Portal de Periódicos da CAPES

A seismic-refraction profile in and around Nagano Prefecture, central Japan.

1986; Volume: 34; Issue: 6 Linguagem: Inglês

10.4294/jpe1952.34.457

1884-2305

Akira IMAI, Toshikatsu YOSHII, Susumu Kubota, Yoshimi Sasaki, Akiko Hasemi, Takeo Moriya, Hiroki Miyamachi, Ritsuko MATSU'URA Segawa, Kazushige Wada,

earthquake and tectonic studies

In 1981, the Research Group for Explosion Seismology recorded a seismic-refraction profile across northern and central Nagano Prefecture, central Japan, to study the velocity structure of the upper crust. The 220-km-long profile, which consisted of six shot points and 82 temporary stations, extended northeastward from near Mt. Ontake to the southwestern part of Niigata Prefecture, crossing the Itoigawa-Shizuoka tectonic line. The entire region is characterized by the highly heterogeneous upper crust. In the northeastern part of the profile, three thick folded surface layers with P-wave velocities of 2.0, 3.1, and 4.7 km/s overlay the basement with a velocity of 6.0 km/s, that slopes down northeastward. In the Nagano basin, the basement lies at about 1 km depth below the surface. An abrupt change of the basement depth by 4 km is found on the east of the Matsumoto basin. The associated subsurface fault is inferred. to a reverse type to satisfy the constraints from both the travel time and the Bouguer gravity anomalies. This fault may be a part of the Tsunan-Matsumoto tectonic line, which has been geologically recognized. The Itoigawa-Shizuoka tectonic line, which is one of the major tectonic lines in Japan, strikes from north to south in the Matsumoto basin. It is, however, not marked by any distinctive seismic velocity features. Southwest of the Matsumoto basin, Paleozoic-Mesozoic rocks are recognizable, and a basement with a velocity of 5.9 km/s lies at about 1 km depth.