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

蓬田 清

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.111-128, 2013-03-19

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After the 2011 Tohoku-Oki earthquake, the immediate threat of megathrust earthquakes in and around Japan has been suddenly advocated by some researchers. One even points out the possibility of a larger event than ever, an event of Magnitude 10. In this study, we discuss whether an event of M10 will be realistic or not, from not only statistical aspects of earthquake occurrence or macroscopic scaling laws of seismic sources in a conventional manner(e.g., fault size, the amount of slips)but also the diversity of megathrust earthquakes that was revealed after the occurrence of the 2011 Tohoku-Oki earthquake. From a simple extrapolation of seismicity and macroscopic scaling laws, one event of M10 occurs every 500 years all over the world, with its fault length, width and average slip to be 1,200 km, 600 km and 50 m, respectively. The width may not exceed 200 km very much because of the limitation of an elastic region of a plate boundary in a subduction zone. We therefore need either of (1) average slip as large as 100 m or (2) fault length of more than 1,500 km for a possible M10 event. The average slip may be able to exceed 100 m, considering an area of very large shallow slips associated with the 2011 Tohoku-Oki earthquake, but we must take care that this is possible only for an event of the along-dip double segmentation. This type of events is, however, generally adjacent to segments of weak plate coupling with small coseismic slips. In contrast, an event of the conventional along-dip single segmentation may extend its fault into many adjacent segments. A drawback in this case is that the average slip may not exceed 50 m unless there are several segments of very strong plate coupling, resulting in co-seismic slips lager than those with usual events repeating in each segnent. Although a very large strong shallow segment of the double segmentation is a candidate of am M10 event, we cannot find any clear evidence of such a region from the present spatial seismic pattern in the world. The subduction zone in south Chile is the best candidate from its apparent strong plate coupling although it may not occur for a while due to the nearly complete strain release associated with the 1960 earthquake. We consider a very large event will be impossible even if a large portion of subduction zones breaks in the Aleutian trench because of the existence of several segments of weak plate coupling with slow events or aseismic slips in this trench.

著者

一柳 昌義 笠原 稔 高橋 浩晃 岡崎 紀俊 高橋 良 大園 真子

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.77, pp.5-13, 2014-03-19

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An earthquake swarm begun at the end of January, 2012 and the activity reached a peak in middle February, then gradually decreased by the end of April, in the Nigorikawa caldera, Hokkaido. The largest earthquake was the MJMA3.6 event which occurred on 15 February 2012 with maximum intensity 3.Temporal seismic observation with two stations carried out from 20 February to 9 April in the caldera. Detailed hypocenter distribution estimated by using both data temporal and secular observations shows two clusters, one of them is aligned along the northern caldera wall and another is located at eastern outside of caldera. Strike of alignment epicenter of the north cluster shows NE-SW direction, which is in good agreement with one of the nodal plane of focal mechanism of the largest event.

著者

一柳 昌義 高橋 浩晃 山口 照寛 東 龍介 山田 卓司 大園 真子 眞城 亮成 笠原 稔 谷岡 勇市郎

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.37-51, 2015-03-19

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An earthquake swarm begun at 15 July 2012 in Nakagawa town of northern Hokkaido. The largest earthquake with MJMA4.3 occurred on 16 July 2012. We carried out temporal seismic observation with seven stations from 18 July 2012 to the last October 2012. Hypocenters were calculated using the Double-Difference hypocenter determination procedure with a local one dimensional P-wave velocity structure. Precise hypocenter data indicated that epicenters were distributed in very narrow area of 2 km×2 km with shallow depth from 4 km to 7 km. Earthquakes after middle of August occurred only in southern part of the region and depth had got shallower with time. Hypocenters indicated no clear alignment in consistent with any nodal planes of major earthquakes. An independent hypocenter cluster with shallower than 2 km was observed above the main activity area. A slow slip event (SSE) with Mw 5.4 coincidentally detected by GNSS crustal deformation data during the swarm. This swarm was situated at the southeastern end of the fault of SSE. This fact suggested that seismic swarm might triggered SSE or was induced by SSE.

著者

西田 泰典

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.15-86, 2013-03-19

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The self-potential (SP) method is based on measuring the natural electric potential differences, which generally exist between any two points on the ground. The SP fields have their origins in different mechanisms which can be used to identify ore deposits, geological features, ground water flow, and hydrothermal systems. The amplitude has a very wide spectrum, from a few mV/km to a few V/km, and their spatial distribution is correlated with the size of sources that lie at a depth within several hundred meters depth. Despite the fact that SP studies have been carried out by many researchers in the past, especially in field of mine prospecting, the use of the method had been restricted because the sources of SP anomalies were not fully identified and the development of other geophysical methods, such as seismic, electromagnetic and gravimetric methods, was very fast. However, since 1970, the SP method has again risen as a modern prospecting method for delineating the thermal state of geothermal fields. These studies have encouraged the application of the SP method to volcanological, geothermal, hydrological, seismological studies and so on. Spatial and temporal measurements of the SP field may prove to have the advantage of sensing dynamic aspects of the tectonic activities. This article will focus on fundamentals of theoretical and experimental SP studies and will show many field examples to guid for beginners.

