2000 Fiscal Year Final Research Report Summary
Intermediate-Depth Earthquakes and Seismotectonics beneath southern Kyushu, Japan
| Project/Area Number |
11640418
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
固体地球物理学
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| Research Institution | Kagoshima University |
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Principal Investigator |
GOTO Kazuhiko Faculty of Science, Kagoshima Univ. Associate Professor, 理学部, 助教授 (20244220)
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| Project Period (FY) |
1999 – 2000
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| Keywords | Wadati-Benioff zone / southern Kyushu / intermediate-depth earthquake / Philippine-Sea plate / island arc / focal mechanism solution / discontinuity of seismic zone |
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| Research Abstract |
Seismic activities and focal mechanism solutions of intermediate-depth earthquakes and the structure of Wadati-Benioff zone beneath southern Kyushu, Japan are investigated based on the high quality data observed by the micro-seismic network of Kagoshima University and Kyushu University. It is revealed that the Wadati-Benioff zone shows the complicated structure. The most characteristic feature is that the seismic zone slides horizontally, that is, the seismic zone in NNE region of Sakurajima volcano shifts to WNW by about 10km compared to that in SSW region. Sliding occurs gradually along about 50km extent at the depth of 130km and about 80km at the depth of 70km. It is interesting that the extreme high seismic activity at about 160km in depth beneath Kaimon volcano, which is one of the characteristic features in this region, occurs at the rim of the sliding region. The section of seismic zone shows to be continued to the depth of about 150-200km with changing its curvature. In detail, it has the wavelike feature, that is, it is convex upward at about 65km in depth and downward at the depths of about 50km and 100km. Focal mechanism solution indicates that the principal axes of tensional stresses are parallel to and those of compressional ones are perpendicular to the descending direction of slab in the vertical plane. However, in the horizontal plane, the axes of compressional stresses are aligned to the converging direction of oceanic plate. From these facts, it is considered that some force acts to the descending Philippine-Sea plate beneath southern Kyushu adding to the force produced by the descending process as seen in almost all island arc. And such force works the complicated structure of seismic zone and the generation of non-down-dip mechanism of intermediate-depth events.
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