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，例如南海海槽将菲律宾海板块俯冲到欧亚板块之下。在四国岛村田岬的尖端，东京大学的室户地球物理观测站，建立了一个新的绝对重力观测点。我们使用绝对重力仪FG5#210进行绝对重力测量，以获得重力值的绝对变化。分别于2001年2月、2002年2月和2002年9月进行了大约一周的绝对重力测量。这些时期的绝对重力点的重力变化未被认识。然而，考虑到FG-5的性能，它将在几年内通过沉降检测到绝对重力变化。由于日本地理调查研究所的重复水准测量，四国岛的武藤角最近以约1 cm/年的速度沉降。还在室户角周围进行了相对重力测量，方法是在水准路线的基准点上使用La Coste & Romberg重力仪，并在地理测量研究所的GPS连续观测站上进行。相对重力测量的精度不足以反映重力场的变化。最近，有人指出，在不久的将来会发生破坏性的南海地震。利用绝对重力仪观测四国岛室户角的重力变化，对阐明板块沉降机制具有重要意义。

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.