著者

一柳 昌義 山口 照寛 高田 真秀 東 龍介 黒井 和典 山田 卓司 高橋 浩晃 前田 宜浩

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.1-13, 2013-03-19

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On December 2, 2010, MJMA4.6 shallow felt earthquake occurred beneath southeastern part of Sapporo city. Three temporal seismic stations were installed near epicentral area. Reliable hypocenter data estimated by double difference procedure with local seismic velocity structure indicated clear southeastern-dipping distribution. Fault plane from focal mechanism estimated from P-wave polarization well agreed with this hypocenter distribution. Geometry of hypocenter distribution was also well consistent with an anticline structure and possible buried active fault estimated for earthquake disaster damage assumption by local government. Recalculation of hypocenters of the 1985, 1990, and 2010 felt earthquakes under equal condition implied that epicentral locations of these three events were approximately same. These facts implied possible stress concentration in epicentral region.

著者

島倉 広至 三浦 貴生 浜根 大輔 松枝 大治 三浦 裕行

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.19-35, 2015-03-19

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The Teine mine is a epithermal Au-Ag-Cu deposit located at the west part of Sapporo, Hokkaido, Japan. More than 30 veins are distributed in the 5km2 area. So far, 48 mineral species are reported from the Teine mine. However, mineralogical study is not enough to describe the wide variety of mineral occurrence in this mine. The authors perform a descriptive study of ore samples stored at the Hokkaido University Museum and found 15 newly observed minerals from the Teine mine. Especially, cupropavonite,hodrushite,koritnigite, krautite and poubaite are first report from Japan. Poubaite, PbBi2Se2Te2, found in goldfieldite grain, is idiomorphic and less than 30 μm in diameter. It shows creamy white color and metallic luster. Bireflectance is strong. The observed empirical formula, PbBi2(Te2.0Se1.9S0.1) Σ4, is consistent with the ideal formula (Zdenek et al., 1987).

著者

若林環 風間卓仁

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.84, pp.11-20, 2021-03-26

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Campaign relative gravity measurement is one of the most powerful methods to monitor spatiotemporal mass variations associated with volcanic activities. In order to quantify volcanic gravity variations accurately, the systematic gravity error due to scale factor should be corrected from original gravity data. We therefore determined relative scale factors of portable relative gravimeters, using relative gravity values measured at six gravity points between Kyoto University and Mt. Hiei in 2020 frequently. We here calculated the relative scale factor of each gravimeter by dividing the gravity value of the LC-G534 gravimeter by that of the corresponding gravimeter. The scale factor of the LC-G680 gravimeter relative to the LC-G534 gravimeter was obtained to be 0.999900 ± 0.000067; although the gravity difference between Kyoto University and Mt. Hiei (about 167 mGal) was not so large for the scale factor calibration, we succeeded in determining the G680’s relative scale factor with small standard deviation because the measurement error of the gravity data became smaller thanks to the frequent gravity measurements. However, the relative scale factors for LC-G680 and D-58 as of 2020 were found to be different from those in 2018 by more than 0.0002. In addition, the standard deviations of the relative scale factors for LC-D58 and CG-5 were obtained to be greater than 0.0002. These results may be related to significant temporal variations in reading values, originating from the overhaul in 2019 (LC-G680), the manual adjustment of the reading range (LC-D58) and the large instrumental drift (CG-5). Our results also imply that scale factor values for LaCoste-type gravimeters can depend on their reading values, as mentioned in a previous study for Scintrex-type gravimeters.

著者

一柳 昌義 山口 照寛 高田 真秀 東 龍介 黒井 和典 山田 卓司 高橋 浩晃 前田 宜浩

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

no.76, pp.1-13, 2013-03

著者

一柳 昌義 高橋 浩晃 山口 照寛 岡田 和見 大園 真子 岡崎 紀俊

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.79, pp.1-8, 2016-03-19

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An M5.0 shallow earthquake occurred on 4 June 2015 in the eastern Hokkaido. In order to evaluate aftershock activity, three temporal seismic stations had been operated in the focal region from 4 June 2015 to 22 August 2015. Hypocenters calculated with a local seismic velocity structure indicated clear southwestern-dipping distribution. This alignment on a plane was consistent with one of nodal plane of mainshock focal mechanism. This earthquake and historical records suggested higher seismic activity in this region.

