地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
1978年伊豆大島近海地震の震源過程
長宗 留男
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ジャーナル フリー

1980 年 33 巻 1 号 p. 71-78

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The epicentral distribution of aftershocks shows that the source region of the Izu-Oshima-Kinkai Earthquake of January 14, 1978 (origin time: 12h 24m 38.6s, 34°46′N, 139°15′E, depth: 0km), consisted of two areas, i. e., the area in the sea between Izu-Oshima and the Izu-Peninsula, spreading to the west from the epicenter of the main shock, and the one in the Izu-Peninsula, extending to the westnorthwest direction from the east coast of the peninsula.
On the other hand, it has been pointed out that the earthquake was a multiple shock, and that the second event took place about 6sec after the occurrence of the main shock (first event). The location of the second event, however, was somewhat undefinitely.
The records of the JMA 59-type electromagnetic seismographs were examined for locating the origin of the second event.
From the readings of the first arrivals of P-waves, the origin for the second event was estimated as follows:
Origin time: 12h 24m 44.5s (5.9sec after the occurrence of the main shock),
Location: 34°46′N, 139°00′E (approximately 23km west of the epicenter of thee main shock),
Depth: 0km.
It appears that the second event occurred at a place linking two source areas mentioned above, when the rupture which started at the origin of the main shock and propagated westwards, deviated its direction of propagation to the westnorthwest.

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