Crustal structure in the northern part of the Philippine Sea plate as derived from seismic observations of Hatoyama-off Izu Peninsula explosions.

Abstract

In August 1980, observations of seismic waves generated by 35 explosions, one on land and the others at sea, were conducted at about 60 sites on land and four sites at sea bottom in a profile from Hatoyama to off Izu Peninsula to obtain the crustal structure in the northern part of the Philippine Sea plate. The profile, approximately parallel to the Suruga trough, crosses the Kan'nawa fault area, Hakone volcanoes, Izu Peninsula, and the Zenisu ridge. By the time term analysis, a velocity of 5.9 km/s was obtained for the granitic layer and a velocity of 6.8 km/s for the basaltic layer. The P_n velocity of 7.7 km/s was assumed for the time term analysis on the basis of travel time data in this experiment as well as previous results. The crustal structure thus obtained shows an interesting transition from the continental to oceanic type towards the Shikoku basin. It is possible that there are offsets in the upper boundaries of the granitic and the basaltic layers and in the Moho. Since these offsets are located in a narrow zone, this zone might be the boundary between the Philippine Sea plate and the Eurasian plate. Comparison of hypocenter distribution along the profile with the crustal structure supports the above idea since the pattern of hypocenter distribution between the two sides of the offset zone differs. This comparison also shows that earthquakes take place in the granitic layer beneath the Izu Peninsula. The distribution of observed Bouguer gravity anomaly along the profile is consistent with that expected from the crustal structure.