Stress measurements in the vicinity of the Futagawa fault based on drilled rock core samples retrieved from a scientific-drilling borehole

In-situ stress state in the vicinity of source faults is the key to understand the mechanisms of earthquakes. In this study, we aim to determine the post-seismic stress state in the vicinity of the Futagawa fault, which is the source fault of the Mw 7.0 2016 Kumamoto earthquake mainshock. These earthquakes were characterized by right-lateral strike-slip. We applied anelastic strain recovery (ASR) method to determine the in-situ stress state.

ASR method is one of the core-based in-situ stress measurements, which analyzes time-dependent recovery of the anelastic strain of a cored sample after stress release by drilling and enable us to determine the three-dimensional stress tensor. We measured ASR with strain gauges attached to the cored sample under a constant temperature condition. Six core samples, which were retrieved from a scientific-drilling borehole penetrating the Futagawa fault, were successfully used for ASR measurements.

The drilling site is located at Mashiki town, Kumamoto Prefecture, Japan where strong motions and surface ruptures were observed. The six cores were retrieved from the depth of 500 - 670 m.

Almost all the results show that the maximum principal stress (σ₁) deeply plunges and the other principal stresses (σ₂ and σ₃) are in the horizontal plane. This indicates that the normal faulting stress regime is dominant around the Futagawa fault. Pre-seismic stress states constrained using CMT inversion conducted in previous studies indicate that the strike-slip faulting stress regime were dominant in the same area. This suggests the coseismic stress rotation during 2016 Kumamoto earthquakes.

I am grateful to Koichiro Sado, Susumu Shibutani and Hiroaki Aizawa from Chi-ken Sogo Consultants Co., Ltd. for their support during ASR measurements on site.