著者

草野 富二雄 浜田 信生

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.44, no.4, pp.305-313, 1991-12-24 (Released:2010-03-11)

参考文献数

34

被引用文献数

2

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The focal process of the 1964 Niigata Earthquake was reinvestigated on the basis of hypocentral distribution of its aftershocks. This study indicates that the aftershocks are distributed on a fault plane dipping westward.Although it has been clear that the fault strike of the mainshock was in N20°E direction, the dip of the fault was not still clear due to a poor resolution of hypocenter of aftershocks. To resolve the difficulty, we reexamined seismological data obtained by the Japan Meteorological Agency (JMA).Reexamination of seismograms of nearby stations enabled us to supplement more than 1200 new P and S arrivals of aftershocks. We also dentified a number of P and S arrivals from the data which were previously reported as unidentified phases. The Joint Hypocenter Determination method was used to get a more reliable aftershocks distribution. The number of located aftershocks much increased, as about 380 aftershocks are well located by this study.Aftershocks on the vertical cross section which is normal to the fault strike shows that aftershocks are on a westward dipping plane. The dip of the plane is estimated as 50 degrees which is consistent with the focal mechanisms reported by several studies. Although the dip angle depends on the velocity model used in hypocenter location, westward dipping of aftershocks is valid, independent of several different velocity models. Therefore we estimate that the subduction of the Japan Sea under the north-east Honshu does not occur in the southern part of the eastern margin of the Japan Sea.The aftershock activity is found to be low around the hypocenter of mainshock which is located near the bottom of aftershock region, suggesting a large strain release around the nucleation point of mainshock. Relative position of forerunning seismic activity which preceded the mainshock by two years seems to be within the shallow part of the aftershock region east of Awashima-island which is located in the western middle of the focal region. The epicentral distribution of aftershocks indicates that aftershock occurrence is scarce around Awashima-island. A similar relation was reported in the case of the 1983 Nihonkai-chubu earthquake between its aftershocks and Kyurokujima-island, which is situated east of the middle of the aftershock region. Few aftershocks occurred in the area around Kyurokujima-island. In spite of the difference in relative location, that is, Awashima is situated west of the aftershock region while Kyurokujima is in the east, this suggests possibilities that crust around the islands cannot sustain enough strain to generate aftershocks or it behave as an earthquake barrier.

著者

草野 富二雄 吉田 明夫

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.43, no.4, pp.459-469, 1990-12-24 (Released:2010-03-11)

参考文献数

20

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Aftershock activity of the 1987 Chiba-ken Toho-oki earthquake (M6.7) is investigated using JMA data. It is found that aftershocks during two weeks just after the main shock occurred mostly in a region to the east side of the fault plane which is nearly north-south direction with a steep dip to the east. However, aftershock activity in the area to the west side of the fault plane became high since the beginning of January 1988. The contrast between spatial distribution of aftershocks in December 1987 and that after January 1988 is conspicuous. The later activity was concentrated to a rather small area and the largest aftershock occurred on 16 January in the active region. The mechanism of the largest aftershock was reverse type in contrast to the mechanism of the main shock which was strike slip type. Further, pattern of temporal decrease of aftershock activity deviated notably from the Omori's formula when the later activity was started. All these characteristics suggest that most earthquakes which occurred in the region to the west of the fault plane of the main shock after January 1988 are not the so-called aftershocks in a narrow sense, but that they represent an appearance of a new fracture, which occurrence might be caused by the stress concentration due to the fault motion of the main shock. The phenomenon that aftershock activity in the either one side against a fault plane is higher than that in the other side is frequently observed, even for fault motions of strike slip type. It is interesting to note that seismicity before the main shock was also asymmetrical, i. e. it was active in the region to the west of the fault plane of the 1987 earthquake. The seismicity in the recent one year also seems to be active in the west region. These features may show that the western block to the fault plane has taken a positive part in the accumulation process of stress in the focal region of the 1987 Chiba-ken Toho-oki earthquake.