Faults affecting the basement and surface soil in the aftershock area of the 1997 Northwestern Kagoshima Earthquake

Abstract

: In 1997 , two strong earthquakes occurred on March 26 (M j 6 . 6 ) and May 13 (M j 6 . 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 M j 6 . 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 separa-tion 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 expo-sure 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.