ELSEVIER Tectonophysics 302 (1999) 1–7 Seismic observations at a seismic gap in the eastern margin of the Japan Sea using ocean bottom seismometers Toshinori Sato a,Ł , Gou Fujie a , Sadayuki Koresawa a , Junzo Kasahara a , Keisuke Tanaka b , Seiichiro Honda b , Takashi Yokota b , Koji Nakamura c , Hirohito Naito c , Yuzo Ishikawa c , Tomowo Hirasawa d a Earthquake Research Institute, University of Tokyo, Tokyo, Japan b Japan Meteorological Agency, Tokyo, Japan c Meteorological Research Institute, Tukuba, Japan d Faculty ofScience, Tohoku University, Sendai, Japan Received 29 January 1998; accepted 5 November 1998 Abstract The eastern margin of the Japan Sea is a nascent convergent plate boundary. Previous studies proposed the existence of a seismic gap along this boundary between 39ºN and 40ºN. The trend of this gap is reported by Ohtake (Island Arc 4, 156–165, 1995) to be north-northwest to south-southeast, but by Ishikawa (Gekkan Kaiyo, Suppl. 7, 102–107, 1994) and Matsuzawa (Prog. Abstr., Seismol. Soc. Jpn. 2, B92, 1995) to be north-northeast to south-southwest. During one month ocean bottom seismic observations were conducted using nine ocean bottom seismometers to investigate seismicity in and around the seismic gap area in detail. The observations revealed that the earthquake epicentral distribution had an echelon shape and could be divided into three groups. These groups have a north-northeast to south-southwest trend. This trend is consistent with the fault system in this area, which was formed by the back-arc spreading in the Early to Middle Miocene. This suggests that previously formed tectonic structures affect the present seismo-tectonics and that this area has weak planes with a north-northeast to south-southwest trend.  1999 Elsevier Science B.V. All rights reserved. Keywords: seismicity; seismic gap; the Japan Sea; ocean bottom seismometers 1. Introduction The eastern margin of the Japan Sea is a nascent convergent plate boundary between the Eurasian plate and the North American plate (Fig. 1). Naka- mura (1983) proposed that the Eurasian plate has been subducting beneath the North American plate Ł Corresponding author. Fax: C81 3 5689 7234; E-mail: satot@eri.u-tokyo.ac.jp for one to two million years, and estimated the plate boundary on the basis of seafloor topography (the broken line in Fig. 1). In recent years many large earthquakes, such as the 1964 Niigata Earthquake ( M 7.5), the 1983 Ni- honkai-Chubu Earthquake ( M 7.7), and the 1993 Off-southwestern Hokkaido Earthquake ( M 7.8), have occurred in this margin. Focal areas of past large earthquakes with magnitudes greater than 7.5 in this margin compiled by Ohtake (1995) are shown 0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII:S0040-1951(98)00274-1