Crustal structure in Izu Peninsula, central Japan, as derived from explosion seismic observations 2. Ito-Matsuzaki profile.

Abstract

The second experiment of explosion seismic observations in the Izu Peninsula, under the Japanese Earthquake Prediction Project, was conducted in December 1980 along a profile from Ito to Matsuzaki which obliquely crosses the previous Mishima-Shimoda profile. Five shot points and about 70 temporary stations were arranged on this profile and an additional shot point was set up at Yumenoshima, Tokyo, about 100 km northeast of the Izu Peninsula. The observation was also made for two other shots at Kawane, Shizuoka Prefecture and Izu Ohshima which were fired by the Geological Survey of Japan. Most of the seismograms obtained are very good and they provide us with highly accurate travel-time data. The data were analyzed mainly by the use of the time-term method and a very complex and accurate ustal structure down to about 3 km was inferred through the analysis. The structure essentially consists of 3 layers, among which an intermediate layer about 2 km thick with a velocity of about 4.2 km/s characterizes the upper crust of the Izu Peninsula region. This layer may correspond to the main part of the Yugashima Group, which is considered to be submarine deposits of Miocene volcanic products. The thin uppermost layer, with a velocity of about 3 km/s may be a younger one such as the Shirahama Group. The "basement layer" has a velocity of about 5.3 km/s at its top with a rather large vertical velocity gradient. It is plausible that this layer corresponds to the lower part of the Yugashima Group, although the exact geological identification of it is still an open question.