著者

阿部 邦昭 岡田 正実

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.46, no.1, pp.25-34, 1993-06-24 (Released:2010-03-11)

参考文献数

9

被引用文献数

1 1

---

We succeeded in explaining a small tsunami on a vertical displacement field of a pure strike-slip fault. The Izu-Oshima Kinkai Earthquake (M=6.5) of February 20, 1990, being accompanied with a small tsunami, was modeled as a pure strike-slip fault on a focal mechanism solution and aftershock area. The sinistral strike slip fault with a plane of 15×12km2, dip of almost 90° and strike of N0°E was assumed at the epicenter.A numerical experiment on the vertical displacement field reproduced the observed tsunami fairly well. The computation was carried out on a linear framework using a mesh of 1km, time step of 1 sec and an additional fine mesh of 0.25km in the vicinity of tide stations. The assumed dislocation of 1m explained the observed amplitude at the nearest tide station. Comparison of waveforms between model and four observations supports the model.Furthermore, a spectral analysis was attempted to reveal predominant frequencies of the observed tsunami. The observed predominant frequencies were explained from wave lengthes characteristic to the pure stlike-slip fault. A strong azimuth dependence is one of the characteristic properties of the pure strike slip tsunami. An unexplainable predominant frequency was attributed to the reflected wave from the Izu Oshima Island.

著者

岡田 正実

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.35, no.1, pp.53-64, 1982-03-25 (Released:2010-03-11)

参考文献数

26

被引用文献数

1 3

---

Monthly distribution of major shallow earthquakes (M≥7.5) in and near Japan is investigated using earthquake catalogs including pre-instrumental data. Major shallow earthquakes off Hokkaido and Sanriku districts are concentrated in spring, and those along the coast from Miyagi prefecture through Shikoku district occurred more frequently from August to December, as Mogi has pointed out. We suggest with some consideration that these variations are statistically significant at a 95% confidence level. In the other region the records on major earthquakes are not enough to discuss the seasonal variation. But it is able to show that large earthquakes with magnitude 7.0 or over occurred more frequently from March to September in the inland regions where no effect of the major earthquakes on the Pacific coast is expected. It seems that the decrease in the weight of water on land and lowering of the mean sea level trigger the occurrence of major earthquake between the oceanic and continental plates, when the strain accumulates closely to the limit. It is also considered that the increase of ground water in the fault zone has an influence on the occurrence of large shocks inland in their critical states.

著者

溜渕 功史 山田 安之 石垣 祐三 高木 康伸 中村 雅基 前田 憲二 岡田 正実

出版者

SEISMOLOGICAL SOCIETY OF JAPAN

雑誌

地震 第1輯 (ISSN:00371114)

巻号頁・発行日

vol.62, no.4, pp.193-207, 2010

---

We found eight <I>M</I> 5.1 characteristic earthquakes regularly occurring since 1966 on the plate boundary between the Eurasian plate and the Philippine Sea plate near Miyakojima Island, the Ryukyu Arc, Japan. The quake recurrence interval was 5.89 years in average, and the standard deviation was only 0.73 years. The accumulating stress presumably ruptured the same asperity enclosed by the creeping zone repeatedly. Also, we found three other groups of small repeating earthquakes of <I>M</I> 4, which occurred close to the hypocenters of the <I>M</I> 5 events. Those groups also occurred regularly and we can consider them to be 'characteristic' earthquake sequences. Now, we called those groups A, B, and C. It is not clear whether groups A and B had an intrinsic recurrence interval or if they influenced each other. However, two events of group C occurred within one week after the <I>M</I> 5 quakes, indicating that the <I>M</I> 5 events triggered the group C events whose asperity had suffcient strain energy. No earthquake exceeding <I>M</I> 7, which could change the recurrence intervals, has been observed on the subduction zone around the Ryukyu Islands. Therefore, there should be numerous characteristic earthquake sequences in other areas of the Ryukyu district. We expect that the next <I>M</I> 5 earthquake at 50 km depth on the plate boundary near Miyakojima Island will occur between September 2012 and July 2014 with 70% probability, using the small-sample theory with a log-normal distribution model. Moreover, the <I>M</I> 5 event may be accompanied by an <I>M</I> 4 quake that could rupture the asperity of group C within one week.

著者

溜渕 功史 山田 安之 石垣 祐三 高木 康伸 中村 雅基 前田 憲二 岡田 正実

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.62, no.4, pp.193-207, 2010-03-15 (Released:2012-03-26)

参考文献数

26

---

We found eight M 5.1 characteristic earthquakes regularly occurring since 1966 on the plate boundary between the Eurasian plate and the Philippine Sea plate near Miyakojima Island, the Ryukyu Arc, Japan. The quake recurrence interval was 5.89 years in average, and the standard deviation was only 0.73 years. The accumulating stress presumably ruptured the same asperity enclosed by the creeping zone repeatedly. Also, we found three other groups of small repeating earthquakes of M 4, which occurred close to the hypocenters of the M 5 events. Those groups also occurred regularly and we can consider them to be ‘characteristic’ earthquake sequences. Now, we called those groups A, B, and C. It is not clear whether groups A and B had an intrinsic recurrence interval or if they influenced each other. However, two events of group C occurred within one week after the M 5 quakes, indicating that the M 5 events triggered the group C events whose asperity had suffcient strain energy. No earthquake exceeding M 7, which could change the recurrence intervals, has been observed on the subduction zone around the Ryukyu Islands. Therefore, there should be numerous characteristic earthquake sequences in other areas of the Ryukyu district. We expect that the next M 5 earthquake at 50 km depth on the plate boundary near Miyakojima Island will occur between September 2012 and July 2014 with 70% probability, using the small-sample theory with a log-normal distribution model. Moreover, the M 5 event may be accompanied by an M 4 quake that could rupture the asperity of group C within one week.