Can the regional 3D stress field according to the Wallace–Bott hypothesis predict fault slip directions of future large earthquakes?

Abstract

When evaluating strong ground motions and tsunamis from specified source faults, it is required that the input parameters, such as fault geometry, rake angle, and slip amount, do not deviate from those of a real earthquake. Recently, a regional three-dimensional (3D) tectonic stress field was used to estimate rake angles for mapped submarine faults with the Wallace–Bott hypothesis (WBH), the direction of fault slip was parallel to the resolved stress vector on a preexisting fault, and strong ground motions and tsunamis were simulated. However, this modeling technique has not been adequately validated. Additionally, it is necessary to examine how the stress field estimated from seismological data for a limited period (~ 10 years) can be used as a proxy for the long-term tectonic stress field. In this study, to provide such validation, we utilized two catalogs of focal mechanism solutions for earthquakes and compared the observed rake angles with those calculated from the regional 3D tectonic stress field with the WBH by fixing the fault strike and dip angles according to those from the focal mechanism data. The resulting misfit angles between the observed and calculated rake angles are generally small (ranging between − 30° and 30°), excluding several regions (e.g., the source and surrounding regions of the 2011 off the Pacific coast of Tohoku earthquake and swarm-like activities activated after the 2011 quake). We also confirmed that the calculated rake angles and classified fault types are consistent with geomorphologically and geologically evaluated types of faulting for major Quaternary active faults in the Kyushu district of southwest Japan. These results support the validity and effectiveness of estimating rake angles for a specific fault with known geometry from the above method and data, while also showing that close attention is needed to apply this method to, for example, seismically inactive regions where the inverted stress field includes significant uncertainties and/or near sites of recent and large earthquakes where the stress field has been perturbed.Graphical