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

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
三輪田 吾郎 高井 伸雄 清水 学 笹谷 努 前田 宜浩 山本 明夫
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
no.73, pp.195-205, 2010-03

Nonlinear site response was observed at TKCH 07 in Toyokoro and HKD 109 in Urakawa during the mainshock(Mj 8.0)and the largest aftershock(Mj 7.1)of the 2003 Tokachi-oki Earthqua ke. At TKCH 07, peak ground acceleration(PGA)was smaller than PGA at the other strong motion station in Toyokoro. At HKD 109, the accelerogram shows a characteristic spiky waveform. Due to this spiky wave, large PGA was observed compared to the other stations in Urakawa, although PGA at HKD 109 was smaller than PGA at JMA Urakawa station during the main shock and smaller aftershocks.
著者
植竹 富一
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
no.73, pp.49-55, 2010-03

A deep(H=374 km)earthquake of Mj=6.7 occurred in Kyoto-Oki region on July 16, 2007. Broad-band velocity ground motions during this event were observed at many thermal power stations in the Tokyo bay area. Waveforms observed at the stations of the east side of the Tokyo bay show longer duration than those at the west side of the bay. The apparent frequencies of later arrivals are lower than those of S-wave parts. Velocity waveforms observed in the mountain area and in the bedrock of the basin have short duration and no significant later arrivals. This suggests that the later arrivals were excited by the Kanto basin.
著者
前田 宜浩 高井 伸雄 笹谷 努
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
no.73, pp.217-227, 2010-03
被引用文献数
1

The largest aftershock(Mw 7.3)of the 2003 Tokachi-oki earthquake(Mw 8.3)generates larger peak ground acceleration(PGA)values than the main shock in the western side of the epicenters despite their magnitude difference. We study the features of strong ground motions from the largest aftershock in detail using PGA values. Spatial distribution maps of PGA ratio between the largest aftershock and adjacent earthquake shows obvious azimuth dependency compared with the other earthquake pairs occurring around Japan. Attenuation relationship of the largest aftershock shows a large azimuth-dependent scattering at distances from 200 to 300 km compared with three smaller earthquakes occurring around the largest aftershock. Because this large scattering appears only in the largest aftershock, we conclude that the azimuth dependency is a peculiar feature of the largest aftershock. Fourier spectral ratios between the largest aftershock and adjacent earthquake at several stations show azimuth dependency at higher frequencies than 1 Hz, which is consistent with the azimuth dependency of PGA values. The azimuth dependency in high-frequency range cannot be explained simply by the source and path effects, and therefore, we make a hypothesis that the azimuth dependency is an apparent one generated by two earthquakes occurring simultaneously at different regions. Synthetic PGA distributions generated by two simultaneous events calculated by the empirical attenuation relationship are in agreement with the observed PGA distribution of the largest aftershock.
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

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

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.
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

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.