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

这项研究的主要目的是从地表密集的GPS导线测量中弄清日本西南部中构造线(MTL)的深部结构和滑动分布。我们在一条220公里长的线路上建造了22个站点，并在1998-2000年进行了三次战役测量。我们总共测定了站的水平地壳速度，包括全国网相邻的连续站。得到的运动速度非常准确，与连续测量的速度显示出很好的一致性。我们将形变场定量地区分为两类：菲律宾海板块俯冲引起的弹性形变和穿越山体的块状运动。前者通过计算板块俯冲模型的弹性变形并从观测速度场中减去它，提取了跨MTL的块体运动、最上部的断层锁定和深部的稳定滑动。在后者中，我们同时估计了板块俯冲、跨MTL的块体运动和MTL最上部的滑动赤字。两种模拟给出了几乎相同的结果：1.相对于北部，MTL的南块以大约5 mm/年的速度向西移动，与地质右旋滑动速率5-10 mm/年一致。地块边界与MTL的断层痕迹不一致，明显向北移动了20-30公里。上述结果解释为MTL断层面以30-40度的角度向北倾斜，MTL的最上部被锁定在15-20公里的深度，在其下方发生稳定的右旋滑动。这些结果已经在日本和美国的几次会议上公布，并正在准备发表在国际期刊上的手稿。

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.