Abstract
Using 3-D dynamic rupture simulations, we investigated the 2016 Mw7.1 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 Mw7.1 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.
Highlights
Earthquakes cause dynamic and static stress changes in the surrounding area
No rupture was initiated for a Dc value longer than 0.75 m, regardless of the value of SH, because of the long Lc and of the stress shadow due to the foreshocks
We conducted 3-D dynamic rupture simulations of the main shock of the 2016 Kumamoto, Japan, earthquake under an initial stress distribution constructed using the sum of the static stress changes induced by M6-class foreshocks and the regional stress field
Summary
Earthquakes cause dynamic and static stress changes in the surrounding area. Both the dynamic and static stress changes can trigger earthquakes, and a stress shadow appears where the Coulomb failure stress change (ΔCFS) is negative and subsequent seismicity tends to cease (e.g., King et al 1994; Stein et al 1997; Kilb et al 2000; Toda et al 2012).On April 16, 2016, at 01:25 (JST, UT + 9 h), the Mw7.1 main shock of the 2016 Kumamoto, Japan, earthquake sequence occurred along the Hinagu and Futagawa faults (Fig. 1). The largest slip was shown to occur at a portion of the Futagawa fault shallower than 12 km by seismic waveform inversion analyses (Asano and Iwata 2016; Kubo et al 2016; Uchide et al 2016; Yagi et al 2016; Kobayashi et al 2017; Hao et al 2017) as well as in the geodetic data (Himematsu and Furuya 2016). The large slip occurred on the Futagawa fault at a depth of 4–12 km for 6–10 s, and on the shallower region of the Futagawa fault later than 8 s
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