著者

吉田 明夫 古屋 逸夫

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.45, no.1, pp.71-82, 1992-03-24 (Released:2010-03-11)

参考文献数

24

被引用文献数

1 1

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Pattern of occurrence of precursory phenomena differs significantly in various respects for each earthquake. It is not the same even for earthquakes occurring in adjacent areas. Amplitude of precursory change is not necessarily large at nearer sites, and that it is very often larger than that of coseismic change. These features suggest that precursors occur locally at weak points or stress concentrating sites. It seems that desrimination of earthquake precursors from noises which are not directly related to the occurrence of earthquake is intrinsically difficult. Whether some observed change is a precursor or not may be expressed in a probabilistic way. When various kinds of anomalous change are observed at the same time, the possibility that they are true precursors will be increased. It is important to elucidate the occurrence mechanism of precursors in relation with structure and its change of seismogenic field.

著者

細野 耕司 西 政樹 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.59, no.1, pp.29-37, 2006-08-31 (Released:2013-08-05)

参考文献数

22

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We investigated changes in the depth distribution of aftershocks of three large inland earthquakes in Japan, the 2000 western Tottori Prefecture Earthquake of M 7.3, the 2003 northern Miyagi Prefecture earthquake of M 6.4, and the 2004 mid Niigata Prefecture earthquake of M 6.8. For all of them we found that the seismogenic layer extended to the deeper and shallower zones after occurrence of the main shock. The shallow and deep activities decayed faster than the activity in the intermediate depth. This suggests that when stress is built up very fast, brittle fractures can occur in the zone where increase of stress is usually relaxed through a ductile process. Our results also show that a fault motion could extend to the outside of the seismogenic zone estimated from the background seismicity, which should be taken into consideration in the hazard assessment.

著者

吉田 明夫 高山 博之

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.103, no.6, pp.696-705, 1994-12-10 (Released:2009-11-12)

参考文献数

13

被引用文献数

1 1

---

We show that seismic activity decreased clearly in a wide area before the 1953 Boso-oki earthquake and the 1972 Hechijojima-toho-oki earthquake. In both cases the seismic quiescence extended to the Tokyo Metropolitan area. Marking precursory activity appeared several years before the Boso-oki earthquake in the Chiba prefecture and in the sea region off-Ibaraki prefecture. It is also noted that the seismicity in the Kanto region as well as in the sea region south off-Boso Peninsula decreased conspicuously after the Boso-oki earthquake. On the contrary an increase of the seismicity was observed after the Hachijojima-toho-oki earthquake, although deep seismic activities decreased remarkably. We show that seismic quiescences preceding the Boso-oki earthquake and the Hachijojima-toho-oki earthquake were detectable before their occurrences, and no other statistically significant quiescence has appeared in the south off-Kanto region since 1926. It is proposed that the occurrence of a large earthquake may be predicted by monitoring changes of the seismicity in a wide area.

著者

吉田 明夫 伊藤 秀美 細野 耕司

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.105, no.1, pp.77-87, 1996-02-25 (Released:2009-11-12)

参考文献数

22

被引用文献数

2 2

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Several examples are presented which seismic quiescence appeared in a wide area around the foci of intraplate earthquakes before their occurrence. A noteworthy feature is that the quiescence area extends along tectonic zones over more than several times of the focal region of the main shock. This phenomenon suggests that stress in a wide area along tectonic zones changes before the occurrence of major intraplate earthquake. We consider that the stress change is a manifestation of a significant physical process leading to earthquake occurrence. An apparently puzzling fact which earthquake precursors are sometimes observed at distant places from the focal region in the specific direction may be explained by such stress change. Further, it is expected that the precursory seismic quiescence provides us with a promising clue to the short-term prediction of intraplate earthquake.

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.

著者

小林 昭夫 吉田 明夫

出版者

日本測地学会

雑誌

測地学会誌 (ISSN:00380830)

巻号頁・発行日

vol.50, no.1, pp.39-42, 2004-03-25 (Released:2010-09-07)

参考文献数

11

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We investigated crustal deformation after the 1946 Nankai earthquake using tide gauge records. Although resolution of tide gauge records is not so high for seeing the temporal change in detail, vertical crustal movements whose relaxation times are between several months to years can be detected by them. It was reported that there had been a postseismic crustal deformation with a time constant of 4-5 months in southern Kui Peninsula (Kobayashi et al., 2002). In this paper we show that crustal deformations with a longer relaxation time of a few years were observed in northern Shikoku, examining tide gauge records at such stations as Takamatsu, Sumoto and Kobe. These observations suggest that there was a time lag in the development of postseismic slip between shallow and deeper parts on the plate interface after the Nankai earthquake.

著者

小林 昭夫 真砂 礼宏 吉田 明夫

出版者

日本測地学会

雑誌

測地学会誌 (ISSN:00380830)

巻号頁・発行日

vol.48, no.1, pp.1-12, 2002-03-25 (Released:2011-03-01)

参考文献数

28

被引用文献数

1

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We examined tide gauge records at stations in and around the Kui peninsula to find if a precursory crustal deformation occurred before the 1946 Nankai earthquake. First, we removed effects of atmospheric pressure and astronomical tide on the records. Then, we tried to get rid of the effects of the oceanic current by taking the difference of the records betweentwo stations. After these corrections we found that sea level at Uragami where is located on the southeastern coast of the Kui peninsula had risen about 10 cm just before the Nankai earthquake. The sea level change, if we consider that to represent real subsidence of land near Uragami, could be explained by supposing that a slow fault slip corresponding to moment magnitude 6.9 occurred on the plate boundary in the offing of southeast of the Kui peninsula near the hypocenter of the Nankai earthquake.