1994 Fiscal Year Final Research Report Summary
The mechanism of the Iriomote earthquakes swarms
| Project/Area Number |
05452377
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
Natural disaster science
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| Research Institution | KYOTO UNIVERSITY |
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Principal Investigator |
ANDO Masataka Kyoto Univ.DPRI.Prof., 防災研究所, 教授 (80027292)
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| Co-Investigator(Kenkyū-buntansha) |
BABA Hisatoshi Tokai Univ.IRD Research Associate, 沖縄地域研究センター, 助手 (80256180)
OHKURA Takahiro Kyoto Univ.CLAS Research Associate, 総合人間学部, 助手 (40233077)
KATAO Hiroshi Kyoto Univ.DPRI Research Associate, 防災研究所, 助手 (80221878)
OHSHIMAN Naoto Kyoto Univ.DPRI Associate Prof., 防災研究所, 助教授 (70185255)
ITO Kiyoshi Kyoto Univ.DPRI Associate Prof., 防災研究所, 助教授 (80022721)
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| Project Period (FY) |
1993 – 1994
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| Keywords | Iriomote / Earthquake swarm / Normal fault / Extensional stress / Taiwan / Back arc / Geothermal actin'ty / Ryukyu trench |
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| Research Abstract |
The Iriomote swarm activity which started suddenly in November, 1991 had been intermittently occurred in the northern part of the island. The largest event of M5.2 happened in November, 1993. Since then the activity had decreased gradually. Nevertheless events of M>3 sometimes had occurred, sugeesting the swarm holding still its activity. From this reason, the present program was initiated in the last fiscal year. In the last year installation of instruments and the examination of the data thus obtained were made.
Events of the earthquake swarm is distributed like a plane, with a dimension of 20 km x 10 km, which strikes N35W and dips to the southwest with an angle of 70 deg.The focal mechanism of relatively large events M>3.5 have dominantly normal fault component that implies the earthquake swarm have been manifested under tensional stress.
This year have conducted observations and analyzes of the data on the swarm activity. We conclude the process of the earthquake swarm beneath the island as follows : Under the NEE-SWW tensional stress field water (liquid phase) intruded from the upper mantle causing an increase of pore pressure leading to the earthquake swarm activity. A more detailed geochemical study is necessary on the origin of the water although a chemical analysis has implied the upper mantle as an supply source of the water. The plane-like distribution of earthquakes on which the micro-earthquakes are concentrated during the swarm activity is a basically fault plane. The dip and strike of the plane are consistent with the regional pattern of faults throughout the southern Ryukyu trench and the mechanism of several M6 class earthquakes near Taiwan.
In conclusion the tensional stress field caused the Iriomote swarm activity and then the medium-size earthquakes near Taiwan. With the occurrence of the series of Taiwan events the activity completed and died out to the normal level.
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