水準測量データによる1930年伊東沖群発地震のダイク貫入モデル

概要

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We examine vertical deformation observed by leveling during and after the 1930 Ito-oki earthquake swarm in the Izu-tobu volcano group, eastern part of the Izu Peninsula, central Japan. Taking the mechanism of the swarm activity after 1970's in the same area into consideration, we estimate the source model which can explain the observed uplift along the eastern coast of the Izu Peninsula. Although the observed deformation in 1930 allows a wide range of parameters of a tensile fault as a source, it is reasonably explained by a near-vertical tensile fault suggesting dike intrusion east off Ito. The volume increases are estimated to be 8.8 × 10^7m^3 and 1.4 × 10^8m^3 before and after April, 1930, respectively. They are much larger than that for each episode of dike activities after 1970's. Broad uplift with no seismic activity was observed during 1931-1933. It is explained by either a spherical inflation source or a horizontal tensile fault at a depth of 〜 10 km, west of Ito. In both cases, the volume increase is estimated about 6 × 10^7m^3. Calculation of the Coulomb failure stress suggests that the dike intrusion east off Ito triggered the M7.3 Kita-Izu earthquake occurred on November 26, 1930.
2007-06-29