1998 Fiscal Year Final Research Report Summary
Study on Objective Detection and Generating Mechanism of Seismic Gap of the Second Kind
| Project/Area Number |
09680439
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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 |
Natural disaster science
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| Research Institution | Tohoku University |
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Principal Investigator |
MATSUZAWA Toru Tohoku University, Graduate School of Science, Associate Professor, 大学院・理学研究科, 助教授 (20190449)
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| Co-Investigator(Kenkyū-buntansha) |
KATO Naoyuki Geological Survey of Japan, Earthquake Research Department, Researcher, 環境地質部, 研究官
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| Project Period (FY) |
1997 – 1998
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| Keywords | seismic gap / seismicity / focal mechanism / stress change / aseismic slip |
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| Research Abstract |
It has been frequently reported that seismic gap of the second kind (seismic quiescence) before a large earthquake was observed. Its generating mechanism, however, is not clear yet. One of the investigators (N.Kato) proposed a model that the gap is produced by aseismic slip before an earthquake. In this study, we verified this model from seismic observation and tested the numerical model.
In order to estimate focal mechanisms of small earthquakes, we have developed a new technique called 'Master Solution Method.' Using the method, we determined so many focal mechanisms for the intermediate-depth earthquakes to find that normal-fault type events in the upper plane of the double-planed deep seismic zone had disappeared since the occurrence of the 1994 far-off-Sanriku earthquake.
We have also developed 'R value' which is a new indicator for the stress change in the descending plate beneath the northeastern Japan arc. Using R value as well as Z value which is another indicator of the seismicity change, we found that the seismic activity in the upper seismic plane became higher while the activity in the lower plane became lower for around one or two years after the major earthquakes on the plate boundary such as the 1989, 1992 and 1994 events.
From a simulation introducing inhomogeneous frictional parameters, we found that aseismic event occurred episodically in this case. Moreover, it was found that stress field in the plate was also changed episodically according to the episodic aseismic events. The simulation suggests that temporal changes in the focal mechanisms and seismicity will be indicators of the aseismic events.
All the analyses mentioned above show that the intra-plate seismicity is affected by the large inter-plate seismic and aseismic slip, suggesting that aseismic slip before an large earthquake can generate the seismic gap of the second kind.
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