Microgravity survey at Ogasawara Iwo-jima has been repeatedly performed by the National Research Institute for Earth Science and Disaster Prevention in every two years since 1998 by using two Scintrex CG-3M gravimeters (No.212 and No.284). Iwo-jima is an active volcanic island belonging to the Izu-Ogasawara islands, and the purpose of this study is to detect the gravity changes associated with the large scale crustal deformation of volcanic origin in the island. Our gravity survey line consists of one return trip from Tsukuba (Bosai BS) to Iwojima through Iruma air base (Iruma BS). As the reference of standard gravity value, we tied Bosai BS to Tsukuba gravity station (Tsukuba GS) of Geographical Survey Institute, Japan, before and after the Iwo-jima measurements. Because of the large gravity difference between Bosai BS and Iwo-jima, about 870 mGal (1mGal=1×10-5ms-2), we calibrated scale factors of the gravimeters, which were determined twice by calibration surveys along the line connecting the 1st-order gravity stations of JGSN96, Haneda GS, Sapporo GS, Chitose GS and Naha GS, in 1999 and in 2003. The obtained two coefficients for scale factor correction for each gravimeter exhibit temporal change amounted to about 1×10-4 . In the present study we assumed temporally linear change of the scale factor for each instrument, and then interpolated or extrapolated the calibrated values to apply to the Iwo-jima survey . We present here the gravity values of the base benchmark (Iwo101) in Iwo-jima estimated for the four surveys in 1998, 2000, 2002 and 2004. The differences between the estimated gravities of Iwo101 by the two in struments range within 0.01 mGal for the 2000 and 2002 surveys, for which the scale factors were interpolated from the calibrated values. Larger differences ranging between 0 .02 and 0.06 mGal appeared in the 1998 and 2004 surveys, for which they were extrapolated, indicating that the errors in the absolute gravity values at Iwo-jima depend strongly on the accuracy of scale factors. The average gravity values of the two instruments at Iwo101 were 979, 081.670 meal (1998), 979, 081.719 mGal (2000), 979, 081.489 mGal (2002) and 979, 081.473 mGal (2004). The large gravity decrease of 0.230 mGal at Iwo101 during the period from December 2000 to December 2002 is well correlated to the large uplift of 0.955 m with the ratio of gravity change to the uplift, -0.241 mGal/m. This ratio corresponds to the intrusion of materials with the density of 1.6×103 kg/m3 for the case of an infinite flat intrusion, indicating a significant contribution of magma in the 2001 large crustal deformation.
