Departmental Bulletin Paper 2003年十勝沖地震(M8.0)による北海道地域の地震時歪み異常分布の検出
Coseismic strain anomaly induced by the 2003 Tokachi-oki earthquake (M8.0) around Hokkaido, Japan

石森, 健太朗  ,  大園, 真子

80pp.13 - 22 , 2017-03-19 , 北海道大学大学院理学研究院
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.

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