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

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. 延岡衝上断層掘削プロジェクト(NOBELL)で得られたコア試料のP波速度, S波速度および間隙率の流体圧コントロール下での室内計測を行った。試料は変形組織によって千枚岩, 面状カタクレーサイトおよび非面状カタクレーサイトの三つに分類した。得られたP波速度, S波速度, 間隙率の範囲は千枚岩で5.17—5.57 km/s, 2.60—2.71 km/s, 2.78—3.02 %, 面状カタクレーサイトで4.89—5.23 km/s, 2.46—2.57 km/s, 3.58—4.53 %, 非面状カタクレーサイトで4.90—5.32 km/s, 2.51—2.63 km/s, 3.79—4.60 %であった。先行研究による露頭サンプルを用いた乾燥状態での結果とほぼ一致した一方, 掘削孔内計測による物性と比較して大きな速度が得られた。非面状カタクレーサイトの物性のばらつきが相対的に小さく, 変形組織によるものと考えられる。 Characteristic deformation textures of tested samples of (a) phyllite; (b) foliated cataclasite; and (c) non‐foliated cataclasite.