Acoustic properties of deformed rocks in the Nobeoka thrust, in the Shimanto Belt, Kyushu, Southwest Japan

Abstract

Laboratory measurements for compressional and shear wave velocities (Vp and Vs, respectively) and porosity were conducted with core samples from the Nobeoka Thrust Drilling Project (NOBELL) under controlled effective pressure (5-65 MPa at 5 MPa intervals) and wet conditions. Samples were classified according to deformation texture as phyllite, foliated cataclasite, or non-foliated cataclasite. Measured values of Vp, Vs, and porosity are within a range of 5.17-5.57 km/s, 2.60-2.71 km/s, and 2.75-3.10 %, respectively, for phyllite; 4.89-5.23 km/s, 2.46-2.57 km/s, and 3.58-4.53 %, respectively, for foliated cataclasite; and 4.90-5.32 km/s, 2.51-2.63 km/s, and 3.79-4.60 %, respectively, for non-foliated cataclasite, which are all consistent with the previous laboratory experiments conducted with outcrop samples under dry conditions. However, our results also indicate higher Vp and Vs and lower porosity than those measured by the previous studies that adopted the wire-line logging methods. The variations in Vp, Vs, and porosity are controlled by deformation structure and are greater for phyllite and foliated cataclasite than for non-foliated cataclasite. K E Y W O R D S acoustic properties, Nobeoka thrust, Shimanto Belt

Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust, Shimanto Belt, southwest Japan

“…The values measured perpendicular to and parallel to foliation reported by Hashimoto et al () and Tsuji et al () show high velocities compared to those of the logging data. These differences are likely caused more by confining pressure on the samples and factors such as sampling scale than by the effect of dip angle and measurement orientation.…”

“…In contrast, sample resistivity is lower than that of the logs, pos- Tsuji et al (2006) and Hashimoto et al (2017) in the measured sample ( Figure 5). In contrast, the measured sandstone-rich phyllite with a moderate P-wave velocity and resistivity shows a high structural dip angle (~70 ), high sandstone ratio (~0.9), and low foliation density (2.3 per 1 cm) ( Figure 5).…”

“…Tsuji et al () measured velocity under dry conditions perpendicular to and parallel to foliation, reporting a P‐wave velocity of 5.12 km/s (perpendicular to foliation) and 5.89 km/s (parallel to foliation) and an S‐wave velocity of 3.28 km/s (perpendicular to foliation) and 3.69 km/s (parallel to foliation) in the hanging wall, and a P‐wave velocity of 4.18 km/s (perpendicular to foliation) and 4.91 km/s (parallel to foliation) and an S‐wave velocity of 2.75 km/s (perpendicular to foliation) and 3.08 km/s (parallel to foliation) in the footwall at 55 MPa. In contrast, Hashimoto et al () measured velocity under wet conditions perpendicular to foliation, documenting a P‐wave velocity of 5.17–5.57 km/s and an S‐wave velocity of 2.60–2.71 km/s in the hanging wall phyllite, and a P‐wave velocity of 4.89–5.32 km/s and an S‐wave velocity of 2.46–2.57 km/s in the footwall foliated cataclasite. When comparing our results to those of the previous studies, it is notable that the measured S‐wave velocity in this study is generally lower by 1–2 km/s at the maximum.…”

“…Tsuji et al () and Hashimoto et al () measured P‐ and S‐wave velocities under confining pressure (0–65 MPa) using outcrop and drill core samples from the Nobeoka Thrust, respectively. Tsuji et al () measured velocity under dry conditions perpendicular to and parallel to foliation, reporting a P‐wave velocity of 5.12 km/s (perpendicular to foliation) and 5.89 km/s (parallel to foliation) and an S‐wave velocity of 3.28 km/s (perpendicular to foliation) and 3.69 km/s (parallel to foliation) in the hanging wall, and a P‐wave velocity of 4.18 km/s (perpendicular to foliation) and 4.91 km/s (parallel to foliation) and an S‐wave velocity of 2.75 km/s (perpendicular to foliation) and 3.08 km/s (parallel to foliation) in the footwall at 55 MPa.…”

“…The purpose of this study is to investigate the effects of lithologic distribution and structural anisotropy on the physical properties of foliated fault rocks in subduction zones, focusing on the hanging wall phyllites and footwall cataclasites exhumed along the Nobeoka Thrust. Continuous coring and borehole geophysical logging down to 255 m below the ground surface (mbgs) was conducted at the Nobeoka Thrust during 2011 (Fukuchi et al, ; Hamada et al, ; Hamahashi et al, , ; Hashimoto et al, ; Kawasaki et al, ; Kitajima, Takahashi, Otsubo, Saffer, & Kimura, ). The logging data record physical properties (e.g.…”