著者

風間 卓仁

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.83, pp.9-23, 2020-03-19

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Relative gravity data is continuously collected using different types of portable relative gravimeters on a trial basis, toward the future utilization of continuous relative gravity measurements for monitoring spatiotemporal mass variations associated with volcanic activities. The gravity data is first recorded using the Tera Term software installed to a Windows-based PC; although a typical LaCoste & Romberg gravimeter does not have any devices to transmit its gravity value digitally, its analog voltage output from its readout socket can be converted to serial data format using an appropriate analog-digital converter. The recorded gravity data is then sent to an Ubuntu-based computer periodically to draw graphs of gravity time series. The graphs are finally uploaded to a web server so as to monitor temporal gravity variations anytime and anywhere. The above system for continuous relative gravity measurement was applied to the LaCoste G680 gravimeter, which was installed at the Kyoto-A gravity point, Kyoto University in September 2017. The G680’s gravity data in 2018 clearly showed time variations in short-period tide and annual instrumental drift. In particular, the instrumental drift was expressed by the linear gravity decrease of about 10 mGal/year and the additional annual gravity variation of about 1.5 mGal amplitude. These results suggest that typical LaCoste gravimeters can be utilized for continuous relative gravity measurements without major instrumental remodeling.

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

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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

一柳 昌義 シェスタコフ ニコライ 奥山 哲 大園 真子 笠原 稔 高橋 浩晃

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.81, pp.17-26, 2018-03-19

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A M5.6 shallow earthquake occurred on 8 July 2014 in middle east Iburi region, southwestern Hokkaido. Maximum seismic intensity of 5- was observed at Shiraoi town. Three temporal seismic stations had been installed in the focal region from 9 July 2014 to 30 April 2015. We determined 384 aftershock hypocenters by the double-difference tomographic hypocenter determination with proper seismic velocity structure. Relocated aftershocks were clearly distributed on a southeastern ward dipping plane with 5~10 km depth. This configuration agreed with a nodal plane of mainshock focal mechanism. Aftershocks occurred in surrounding part of a low seismicity patch. Mainshock was located deepest part of aftershock zone. These facts suggested that mainshock rupture propagated from deep to shallow, and a low seismicity patch was mainshock slip area. No clear correlation between this earthquake sequence and nearest active Tarumae volcano activity were identified.

著者

一柳 昌義 シェスタコフ ニコライ 奥山 哲 大園 真子 笠原 稔 高橋 浩晃

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.81, 2018-03-19

著者

松本 亜希子 宮坂 瑞穂 中川 光弘

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

no.78, pp.1-9, 2015-03

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We examined the analysis method of the compositions of volcanic glass using WDS-EPMA, focusing on Na migration caused by electron-beam bombardment. As a result of the comparison among the beam current in 10 μm square area, it is concluded that Na migration occurs in any cases at 15 kV. During the first 30 seconds, Na decay is not observed, and therefore, the detection of Na must be finished within the first 30 seconds. Considering the variations of other elements, the 15 kV accelerating voltage and 7 nA beam current with raster scanning of 10 μm square area is the best condition for the determination of volcanic glass compositions. This can prevent "grow-in" of Si and Al, as well as can make smaller the deviations of the minor elements. Using this condition, we can discuss the variations of volcanic glass compositions (except for Na) without any corrections.

著者

一柳 昌義 高田 真秀 高橋 浩晃

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.80, pp.1-11, 2017-03-19

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A M5.3 shallow earthquake occurred on 16 June 2016 in Minamikayabe region, southern Hokkaido. Three temporal seismic stations had been operated in the focal region from 18 June 2016 to 29 August 2016. Initial hypocenters and 1-D P-wave velocity structure were estimated from waveform data of temporal and permanent seismic stations. Fine aftershock distribution inferred from the double-difference procedure showed that the hypocenters were distributed in NW-SE direction, which might be agree with a fault plane of mainshock focal mechanism. Five foreshocks occurred near mainshock hypocenter showed similar waveform with high correlation. Characteristic persistent background seismicity and latest middle size earthquake might be related to high geothermal gradient in this region.

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.80, 2017-03-19

著者

石森 健太朗 大園 真子

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.80, pp.13-22, 2017-03-19

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Observed coseismic crustal deformation is an answer of the elastic response induced by the seismic fault dislocation. Because real subsurface structure is not pure elastic space, thus the observed coseismic displacement distribution probably different from the predicted displacement field when we assume the pure elastic medium. By the coseismic strain analysis using dense GNSS network at the 2011 Tohoku-oki earthquake, a supporting result, which shows that the strain anomaly related to inhomogeneous subsurface structure, is proposed. However, there is no other previous study. Therefore, we try to detect the same relationship from coseismic displacement at the 2003 Tokachi-oki earthquake. After an appropriate rectangular coseismic fault is estimated, we obtained difference between observed and predicted dilatation as the strain anomaly field. As a result, the strain anomaly shows excess extension around the Ishikari Plain. There are two possible causes for this region. One is inelastic response by the existence of thick sedimentary layer. Another is different coseismic behavior induced by clear basement boundary around this region. The east side of this excess extension area, extension deficit area is also detected. Around this area, high seismic velocity area corresponds to the Hidaka metamorphic belt is distributed. Therefore, this relatively rigid elastic medium possibly generates the extension deficit. The coseismic crustal strain distribution at the 2003 Tokachi-oki earthquake also suggest the existence of the coseismic strain anomaly induced by inhomogeneous subsurface structure.

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.80, 2017-03-19

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.