Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Abstract

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three‐dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre‐condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low‐pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations.

Tomography of the 2016 Kumamoto earthquake area and the Beppu-Shimabara graben

“…A unique tectonic feature of Kyushu is that the central part of the island is subject to north-south extensional stress, whereas the crust of the Japan Islands is generally affected by compressive regional stress 8 , 9 . Extensional deformation runs east-west across central Kyushu, resulting in the Beppu-Shimabara graben (BSG) where four active volcanoes (Tsurumi, Kuju, Aso and Unzen) exist, in particular, Mt.…”

“…Aso with a large caldera whose area is ~400 km 2 . A number of M6 class historical earthquakes occurred in and around the BSG 7 , 10 that is under a heterogeneous stress regime associated with both right-lateral faults and normal faults whose tensional axes trend north-south 8 . Concerning the driving force of the extensional deformation in central Kyushu, two hypotheses have been proposed: northward extension of the opening Okinawa Trough 11 , and transient variations in the convergence direction of the PHS plate 12 .…”

Electrical Conductive Fluidized Zones and Their Influence on the Earthquake Nucleation, Growth, and Arrest Processes of the 2016 Kumamoto Earthquake Sequence, Kyushu Island, Japan

“…The relationship between resistivity structure and earthquake rupture process suggested that the distribution of mid-crustal uids contributes to the nucleation, growth, and arrest of crustal earthquakes. In addition to the pre-failure PT gradient, heterogeneities in the stress eld, fracture strength, and crack density may also contribute to the nucleation and arrest of earthquakes (Umeda et al, 1996;Kame and Yamashita, 1999;Matsumoto et al, 2018). Furthermore, all of the parameters depend upon each other, and can undergo temporal changes during rupture growth.…”

“…Furthermore, the seismicity before the 2016 Kumamoto earthquake sequence has been captured by a densely deployed seismometer network (Matsumoto et al, 2015;Matsumoto et al, 2018). In this study, we precisely relocate the detected earthquakes during the January 1993-June 2019 via the doubledifference method and a detailed three-dimensional (3-D) seismic velocity structure (Shito et al, 2017).…”

P-wave first-motion polarity determination of waveform data in western Japan using deep learning

“…In Japan, the WIN system (Urabe and Tsukada 1991;Urabe 1994;Uehira 2001), a useful software package for data acquisition and storage to deal with multichannel seismic waveform data, has been widely used; with a WIN system software, the P-wave first-motion polarity can be determined automatically. Horiuchi et al (2009) also developed an algorithm for the automatic determination of P-wave first-motion polarity, which has worked quite well and hugely helped to determine first-motion polarities in many studies (e.g., Matsumoto et al 2018;Katoh et al 2018;Okada et al 2019). However, there is still need for human experts to check the obtained results for accuracy even for the algorithm developed by Horiuchi et al (2009).…”