[Abstract]

From May 8 to 13 in 2002, a small-scale earthquake swarm took place off Ito City, eastern Izu Peninsula, central Japan. Associated with the swarm, changes in the ground tilt as much as 2.8 rirad were detected by several tiltmeters near the swarm area. The change can be approximately interpreted by a tensile fault in the vicinity of the swarm area. We estimated the fault parameters from the tilt and GPS data, assuming that the fault length is 2 km. The depth of the upper bound of the tensile fault and the width are 10 km and 2 km, respectively. The estimated volumetric increase is 7 x 10-3 km3. This tensile fault is confirmed to be a dike intrusion from a deep seated magma reservoir, that is, the same process which had been observed in 1980's to 1990's. The tilt vectors of stations ITO and YOS are quite similar in shape in the period from May 8 to 11, suggesting thickening of the dike without upward elongation during the period. Keywords: the eastern Izu Peninsula, earthquake swarm, crustal deformation, tiltmeter, GPS

e-mail:

ueda@bosai.go.j