Seismic observations at a seismic gap in the eastern margin of the Japan Sea using ocean bottom seismometers

The eastern margin of the Japan Sea is a nascent convergent plate boundary. Previous studies proposed the existence of a seismic gap along this bounda ry between 39 degrees N and 40 degrees N. The trend of this gap is reported by Ohtake (Island Are 4, 156-165, 1995) to be north-northwest to south-sou theast, but by Ishikawa (Gekkan Kaiyo, Suppl. 7, 102-107, 1994) and Matsuza wa (Prog. Abstr., Seismol. Sec. Jpn. 2, B92, 1995) to be north-northeast to south-southwest. During one month ocean bottom seismic observations were c onducted using nine ocean bottom seismometers to investigate seismicity in and around the seismic gap area in detail. The observations revealed that t he earthquake epicentral distribution had an echelon shape and could be div ided into three soups. These groups have a north-northeast to south-southwe st trend. This trend is consistent with the fault system in this area, whic h was formed by the back-are spreading in the Early to Middle Miocene. This suggests that previously formed tectonic structures affect the present sei smo-tectonics and that this area has weak planes with a north-northeast to south-southwest trend. (C) 1999 Elsevier Science B.V. All rights reserved.