1997年鹿児島県北西部地震余震域内の基盤から土壌までを切る断層の露頭

• Published in: Chishitsugaku zasshi Vol. 124; no. 5; pp. 361 - 366
• Main Authors: 関根, 秀太郎, 笠原, 敬司, 林, 広樹, 内田, 嗣人, 武田, 哲也, 吉崎, 那都, 向吉, 秀樹, 後藤, 和彦
• Format: Journal Article
• Language: Japanese
• Published: Tokyo 一般社団法人 日本地質学会 15.05.2018; Japan Science and Technology Agency
• Subjects: 1997 Northwestern Kagoshima Earthquake; active fault; Earthquakes; Fault lines; Faults; Loam; Miocene; Outcrops; Sandy loam; Seismic activity; Soil; Soil surfaces; Soils
• ISSN: 0016-7630; 1349-9963
• DOI: 10.5575/geosoc.2018.0006

In 1997, two strong earthquakes occurred on March 26 (Mj6.6) and May 13 (Mj6.4) in the northwestern part of Kagoshima Prefecture, Japan (termed the 1997 Northwestern Kagoshima Earthquakes). However no seismogenic faults associated with these earthquakes had previously been recognized at the surface. In this study, we report structural observations from newly recognized fault outcrops located 1.5 and 2 km southwest of the epicenter of the Mj6.6 earthquake. One outcrop consists of the Miocene Shibi-san Granodiorite, which is unconformably overlain by lower sandy loam, lower humic soil, upper sandy loam, and upper humic soil layers that are clearly offset by steeply dipping faults. We measured ~ 40 cm of vertical separation of the contact between the lower sandy loam and lower humic soil layers along a WNW-trending fault, above which the ground surface flexed upward by about 20 cm. The second outcrop is a streambed exposure of the Shibi-san Granodiorite that shows a 30-cm-thick layer of foliated cataclasite along a fault plane, with textural evidence of sinistral slip.