使用纵波和横波走时层析成像研究菲律宾海板块西边缘带和南海地区的深部结构

Deep structure beneath the west margin of Philippine Sea Plate and South China Sea from P and S wave travel time tomography

使用纵波震相及同台同源纵波和横波震相的走时资料及层析成像反演方法，分别给出了菲律宾海板块（PHP）和南海地区的纵波速度Vpf及同台同源的纵波和横波速度Vp和Vs结构．结果表明，（1）PHP与欧亚板块（EUAP）的俯冲接触关系随地段而异，在琉球海沟，PHP向EUAP之下俯冲深达400km；在台湾岛，EUAP先近陡直地俯冲到深度400km，然后折向PHP之下达到660km左右；在马尼拉海沟北段，俯冲板片几乎垂直达到660km附近；在菲律宾海沟，PHP向EUAP之下的俯冲深度超过660km．（2）南海地区之下是一个深达300～400km的宽阔低速区，并且大致在莺琼海盆与700km深处另一低速区曲折相通；在该宽阔低速区内部，有两个小而明显的低速区分别在海口火山和珠江口盆地下方．（3）对Vp和Vs及据其算出的容声速度Vb作分析发现，Vs和Vb的平均扰动量对深度的变化在一些深度范围内是反向的；年龄较大的太平洋板块俯冲体的Vs相对扰动量的平均值大于K的，而在较年轻的PHP俯冲体中则正好相反．

The velocity model Vpf from a data set of P waves, and 3-D models of Vp and Vs estimated from P and S wave data with the same source-receiver pairs, were respectively determined for the area of Philippine Sea Plate （PHP） and South China Sea （SCS）. The results obtained revealed the following features： （1） The interactions between the PHP and Eurasia plate （EUAP） are different in different parts of the west boundary of PHP. Along the Ryukyu Trench, the PHP subducts beneath EUAP down to a depth of 400 km. Under Taiwan island, the EUAP steeply subducts down to about 400 km depth, and then toward PHP until it reaches a depth of about 660 km. At the north segment of Manila Trench, a nearly vertical slab inserts down to about 660 km depth. Underneath Philippine Trench, the subducting PHP slab inserts into EUAP, and penetrates below the 660＇-discontinuity. （2） Under the SCS, there is a low-velocity area （LVA） of large width which is visible at depths of 300 - 400 km. It wri es and reaches to another LVA at depth about 700 km under Yingqiong Basin. Within the LVA under SCS, there are two small but prominent low velocity anomalies： one beneath Haikou Volcano, and another beneath Zhujiangkou Basin. （3） The statistical results of the perturbations of Vs and Vb （the bulk-sound velocity calculated from Vp and Vs ） show that： Firstly, the change rates of average perturbations of Vs and Vb with depth are reverse within some depth ranges. Secondly, the averages of relative perturbations of Vs are bigger than that of Vb in the old subducting Pacific slab. In the young subducting PHP slab, however, this contrast relationship is opposite.