著者

角田 寿喜 後藤 和彦

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.55, no.3, pp.317-336, 2002-12-15 (Released:2010-03-09)

参考文献数

107

被引用文献数

2

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We reviewd characteristics of seismic activities in and around Kyushu and the northern half of Nansei Shoto (Ryukyu Islands), mainly on the basis of hypocenter distributions and focal mechanisms. Interplate and intraplate earthquakes, inclusive of intermediate-depth events, considerably differ in seismicity from region to region along the arc. Most of the boundaries between the seismic regions correspond to left-lateral tectonic lines geologically inferred and/ or narrow zones of crustal earthquakes of left-lateral strike slip types across the arc. Thus we surmise that the Philippine sea plate spiitted into several segments subducts with low coupling under the Kyushu-Ryukyu arc.Not only rifting along the Okinawa trough but also southeastward spreading from the Shanshi graben with left-lateral movements is probably important for crustal earthquakes of normal and strike-slip fault types dominant in an extensional stress field in the marginal sea and inland areas.

著者

八木原 寛 角田 寿喜 後藤 和彦 清水 洋

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.47, no.1, pp.53-61, 1994-06-14 (Released:2010-03-11)

参考文献数

30

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On January 30, 1992, a shallow earthquake of magnitude 4.9 followed by about 300 aftershocks occurred in a northern area of the Koshiki channel, north-western Kagoshima Prefecture. We located seismic events observed at two stations of NOEV (Nansei-toko Observatory for Earthquakes and Volcanoes) and four stations of SEVO (Shimabara Earthquake and Volcano Observatory), using Joint hypocenter determination (JHD). Hypocenters of the mainshock and its aftershocks were nearly vertically distributed at depths from 5km to 13km in a small area.Initial motions at the seismic stations of NOEV, SEVO and FMO (Fukuoka Meteorological Observatory) suggest a focal mechanism of strike slip fault type with a T-axis of NNW-SSE direction: the mechanism is very similar to those reported for the earthquakes in and around the area. The nodal plane striking in NE-SW agrees with trends of the fault system in the channel and the other WNW-ESE plane is parallel to the earthquake alignment along Amakusanada-Izumi-Kakuto areas. Hypocenters of the event and aftershocks nearly vertically distributing are, however, not consistent with any of the planes.In March of 1991, about 10 months before the M 4.9 event, an earthquake swarm (Mmax 2.9) occurred at depths around 5km almost within the same epicentral area. Namely, two different types of earthquake sequence occurred at different depths in the same area: the swarm occupied a shallower zone than the focal zone of the M 4.9 event. Although some volcanic process may be inferred from hypocenters vertically aligning, it is probably difficult to explain the fact that the earthquake swarm at shallow depths occurred about 10 months before the M 4.9 event at a deeper depth without accompanying any notable foreshocks.

著者

角田 寿喜 後藤 和彦 宮町 宏樹 平野 舟一郎 清水 力 岩切 一宏 中辻 剛 立山 清二

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.106, no.4, pp.476-485, 1997-08-25 (Released:2009-11-12)

参考文献数

26

被引用文献数

1

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A shallow earthquake of MJMA 6.7 on October 18, 1995, at about 50km southeast off Kikai-jima in the Ryukyu Islands caused slope landslides, subsidence of concrete slabs at fishing ports and collapses of walls piled up with coral ragged stones. Several fishing boats moored to wharves were damaged by tsunami; four boats were turned over at ports in Amami-Oshima.From May 19 to 30, 1995, earthquakes of MJMA≤4.1 had occurred in the focal area after a long seismic quiescence since the 1911 Kikai-jima Earthquake of M=8.0. Some of them were felt in JMA intensity 1 or 2 at Kikai-jima. Numerous felt shocks also struck the island immediately after the main shock; 62 and 125 shocks on October 18 and 19, respectively. Numbers of aftershocks decreased rapidly, though they lasted through the end of 1996.The damage distribution of the 1911 event being compared with that of the 1995 event, their focal areas were considered to overlap with each other. It was, however, possibly an interplate earthquake of a thrust fault type in contrast with the 1995 event of a normal fault type in the subducting plate, because the first motions of P wave at Naze and of tsunami wave at Kikai-jima were reversed between the two events.

著者

八木原 寛 角田 寿喜 宮町 宏樹 後藤 和彦 平野 舟一郎 日野 亮太 金澤 敏彦 海宝 由佳 塩原 肇 渡邊 智毅 望月 将志 根本 泰雄 島村 英紀

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.49, no.2, pp.227-239, 1996-08-23 (Released:2010-03-11)

参考文献数

23

被引用文献数

1 1

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We investigated seismic activity around Tokara Channel north off Amami Oshima, Nansei Syoto (Ryukyu) Islands of western Japan, using 12 ocean-bottom seismographs (OBS), as well as two temporal stations at Yaku Shima and Amami Oshima islands, operated from April 16 to May 10, 1992. One-dimensional velocity structure and station corrections were inverted from P and S times of 51 events provisionally well-located in the OBS network. We then relocated precisely 239 events in the studied region, using the inverted velocity structure and station corrections.Seismicity was highest in an area of about 10km×10km near the trench axis northeast off the OBS network: the largest event of MJMA 5.6 and other 40 events (probably aftershocks) were located at shallow depths. A mechanism solution of normal fault type with a T-axis of NW direction for the largest event was concordant with bending process of the Philippine sea plate. On the other hand, 18 events at depths of about 30km in a small area north of the OBS network were presumably due to interplate thrusting, because a composite mechanism solution for three events was of reverse fault type with a P-axis of ESE direction. A cluster of 17 events at depths from 10km to 25km was found in a southwest area of the network. These shallow events were probably crustal earthquakes within the Eurasian plate.We found an area of very low seismicity in the southeast of the network during the period studied. It is also identified at the nearly same location in the epicenter distribution from 1984 through 1991 obtained by Japanese Meteorological Agency (JMA) and possibly corresponds to the aftershock area of the 1911 Kikaijima Earthquake (M 8.0).Although we could not confirm any discernible alignments of shallow earthquakes along the Tokara Channel which is a notable tectonic line, the dipping angle of the intermediate-deep seismic zone changes discontinuously from about 65° on the north of the channel to about 40° on the south.