Subsurface structure and faulting of the Median Tectonic Line, southwest Japan

本研究的主要目的是利用密集的地表GPS横贯测量资料，厘清日本西南部构造中位线（MTL）的深部构造和滑动分布。1998-2000年，我们在220公里长的铁路线上建设了22个站点，并进行了三次运动测量。我们总共测定了64个测点的水平地壳速度，包括全国网络中相邻的连续测点。获得的运动速度非常精确，与连续测量的速度非常吻合。我们定量地将变形场分为两类：菲律宾海板块俯冲引起的弹性变形和横跨MTL的块状运动。我们进行了正演和反演建模。在前者中，我们通过计算板块俯冲模型的弹性变形并从观测到的速度场中减去弹性变形，提取了横跨MTL的块体运动、最上部的断层锁定和深度的稳定滑动；在后者中，我们同时估计了板块俯冲、横跨MTL的块体运动和MTL最上部的滑动亏缺。两个模型给出了几乎相同的结果：1。MTL南地块相对于北地块以约5mm/年的速度向西移动，与5-10mm/年的地质右侧滑动速率一致2。地块边界与MTL的面迹不一致，明显向北偏移了20 ~ 30km。上述结果解释为，MTL断裂面向北倾斜30 ~ 40度，MTL上部被锁定在15 ~ 20km深度，下部发生稳定的右侧滑动。这些结果已经在日本和美国的几次会议上提出。在国际期刊上发表的稿件正在准备中。

Main purpose of this study is to clarify deep structure and slip distribution of the Median Tectonic Line (MTL) in southwest Japan from dense GPS traverse measurements at the surface. We constructed 22 stations along a 220km-long line and conducted three campaign measurements in 1998-2000. We determined horizontal crustal velocities at 64 sites in total including adjacent continuous stations of the nationwide network. Campaign velocities obtained are so accurate as to show very good accordance to the velocities from continuous measurements.We quantitatively discriminated deformation field into two categories : elastic deformation caused by the subduction of the Philippine Sea plate and block-like movements across the MTL.We made both forward and inversion modelings. In the former we extracted block motion across the MTL, fault locking in the uppermost part and steady slip at depth by calculating elastic deformation from a plate subduction model and subtracting it from the observed velocity field In the latter we simultaneously estimated plate subduction, block motion across the MTL and slip deficit in the uppermost part of MTL.Two modelings gave the nearly same results :1. Southern block of the MTL moves westward at a rate of about 5mm/yr relative to the northern one, consistent to the geological right-lateral slip rate of 5-10mm/yr.2. The block boundary does not agree with the stuface trace of the MTL, clearly displaced 20-30km to the north.3. The above results are interpreted that the fault plane of MTL dips into the north at an angle of 30-40 degrees, and the uppermost part of MTL is locked to the depth of 15-20km and steady right-lateral slip occurs below. These results have been presented at several meetings in Japan and USA.The manuscript for the publication in the international journal is being prepared.