Crustal Structure beneath Northeastern Japan Derived from Explosion Seismology and Gravity Anomalies

In 1997, one of the most extensive explosion seismological investigation was conducted along the Kamaishi-Iwaki profile in the northern part of Honshu Island, Japan, and Iwasaki et al.(1999) presented a detailed P-wave crustal velocity structure in this region. Our group had about 35,000 gravity data points covering northeastern Japan, and now we added the gravity data along the Kamaishi-Iwaki profile. Using this data set, we constructed a detailed gravity profile along this line. We obtained a two dimensional crustal structures applying 2D-Talwani?s method as we describe below. As a starting model, we converted a P-wave velocity structure given by Iwasaki et al. (1999) into a density structure model employing an empirical relationship between seismic velocity and rock density (Nafe and Drake, 1957). This model, however, did not agree with the gravity anomalies. Therefore we modified the starting model in the following order: (a) we changed the depth of the Moho and the Conrad boundary in order to achive a better fit to the long wavelength component of gravity anomalies; (b) modify the density of the sedimentary rocks into heavier one. However we could not obtain consistent theoretical gravity anomalies over the Tono region where large granitic bodies is thought to have intruded during the Cretaceous period exist. Therefore we assume a granite pluton whose density is lighter than surrounding rocks?, 2.64 g/cc, and insert in an advanced model. In this model, the depth of the diapir-shaped granite pluton is considered to be no deeper than 8-10 kilometers. This model is consistent with epicenter distribution of earthquakes and geology in this region.