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

本研究的主要目的是澄清深层结构和滑动分布的中央构造线（MTL）在日本西南部的密集GPS导线测量在表面。我们在220公里长的线路上沿着建造了22个车站，并在1998-2000年进行了三次活动测量。我们确定了水平地壳速度在64个网站，包括相邻的连续站的全国网络。我们将变形场定量地分为两类：菲律宾海板块俯冲引起的弹性变形和横穿MTL的块体运动，并进行了正演和反演模拟。前者通过计算板块俯冲模型的弹性形变，并从观测速度场中减去弹性形变，提取出跨MTL的块体运动、最上部的断层闭锁和深部的稳定滑动;后者同时估计了板块俯冲、跨MTL的块体运动和MTL最上部的滑动亏损。MTL南部块体相对于北方块体以约5 mm/yr的速率向西移动，与地质右行滑动速率5- 10 mm/yr相一致。块体边界与MTL的stuface轨迹不一致，明显向北偏移了20- 30 km。上述结果表明，MTL断层面向北倾斜30-40 °，MTL最上部被锁定在15- 20 km深度，下部发生稳定的右旋走滑。这些结果已在日本和美国的几次会议上介绍，目前正在准备在国际期刊上发表的手稿。

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.