著者

小山 順二 都筑 基博 蓬田 清 吉澤 和範

出版者

北海道大学大学院理学研究院

雑誌

北海道大学地球物理学研究報告 (ISSN:04393503)

巻号頁・発行日

vol.76, pp.129-146, 2013-03-19

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2011 年3 月11 日マグニチュード9.0 の超巨大地震が東北地方太平洋沿岸をおそった.この地震は過去千年以上にわたる日本付近で発生したどの地震よりも大きな津波を励起し,地震動災害ばかりではなく歴史に残る甚大な津波災害を発生させた.従来,このような超巨大地震が日本付近で発生することは,地震学的に想定されてこなかった.我々は,この超巨大地震の発生を考えるうえで,今まで見過ごされてきた超巨大地震の発生場には二つの異なった特徴があることに気が付いた.それはAlong-dip Double Segmentation(ADDS)とAlong-strike Single Segmentation(ASSS)という異なった地震活動である.我々はこの考えに基づき,世界中で発生した超巨大地震を調べなおし,超巨大地震の発生場を,地震活動の特徴(ADDS/ASSS),地震メカニズム,破壊様式,沈み込み帯の形状,上盤プレートの性質や背弧海盆の活動といった性質から,明らかにする.

著者

小山 順二 都筑 基博 蓬田 清

出版者

北海道大学大学院理学研究院自然史科学部門(地球物理学)

雑誌

北海道大学地球物理学研究報告 (ISSN:04393503)

巻号頁・発行日

vol.75, pp.161-174, 2012-03-19

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Since the 2011 Tohoku-oki megathrust earthquake, Japan, it has been recognized that there is a variety of megathrust earthquakes occurring in the world not just only the Chilean type megathrust earthquake. In the variation, one end member is the 1960 Chile earthquake and the other is the 2004 Sumatra-Andaman earthquake, while the former is characterized by subduction zone of a young plate forming the Cordilleran orogeny, the latter is by an obliquely subducting plate along a continental margin with active back-arc activity. We study in detail megathrust earthquakes along such oblique subduction zones, considering characteristics of earthquake activities, focal mechanisms, rupture patterns, geometry of subduction zones, types of overriding plates and back-arc activities. Discussions are further made on one of the oblique subduction zones near Japan Islands, the Sagami Trough, in order to derive some information and the possibility of future large earthquakes there from the seismological data at hand. We found that there is a variety of large earthquakes in the oblique subduction zones in the world. Since we have no hand to suspect the future activity of a particular subduction zone, comparative studies on seismic activities in different oblique subduction zones are inevitable.

著者

小山 順二 都筑 基博

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.66, no.4, pp.83-95, 2014-03-25 (Released:2014-05-20)

参考文献数

59

被引用文献数

1 1

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After the 2011 Tohoku-oki megathrust earthquake of Mw 9.0, we observed tens of thousands of inland and volcanic earthquakes all over the Japan Islands, which are not confined within the aftershock area of off the Pacific coast of Tohoku region. This reminds us to evaluate the earthquake activity in a much wider sense, discarding an ordinary idea of the foreshock-mainshock-aftershock activity within a limited aftershock area of a particular earthquake. There occurred several megathrust earthquakes worldwide in the last one hundred years. We have studied their significant events before and after the earthquakes based on our new hypothesis on the earthquake generating zone with the distinct difference between Along-dip Double Segmentation (ADDS) and Along-strike Single Segmentation (ASSS). In summary, some of significant aftershocks (larger than Mw 7.5) of ADDS megathrusts are those (Type I) in and near outer-rises, where some are dip-slip normal faultings and some are strike-slip faultings within subducting oceanic plates with component of normal fault. These outer-rise earthquakes are considered to be controlled by the slab-pull of descending slabs of oceanic plates induced by the reduction of plate couplings by megathrusts. In addition, some are also significant (Type II) along the plate interfaces within aftershock areas and those extending aftershock areas similarly to the corresponding megathrusts. On the other hand, aftershocks of ASSS occurred in and near corresponding aftershock areas and significant and or disastrous outer-rise events are rare. Some large (not significant) inland earthquakes are also known for both ADDS and ASSS megathrusts. All these results are not dependent on whether the subduction geometry is oblique or orthogonal.

著者

小山 順二 都筑 基博

出版者

北海道大学大学院理学研究院

雑誌

北海道大学地球物理学研究報告 (ISSN:04393503)

巻号頁・発行日

vol.76, pp.87-96, 2013-03-19

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After the 2011 Tohoku-oki megathrust earthquake, we have proposed a hypothesis on the earthquake activity characterizing the distribution of earthquakes into Along-dip Double Segmentation (ADDS) and Along-strike Single Segmentation (ASSS): The apparent absence of earthquakes in the trench-ward segments as opposed to the Japan Island-ward segments that generate repeated smaller earthquakes (ADDS), where the 2011 Tohoku-oki megathrust occurred. In the mean time, a young and buoyant plate is subducting rapidly under the overriding plate where there is weak seismic activity before the main event all over the plate interface of the subduction zone (ASSS). The 1960 and 2010 Chile megathrusts occurred in ASSS. In and near Japan, ADDS earthquake activity is restrictively found along the Pacific side of Hokkaido and Tohoku regions and the Hyuganada, Kyushu. The rest of seismic activity near Japan is classified into ASSS. We found a distinct difference in tsunami excitations between earthquakes in ADDS and ASSS, comparing tsunami magnitude m from local tsunami heights and seismic moment Mo from long-period surface-waves. Tsunami wave heights of ASSS earthquakes are almost two times larger than those of ADDS's. This is also confirmed by studying tsunami magnitude Mt calculated from teleseismic tsunami wave heights. The reason of this different excitation of tsunamis is also considered.