1988 Fiscal Year Final Research Report Summary
Determination of depth and fine structure of the upper mantle discontinuities
| Project/Area Number |
62540285
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| Research Category |
Grant-in-Aid for General Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Research Field |
固体地球物理学
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| Research Institution | Hokkaido University |
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Principal Investigator |
NAKANISHI Ichiro Hokkaido University Fac.of Science, 理学部, 助教授 (10164229)
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| Project Period (FY) |
1987 – 1988
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| Keywords | Upper mantle / reflected wave / body wave / deep earthquake / transition zone / phase transition / seismograph array / ビームフォーミング |
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| Research Abstract |
1. The travel time anomalies of teleseismic P waves are inverted to three dimensional P velocity model for the crust and uppermost mantle beneath the Hokkaido region. It is shown that a low velocity zone of 8 % velocity contrast exists under the Hidaka mountains.
2. We analyzed the precursors to P'P' waves. The P'P' waves and their precursors are analyzed by using stacking and filtering techniques. It is shown that the correction of the station anomalies is indispensable for the analysis. A good data set of the precursors is obtained from a large deep earthquake in the Fiji Islands and is analyzed in detail. The analysis shows that there exists velocity discontinuities at depths of 415 and 655 km beneath a region near the Mid-atlantic Ridge. Since this study uses digital data, it is possible to analyze shallow earthquakes and to extend the study area as compared with previous studies. No significant change (larger than 10 km) of the depth of the 650 km discontinuity is found beneath regions surrounding the Mid-Atlantic Ridge except right beneath the Ridge.
3. We analyzed the first P and later arrivals from shallow earthquakes which occurred in the Kurile and Aleutian Islands and determined the P velocity structure at depths down to 800 km beneath the northwestern margin of the Pacific Ocean. We used a trial and error method and -p inversion. The analysis shows that no low velocity zone exists in the uppermost mantle beneath this old region. The depth of the 650 km discontinuity is determined to be 700 km. This may suggest that it is deeper in the subduction region than in other regions and may provide an important implication on the destination of the subducted slab.
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