Subsurface structure under a basaltic monogenetic volcano near the active Atera fault T. Tanaka a, * , H. Aoki a , A.J. Martin b , K. Oshita c , K. Nozaki d , M. Onishi e a Tono Research Institute of Earthquake Science, Association for the Development of Earthquake Prediction, 1-63 Yamanouchi, Akeyo-cho, Mizunami 509-6132, Japan b Mizunami Geoscience Academy, Japan Nuclear Cycle Development Institute, 1-63 Yamanouchi, Akeyo-cho, Mizunami 509-6132, Japan c Chubu Branch Office, OYO Corporation, 102 Nakashima, Seko, Moriyama, Nagoya 463-8541, Japan d Tsukuba Technical Research and Development Center, OYO Corporation, 43 Miyukigaoka, Tsukuba 305-0841, Japan e JGI, Incorporated, Meikei Building, 1-5-21 Otsuka, Bunkyo, Tokyo 112-0012, Japan Accepted 11 September 2003 Abstract In this study, we examined the subsurface structure in the vicinity of the active Atera fault beneath at least one monogenetic volcano that forms the Sakashita body of the Ueno basalts, central Japan. Microgravity and seismic reflection surveys over an area of about 3  3 km (horizontal)  1 km (vertical from surface) revealed that (1) a low-density body interpreted as the damaged zone of the Atera fault reaches a depth more than 200 m below sea level; and (2) a weak zone of crust that may exist about 1.5 km southwest of the Atera fault in a direction parallel to it. By combining the shape of the Sakashita body and (2), we propose that a dyke extending in a NW – SE direction with several vent alignments in the same direction may exist along this weak zone. This suggests that the studied area was governed in the NW – SE compressional and NE – SW extensional stress field when the Sakashita body formed. D 2003 Elsevier B.V. All rights reserved. Keywords: Atera fault; Ueno basalts; Gravity survey; Seismic reflection; Monogenetic volcano; Stress field 1. Introduction Since the Atera fault zone is one of the most active regions both geomorphologically and geologically in Japan, it has been the focus of many studies (e.g., Kato et al., 1987; Kanaori et al., 1990; Tsukuda et al., 1993; Yasue and Hirouchi, 2002). From a geophysical prospecting point of view, Yamamoto et al. (1982) indicated that the fault is not only a morphological boundary but also probably a gravimetric boundary. Present seismicity is low in and around the fault zone (Kimura and Observation and Research Group of Crustal Activities in the Kanto-Tokai District, 2001), therefore, a large-scale detailed seismic survey is needed. Vibroseis surveys have not been carried out in the vicinity of the Atera fault due to poor road access and the severe topography. In recent years, however, some vibroseis observations have been carried out under similar difficult conditions (e.g., Arita et al., 1998; Tono Research Institute of Earthquake Science [TRIES], 2000). Moreover, a drilling project by the National Research Institute for Earth Science and 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.09.014 * Corresponding author. Fax: +81-572-67-3108. E-mail address: tanaka@tries.jp (T. Tanaka). www.elsevier.com/locate/tecto Tectonophysics 378 (2004) 197– 208