2006 Fiscal Year Final Research Report Summary
Elastic wave velocity and Poisson's ratio of shallow crustal seismogenic zone
| Project/Area Number |
16340167
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Petrology/Mineralogy/Science of ore deposit
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| Research Institution | Yokohama National University |
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Principal Investigator |
ARIMA Makoto Yokohama National University, Graduate School of Environment and Information Sciences, Professor, 大学院環境情報研究院, 教授 (10184293)
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| Co-Investigator(Kenkyū-buntansha) |
ISHIKAWA Masahiro Yokohama National University, Graduate School of Environment and Information Sciences, Associate Professor, 大学院環境情報研究院, 助教授 (70232270)
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| Project Period (FY) |
2004 – 2006
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| Keywords | Elastic wave velocity / Poisson's ratio / Seismogenic zone / Crust structure / High temperature and pressure |
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| Research Abstract |
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
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