3-D dynamic rupture simulations of the 2016 Kumamoto, Japan, earthquake

Abstract

Using 3-D dynamic rupture simulations, we investigated the 2016 M w 7.1 Kumamoto, Japan, earthquake to elucidate why and how the rupture of the main shock propagated successfully, assuming a complicated fault geometry estimated on the basis of the distributions of the aftershocks. The M w 7.1 main shock occurred along the Futagawa and Hinagu faults. Within 28 h before the main shock, three M6-class foreshocks occurred. Their hypocenters were located along the Hinagu and Futagawa faults, and their focal mechanisms were similar to that of the main shock. Therefore, an extensive stress shadow should have been generated on the fault plane of the main shock. First, we estimated the geometry of the fault planes of the three foreshocks as well as that of the main shock based on the temporal evolution of the relocated aftershock hypocenters. We then evaluated the static stress changes on the main shock fault plane that were due to the occurrence of the three foreshocks, assuming elliptical cracks with constant stress drops on the estimated fault planes. The obtained static stress change distribution indicated that Coulomb failure stress change (ΔCFS) was positive just below the hypocenter of the main shock, while the ΔCFS in the shallow region above the hypocenter was negative. Therefore, these foreshocks could encourage the initiation of the main shock rupture and could hinder the propagation of the rupture toward the shallow region. Finally, we conducted 3-D dynamic rupture simulations of the main shock using the initial stress distribution, which was the sum of the static stress changes caused by these foreshocks and the regional stress field. Assuming a slip-weakening law with uniform friction parameters, we computed 3-D dynamic rupture by varying the friction parameters and the values of the principal stresses. We obtained feasible parameter ranges that could reproduce the characteristic features of the main shock rupture revealed by seismic waveform analyses. We also observed that the free surface encouraged the slip evolution of the main shock.

Stress Release Process Along an Intraplate Fault Analogous to the Plate Boundary: A Case Study of the 2017 M5.2 Akita‐Daisen Earthquake, NE Japan

“…In this study, we focused on an M 5 earthquake sequence that occurred on a single fault plane. Fault structures inside the plate, however, are usually more complex, and their interactions play important roles in the 3‐D release of intraplate stress (Urata et al, 2017). When faults are randomly distributed in space, shear strain energy density can be a measure of the average shear stress over the faults (Saito et al, 2018).…”

The 3‐D Spatial Distribution of Shear Strain Energy Changes Associated With the 2016 Kumamoto Earthquake Sequence, Southwest Japan

“…One is based on the observation of stress rotation due to the earthquake, which suggested that the maximum shear stress is smaller than 10 MPa (Yoshida et al, ). Another is proposed from the dynamic rupture simulations reproducing the foreshock and main shock, indicating that the maximum shear stress should range from 15 to 30 MPa (Urata et al, ). Our estimation of the lower limit of maximum shear stress supports the latter background stress state and dismisses the extraordinarily low magnitude of stress in the former model.…”

Shallow Temporal Changes in S Wave Velocity and Polarization Anisotropy Associated With the 2016 Kumamoto Earthquake Sequence, Japan

“…The Kumamoto earthquake sequence began with a M w 6.2 event at 21:26 JST (UTC + 9) on 14 April 2016, and the M w 7.0 mainshock occurred at 01:25 JST on 16 April 2016 (e.g., Urata et al, 2017). We refer to these two events hereafter as the "largest foreshock" and "mainshock," respectively.…”