| Co-Investigator(Kenkyū-buntansha) |
NAKAHARA Hisashi Graduate School of Science, Tohoku University, Research Associate, 大学院・理学研究科, 助手 (20302078)
NISHIMURA Takeshi Graduate School of Science, Tohoku University, Research Associate, 大学院・理学研究科, 助手 (40222187)
SATO Haruo Graduate School of Science, Tohoku University, Professor, 大学院・理学研究科, 教授 (80225987)
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| Research Abstract |
We studied the remote triggering mechanism of micro-earthquakes based on analyses of the seismic data obtained at geothermal areas. We started a high resolution seismic observation at the Kakkonda geothermal area, Iwate Prefecture, Japan, in May 1998 by using a bore-hole seismometer. However, the Kakkonda area were severely damaged by the M6.1 earthquake on September 3, 1998, and we could not continue the observation after the earthquake. In this study, therefore, we analyzed : (1) continuous records of a broad-band seismometer that was installed 5 km east of the Kakkonda area, and (2) past records of bore-hole seismometer at Kakkonda that were recorded in 1986-1990, July 1993 and December 1994, (3) remote triggering phenomena reported in the world. We further calculated theoretical dynamic strain, static strain and maximum acceleration associated with a large earthquake. Based on these observations and theoretical studies, we obtained the following results :
1. From the observed relations between seismic magnitude and epicentral distance of the earthquakes that remotely triggered micro-earthquakes, we find that dynamic strain associated with a large earthquake plays a main role to trigger micro-earthquakes, and that dynamic strain of more than 10ィイD1-7ィエD1 is necessary to remotely trigger micro-earthquakes.
2. The earthquake swarms are remotely triggered 0.1-1 hour after the maximum dynamic strain due to a large earthquake.
3. Remote triggering phenomena are often observed at geothermal and volcanic areas. This suggests that an increase of pore pressure of magnetic fluid and hot water in the crust may play an important role in the remote triggering mechanism.
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