ῒΐ  /*  (,**/)
 0  /1/ῌ/2. ῕ ῔,**.ῌ,**/ ,**/  /  +-  ,**/  ++  . !"Ground Deformation Associated with the ,**.ῌ,**/ Unrest of Asama Volcano, JapanYosuke AOKI῍, Hidefumi WATANABE῍, Etsuro KOYAMA῍῍, Jun OIKAWA῍ and Yuichi MORITA῍
Ground deformation associated with the most recent eruptions in Asama volcano started on September +,,**., is reported. The ground deformation observed by continuous Global Positioning System measurements ismodeled by dike intrusion for two di#erent periods ; one is between July, ,**., to March, ,**/, which represents
overall deformation during the unrest, and the other is between November, ,**., to March, ,**/, which represents
the deformation during the latter half of the unrest. We assumed a rectangular dike opening uniformly in elastic,
homogeneous, and isotropic medium to model the deformation ﬁeld. To solve a nonlinear optimization problem
in which model parameters are nonlinear to observed deformation ﬁeld, the Simulated Annealing inversion wasemployed. The uncertainties of and trade-o#s between the model parameters are estimated by the Bootstrap
method.
The results show that the deformation ﬁeld is well modeled by a dike striking roughly east-west, strike ofwhich is consistent with the regional stress ﬁeld. Shape of the dike, that is, length, width, and thickness, is notwell constrained due to the small amount of deformation, up to +* mm, and the scarcity of GPS sites, but volumeof the dike is well constrained to be 0.2, and ..0- million cubic meters for the whole period and the latter half,respectively. The estimated depth of the dike tip is roughly + km below the sea level ; the depth of hypocenters isconsistent with a theory of dike-induced earthquakes that they occur near the dike tip due to the stressconcentration. However, the comparison of the location of the modeled dike and the distribution of earthquakes
clearly shows that the hypocenter distribution is inconsistent with the theory described above, that is, thehypocenters are distributed only in the eastern half of the modeled dike tip. The possible reasons for thisinconsistency are either +) earthquakes exists in the west half of the dike tip as well, but they are not detected dueto the sparse distribution of seismometers at the west of the ﬂank, ,) the western half of the modeled dike is notcapable of generating earthquakes because the temperature is too high for brittle failure of rocks, or -) thedi#erential stress in the western half is so low that the area cannot reach the critical stress ﬁeld even by theintroduction of dike-tip stress concentration. Current geophysical observations cannot identify the reason but
future development of geophysical observations is expected to solve the puzzle.
Key words : ground deformation, Asama volcano, dike intrusion, Global Positioning System, geodetic inversion+ῌ ῐ ῎ ῑ ῏

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Asama Volcano Observatory of Earthquake ResearchInstitute, University of Tokyo, ,+,/ Nagakura, Karuizawa, Nagano -23ῌ**++, Japan.
Corresponding author : Yosuke Aoki
e-mail : yaoki@eri.u-tokyo.ac.jp