Fine structure of P-wave velocity distribution along the Atotsugawa fault, central Japan

A seismic experiment with five explosive sources and 396 seismic stations was conducted in August 2005 in the Atotsugawa fault area, central Japan. The 396 seismic stations were located on a survey line with a length of 47 km. The profile line was located along the Atotsugawa fault. The average spacing of the seismic stations was about 119 m. The spatially high-density linear-array was used to reveal the seismic structure of the fault zone. The fine structure along the Atotsugawa fault was obtained at depths shallower than 6 km. A large lateral variation of P-wave velocity, which was consistent with the geological structure, was found at the shallowest layer. The heterogeneous variation with a velocity range of 5.7 km/s–6.2 km/s was obtained at the second layer. The second layer was divided into three parts. The western part of the research area was determined to be high velocity. The low-velocity area was detected at the central part. At the east end of the profile line, the P-wave velocity was higher than that of the central part. The low-velocity area coincided with a low-seismicity area. The depth section of the reflection profile showed a boundary located at a depth of around 13 km. The boundary was roughly consistent with the cut-off depth of the seismicity of the microearthquakes. The high-seismic activity area was characterized to be reflective.