Lower crustal fluid distribution in the northeastern Japan arc revealed by high-resolution 3D seismic tomography Makoto Matsubara a, * , Naoshi Hirata b , Hiroshi Sato b , Shin’ichi Sakai b a National Research Institute for Earth Science and Disaster Prevention 3-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan b The Earthquake Research Institute, the University of Tokyo 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan Received 10 September 2003; received in revised form 21 January 2004; accepted 13 June 2004 Available online 26 August 2004 Abstract The Ou Backbone Range strikes northwards through the central northeastern Japan arc and is bounded on both sides by the active reverse Uwandaira and Sen’ya faults. We have applied a traveltime inversion method (seismic tomography) with spatial velocity correlation to active and passive seismic data in order to investigate a three-dimensional (3-D) velocity structure. The data set contains 33,993 P- and 18,483 S-wave arrivals from 706 natural sources and 40 blasts, as well as 2803 P-wave traveltime data from 10 explosions detonated during the 1997 controlled source experiment. The traveltime inversion reveals a zone beneath the Ou Backbone Range in which P-wave velocities (V P ) are approximately 6–8% lower than the average velocity at equivalent depths. The low V P and a low V P to S-wave velocity (V S ) ratio (V P /V S ) of about 1.65 suggest the presence of aqueous fluids in the middle crust. D 2004 Elsevier B.V. All rights reserved. Keywords: Island arc; Volcanic front; Middle crustal aqueous fluids; Tomographic method; Northeastern Japan arc 1. Introduction The northeastern Japan arc is one of the typical and most active convergent plate boundaries with an active volcanic chain parallel to the arc and the Japan Trench (Fig. 1, Sato, 1994). The structures of the arc crust are important to understand active tectonics ongoing in the subduction zone. The generation mechanism of a large intra-arc earthquake is one of the most important issues to be addressed in the intra- arc tectonics. Ague et al. (1998) found that liquid migrating into less permeable crust might trigger the intraplate earthquakes. It is important to distinguish aqueous liquid from molten rock in the crust to understand mechanical properties of the crust. We observe lateral variation of volcanic activity along the volcanic chain and the large intra-arc earthquakes correlate with thermal distribution: large earthquakes occurred where few volcanoes exist. Seismic methods, including reflection and refrac- tion seismic methods (Iwasaki et al., 1999, 2001; Sato 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.07.046 * Corresponding author. Tel.: +81 29 863 7674; fax: +81 29 860 2317. E-mail address: mkmatsu@bosai.go.jp (M. Matsubara). Tectonophysics 388 (2004) 33– 45 www.elsevier.com/locate/tecto