated under the assumption that the rupture propagates symmetrically within a circular fault. However, recent studies have observed directivity effects on seismic waves even for small earthquakes. In this study, rupture directivity was investigated systematically for small to moderate-sized earthquakes (M 3.5-5.5) that occurred beneath inland Japan from 2004 to April 2019. Apparent moment rate functions were determined for 1,463 earthquakes, and their corner frequencies were inverted for their rupture parameters.

The results indicate that most of the analyzed earthquakes are characterized by significantly asymmetrical rupture propagation. The stress drops of the earthquakes estimated by considering asymmetrical rupture propagation was 16.8 MPa, which is almost twice the estimates based on the commonly used symmetrical rupture model. This shows the importance of recognizing the diversity of ruptures, even for small earthquakes, for extracting information about earthquake sources and the Earth’s structure. The prevailing rupture directivity can provide useful information on source parameters including the stress drop, fault size, fault geometry, and rupture velocity of small earthquakes. It should be noted that the use of the generalized model enable to estimate the radiation efficiency, which may be helpful to examine the maturity of fault.