Elastic wave velocity and Poisson's ratio of shallow crustal seismogenic zone

点击翻译按钮获取中文摘要

The recent development in seismic tomographic study has demonstrated presences of seismic low velocity anomalies within crust and upper mantle beneath various areas of island arcs. In addition, theoretical studies have suggested significant negative effects of melt/fluid on Vp and Vs as a function of melt/fluid fraction and its geometric characteristics. These studies collectively suggest a presence of melt/fluid phase in such areas defined by low velocity anomalies. The seismic tomographic studies also suggested that some areas with low velocity anomalies are spatially associated with seismogenic zones observed at relatively shallower level of arc crust.To evaluate the effect of melt/fluid on seismic velocities and Poisson's ratio of rocks under high temperatures and pressures comparable to the middle to lower crustal conditions of volcanic arc crust, we developed the new experimental method that enable us to make simultaneous measurements of Vp and Vs in rocks at high pressures (up t … More o 900℃) and temperatures (up to 1 GPa). Using this new experimental method, we measured Vp and Vs of various lower crustal and upper mantle rocks including amphibolite, gabbro, peridotite, and tonalite, which were collected from Tanzawa, Ichino-megata, Takashima, and Kohistan. Our data demonstrate that Vp and Vs significantly decrease with elevating temperature and highly depend on melt/fluid proportion in the rock sample. The data also suggest that phase the transformation of high-low plagioclase is an additional factor, which significantly controls seismic velocity of crustal rocks at middle to lower crustal levels.Employing the present experimental data and the seismic tomographic images, we constructed petrological models for the northern part of Izu arc crust and the northeastern Honshu arc crust. Our data suggest that the 〜6 km Vp middle crustal layer defined for the Izu arc is composed of hornblende-hearing felsic rock (tonalite). The data also suggest that the lower crust of the northeastern Honshu arc consists of hornblende-bearing gabbro. The area of low velocity anomalies defined by the seismic tomographic studies are interpreted as a zone having considerable amounts of melt phase. Less