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

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

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
武村 雅之 小山 順二
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.36, no.3, pp.323-336, 1983-09-25 (Released:2010-03-11)
参考文献数
37
被引用文献数
1

Classification of low-frequency earthquakes has been made quantitatively by using a diagram of seismic-moment factor Me versus characteristic period Tc. Tc and Me correspond to period of corner frequency and seismic-moment density at Tc of each earthquake. About 3, 000 earthquakes from 1926 to 1978 along the Kurile, Japan, and Ryukyu trenches have been analyzed and MS's of those earthquakes cover the range from 3 to 8.3. Most of earthquakes beneath the inner trench slopes have been classified into low-frequency events which show large Tc for the same Me, while ordinary earthquakes have been commonly found in the frontal arc regions. Relations among magnitudes and seismic moment: MS-MJMA, MS-mb, and MS-Mo, for low-frequency events are also different from those for ordinary events. The relations, therefore, cannot be explained by a scaling model for ordinary earthquakes. A scaling model has been derived for low-frequency earthquakes in a statistical manner, taking into account constraints based on the relations among magnitudes and seismic-moment mentioned above. Corner frequencies of source spectra of the low-frequency model are always one half of those of the ordinary model with the same seismic-moment. This model has been also justified by the data of so called tsunami earthquakes in other subduction regions in the world, suggesting a similarity relation among destructive tsunami earthquakes and low-frequency earthquakes with small magnitude.
著者
小山 順二
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.78, pp.53-68, 2015-03-19

After the 2011 Tohoku-oki megathrust earthquake of Mw 9.0, one of the great concern is whether or not the megathrust earthquake induces large volcanic eruptions and disastrous aftershocks in Japan. There exist 110 active volcanoes in Japan and large eruptions including historical events of each volcano have been compiled in terms of Volcanic Explosivity Index (VEI; Newhall and Self, 1982) from 0 (Non-explosive) to 8 (Mega-colossal). According to “Volcanoes of the World” by Siebert et al. (2010), we could find 50 eruptions in Japan since 600 A.D. classified as VEI 4 (Large) and 5 ~ (Very Large). Speaking about great earthquakes whose earthquake (moment) magnitude M (Mw) is equal to or larger than 8.0, there occurred 34 times since 684 A.D. in Japan. I have compared the time of occurrences of Large volcanic eruptions and large earthquakes (M>7.5) at first, taking examples of Mt. Fuji, Tokachidake, Hokkaido Komagatake and Sakurajima volcanoes. Since I could not find any systematic correlation between them, I decided to compare all the Large (and Very Large) volcanic eruptions and the great earthquakes (M>8.0) in Japan. In order to develop the quantitative discussion, we need to introduce cross-correlogram analysis (Perkel et al., 1967; Grun, 2009) for these two discrete point-processes. All that I could find was very poor correlation between great earthquakes and Large volcanic eruptions in Japan. It is true that many papers have been showing us the correlation between large earthquakes and volcanic eruptions. However, speaking about Large eruptions (not speaking about volcanic disasters), it is not true for the correlation between them. The reason is that some small-scale volcanic activity out of many volcanoes has been activated by great earthquakes almost all the time in the world. Even just after the 2011 Tohoku-oki megathrust, it has been reported that the volcanic activity of about 20 volcanoes in Japan has been enhanced. It is also true that some volcanoes are activated in the present, about 4 years after the megathrust. The evidences should be added, which are the eruptions of Mt. Fuji in 1707 (VEI 5), 49 days after the 1707 Hoei earthquake of M 8.6 and of Bezymianny in Kamchatka in 1956 (VEI 5), 4 years after the 1952 Kamchatka earthquake of Mw 9.2. Both of the volcanoes have spent about 900 ~ 1200 years of quiescence in the volcanic activity. All these would suggest that great earthquakes might induce Large volcanic eruptions nearby, however it is totally dependent on its own potential and preparedness to erupt of each volcano and the earthquakes just play a role of a trigger.
著者
小山 順二 武村 雅之 鈴木 次郎
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.33, no.2, pp.187-198, 1980-06-25 (Released:2010-03-11)
参考文献数
21

A simple method is developed to determine seismic moments of earthquakes by using tabulated data in usual seismological bulletins. The method is qualified through the criteria such as simplicity of calculations, coverage of wide magnitude range, and insensitivity of the instrumental response: At first, characteristic period Tc of each earthquake is defined as the average value of apparent periods of wavelets which give maximum amplitudes of ground displacement at epicentral distances between 200 and 700km. Secondly, amplitude information is taken into consideration, making a product of maximum amplitude, its period and epicentral distance. Seismic-moment factor Me for a given earthquake is defined at the characteristic period Tc as the average value of those products evaluated from horizontal components at stations within epicentral distance range from 200 to 400km. The narrow range of epicentral distance in evaluating Me is taken so as to reduce the uncertainty due to seismic-energy attenuation into a permissible range and to be able to obtain equal number of observations for small earthquakes to that for large ones. The relation between the seismic-moment factors and characteristic periods for 163 intraplate earthquakes in Japan from 1926 to 1977 clearly demonstrates that Me is proportional to the cube of Tc. A scaling model of earthquakes that satisfies the empirical relations among surface-wave magnitude, JMA magnitude, and body-wave magnitude facilitates the estimate of static seismic-moments from calculated Me's. The seismic moments of 16 earthquakes determined by conventional analyses from near- and/or far-field observations are consistent with static seismic-moments thus estimated. This shows the potential in practice of the present method especially in routine processing of seismic data.
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

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.