Seismic velocity structure in the source region of the 2016 Kumamoto earthquake sequence, Japan

Abstract

We investigate seismic wave velocity structure and spatial distribution of the seismicity in the source region of the 2016 Kumamoto earthquake sequence. A one‐dimensional mean velocity shows that the seismogenic zone has a high‐velocity and low‐Vp/Vs ratio relative to the average velocity structure of Kyushu Island. This indicates that the crust is relatively strong, capable of sustaining sufficiently high strain energy to facilitate two large (Mj > 6.5) earthquakes in close proximity to one another in rapid succession. Three‐dimensional tomography of the seismogenic zone around the source of the 2016 Kumamoto earthquake sequence yields Vp = 6 km/s and Vs = 3.5 km/s. Most large‐displacement areas (asperities) of the Mj 7.3 event overlap with the seismogenic zone and the overlying surface layer. Aftershock seismicity is distributed deeper than the conventional seismogenic zone, which suggests decreased strength due to fluids or increased stress, both caused by coseismic slip.

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

“…From the slip distributions, we calculated coseismic deviatoric stress changes , assuming an elastic half‐space model with Poisson's ratio = 0.25 and G = 34.3 GPa, which is compatible with Shito et al ().…”

“…From the slip distributions, we calculated coseismic deviatoric stress changes Δσ ′ ij , assuming an elastic halfspace model with Poisson's ratio = 0.25 and G = 34.3 GPa, which is compatible with Shito et al (2017). Terakawa and Matsu'ura (2010) have estimated the 3-D spatial pattern of background crustal stress field in and around Japan from the inversion analysis of CMT data of seismic events.…”

The 2018 Hokkaido Eastern Iburi earthquake (MJMA = 6.7) was triggered by a strike-slip faulting in a stepover segment: insights from the aftershock distribution and the focal mechanism solution of the main shock

“…This range is extraordinarily deeper than other intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Table S1 in Additional file 3 shows that in the case of the 1995 Hyogoken-Nanbu earthquake (M JMA 7.2), the depth range of the aftershock area is from 0 to 20 km (Hirata et al 1996); the 2000 Western Tottori (M JMA 7.3), 2-13 km (Shibutani et al 2005); the 2004 mid-Niigata (M JMA 6.8), 2-13 km (Okada et al 2005); the 2005 West Off Fukuoka (M JMA 7.0), 2-16 km (Shimizu et al 2006); the 2007 Chuetsu-oki (M JMA 6.8), 5-22 km (Nakahigashi et al 2012); the 2007 Noto Hanto (M JMA 6.9), 1-13 km (Sakai et al 2008); the 2008 Iwate-Miyagi Nairiku (M JMA 7.2), 2-8 km (Okada et al 2012); the 2011 Iwaki (M JMA 7.0), 2-15 km (Kato et al 2013); the 2016 Kumamoto (M JMA 7.3), 2-18 km (Shito et al 2017); and the 2016 Tottori (M JMA 6.6), 5-15 km (Ross et al 2018). On the other hand, the aftershock area is deep in and around the HCZ, Hokkaido.…”

Crustal Extension and Graben Formation by Fault Slip‐Associated Pore Opening, Kyushu, Japan

“…Considering the importance of the consistency of 1‐D initial velocity model with the arrival‐time data, we examined a number of initial models proposed by the previous studies (e.g., Wang and Zhao, 2006; Saiga et al, ; Shito et al, ). The starting velocity model used in the present study is adopted from a previous study (Wang & Zhao, ) based on the maximum reduction of the root mean square of the travel time residuals for the inverted model.…”

“…There have been several tomographic studies for the crust and uppermost mantle in the Kyushu region (e.g.,Saiga et al, ; Saita et al, ; Shito et al, ; Wang & Zhao, ; Xia et al, ), yet the structure of BSG has been poorly resolved. Our seismic tomography uses a total of 368,340 pairs of P and S arrival times reported from 207 stations in Kyushu for the 6,014 aftershocks of the Kumamoto earthquake and for 6,114 ordinary earthquakes.…”