Toward elucidation of behavior in updip limit of seismogenic zone

2020 Fiscal Year Final Research Report

• Project/Area Number: 17H02952
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Solid earth and planetary physics
• Research Institution: The University of Tokyo
• Principal Investigator: Kato Aitaro 東京大学, 地震研究所, 教授 (20359201)
• Project Period (FY): 2017-04-01 – 2021-03-31
• Keywords: 地震発生帯の上限 / 地震活動 / 超高密度地震観測

Outline of Final Research Achievements

We deployed a very dense seismic array near the surface traces of Futagawa active fault, which hosted the 2016 Kumamoto earthquake in Kyushu, to investigate the detailed distribution of earthquakes around the upper limit of the seismogenic zone. After analyzing the continuous seismic waveform data during about 50 days, we revealed that hypocenters roughly aligned along a NW-dipping plane, which is consistent with the inclination of the Futagawa fault and the aftershock distribution of the Kumamoto earthquake. We have not detected any earthquakes at depths shallower than about 2 km beneath the fault surface traces. The magnitude of earthquakes at shallow depths is smaller than that at deep earthquakes, indicating a transition of frictional property along the fault-dip.

Free Research Field

地震学

Academic Significance and Societal Importance of the Research Achievements

2016年熊本地震を引き起こした布田川断層帯の直上における観測により，地表地震断層から深さ約2 kmまで顕著な地震活動が起きていないこと，地表に近い地震ほど規模が相対的に小さくなることを示した。これらは，地震発生帯の上限付近において地表に向かって断層の摩擦特性が徐々に変化することを示唆する。一方で，熊本地震発生時のすべりは断層深部から地表まで到達しており，摩擦特性の変化はそれほど強いものではなく，勢いのある加速した動的破壊すべりが伝わってくる場合には，地震性すべりが起きることを意味する。