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

• Published in: Geophysical research letters Vol. 44; no. 15; pp. 7766 - 7772
• Main Authors: Shito, Azusa, Matsumoto, Satoshi, Shimizu, Hiroshi, Ohkura, Takahiro, Takahashi, Hiroaki, Sakai, Shinichi, Okada, Tomomi, Miyamachi, Hiroki, Kosuga, Masahiro, Maeda, Yuta, Yoshimi, Masayuki, Asano, Youichi, Okubo, Makoto
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.08.2017
• Subjects: Aftershocks; Average velocity; double‐difference tomography; Earthquakes; Ecological succession; Energy; Fluids; Seismic activity; Seismic velocities; Seismic wave velocities; Seismic waves; Seismicity; Sequencing; Spatial distribution; Surface boundary layer; Surface layers; the 2016 Kumamoto earthquakes; Tomography; Velocity; velocity structure; Wave velocity; Japan
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL074593

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. Key Points One‐dimensional mean velocity structure suggests high crustal stress in the source region, sufficient for the earthquake sequence A large‐displacement area of the Mj 7.3 event corresponds to the region with moderate seismic velocities (Vp = 6.0 km/s, Vs = 3.5 km/s) Penetration of the aftershock seismicity into conventional aseismic depth suggests existence of fluids or stress concentration