Tectonics of the southeastern Philippine Sea using GPS

为了研究马里亚纳弧的瞬时运动，继1992年和1994年第一次占领北马里亚纳群岛后，我们于1998年和1999年在北马里亚纳群岛进行了两次GPS观测。我们在1998年1月至2月和1999年2月使用了7台Trimble 4000SSE GPS接收器进行了为期四天的会议。使用Bernese Ver. 0.0基线分析软件结合1994年的数据对结果进行分析。结果表明，这些岛屿相对于菲律宾主板块正在向东移动。每个岛屿的估计速度为；格里汉的N113E为10.4mm/年，Pagan1的N70E为20.0mm/年，Guguan的N73E为20.9mm/年，Anatahan的N75E为40.0mm/年，塞班岛的N83E为40.3mm/年，关岛的N93E为50.8mm/年。因此，估计的速度可以代表马里亚纳海槽弧后扩张的速度。值得注意的是，当我们去南部岛屿时，速度更大，而且向东移动。这可能暗示了弧后目前是如何扩展的。虽然我们的研究进行得很成功，但我们可能不得不考虑以下几点。1)我们应该对更北的岛屿进行GPS测量，因为这些岛屿尚未被重新占领，这对于明确弧后向北延伸的距离很重要。2)考虑到许多岛屿是火山，我们应该对所有这些岛屿进行重复调查，因此监测这些岛屿的局部变形对于减少火山危害至关重要。

In order to investigate the instantaneous motion of the Mariana arc, we have conducted twice of the GPS obserbation in the Northern Mariana Islands in 1998 and 1999 following the first occupation in 1992 and 1994. We have conducted four day sessions in January-February of 1998 and February of 1999 using seven Trimble 4000SSE GPS receivers. The results were analyzed using the Bernese Ver. 0.0 baseline analysis software togeher with 1994 data. Results suggests that the islands are moving toward east relative to the main Philippine Aes plate. Estimated velocities at each islands are ; 10.4mm/yr with N113E for Agrihan, 20.0mm/yr with N70E for Pagan1 20.9mm/yr with N73E for Guguan, 40.0mm/yr with N75E for Anatahan, 40.3mm/yr with N83E for Saipan, and 50.8mm/yr with N93E for Guam, respectivery. Thus estimated velocities may represent the velocity of back-arc spreading at the Mariana trough. It would be worthy to note that the velocity is larger and move eastward when we go to southern islands. This may suggest how is the back-arc spreading at present time.Although our research was successfully conducted, we may have to consider the followings. 1) we should conduct GPS surveys for the islands further north because these islands are not yet re-occupied, which is important to clarify how far the back-arc is reaching to north. 2) we should repeat surveys for all of these islands considering that many of islands are volcanic so that the monitoring of local deformation of these islands are important in terms of volcanic hazard reduction.