Detection of the gravity changes in the subducting zone by means of precise gravity measurements

大地震经常发生在板块边界，如南开海槽，将菲律宾海板块俯冲到欧亚板块之下。我们试图探测俯冲板块引起的绝对重力变化。位于四国岛村田角海角尖端的东京大学室本地球物理观测站新测定了绝对重力点。利用FG5#210绝对重力仪进行了绝对重力测量，得到了重力值的绝对变化。绝对重力测量分别在2001年2月、2002年2月和2002年9月进行了约1周。没有认识到这些时期绝对重力点的重力变化。然而，考虑到FG-5的性能，它将在几年内通过下沉检测到绝对重力变化。四国岛的武藤角最近正以每年约1厘米的速度下沉，这是日本地理测量研究所反复进行水准测量的结果。还在Muroto角周围使用了La Coste&Romberg重力仪，在水准测量路线的基准点上以及在地理勘测所的GPS连续观测站上进行了相对重力测量。所获得的结果不能比相对重力的精度更能反映重力的变化。最近，南开破坏性地震被指出在不久的将来会发生。利用绝对重力仪观测四国岛室东角的重力变化，对于阐明板块下沉机制具有重要意义。

Great earthquakes frequently occur at plate boundaries such as the Nankai Trough subducting the Philippine Sea Plate beneath the Eurasian Plate. We have attempted to detect absolute gravity changes caused by subducting plate.New absolute gravity point was made at the Muroto Geophysical Observatory of University of Tokyo where located at the tip of a promontory of the Cape Murata in the Shikoku Island. We carried out the absolute gravity measurements to obtain the absolute change of the gravity values using the absolute gravimeter FG5#210. The absolute gravity measurements were performed about 1 week, in February 2001, February 2002 and September 2002, respectively. It was not recognized the gravity changes at absolute gravity point in these periods. However, considering to performance of FG-5 it will be detectable to absolute gravity changes by subsiding within several years.Cape Mutoto in the Shikoku Island is subsiding recently at about 1 cm/year as a result the repeated leveling survey by the Geographical Survey Institute of Japan. The relative gravity measurements were also carried out around the Cape Muroto by employing La Coste & Romberg gravimeters on bench marks of the leveling routes as well as on the GPS continuous observation stations by the Geographical Survey Institute. Obtained results could not show the gravity changes more than precision of relative gravimetry.Recently, destructive Nankai earthquake has been pointed out to occur in the near future. It is important to observe gravity changes employing absolute gravimeter at the Cape Muroto in the Shikoku Island for clarify the mechanism of subsiding plate.