中国大陆Pn波速度结构与强震孕育的深部背景

在作者以往工作的基础上，增加了中国地震年报（1997～1998年）、朝鲜以及我国各区域地震台网的数据，使Pn到时数据增加到近14万条，从中选取符合反演条件的8043次地震在581个地震台站上记录到的Pn射线99440条,重新作了中国大陆Pn波速层析成像,空间分辨率达到3°×3°. 文中将Pn波速度横向变化与活动地块的划分进行了对比,结果表明: Pn波速度横向变化与Ⅰ级活动地块区之间关系密切，大致以105°E为界，西部地区速度以正异常为主，其中西域地块区速度正异常最大，其次是青藏地块区，而滇缅地块区为负异常区；东部地区速度以负异常为主，华北地块区速度负异常最大，其次是东北亚地块区. 西部的地块区上地幔顶部Pn波速度各向异性的平均快波速度方向大体为NWW—SEE向;东部的地块区大体为NNW—SSE向，大致与相应的张应力优势方向一致. Pn波速度横向变化与强震活动间的关系也较明显，所有MS≥7.0的强震大部分发生在上地幔顶部Pn波速度的低速异常区上面的地壳内，少数发生在高、低速区域的过渡地带或两个高速区之间的相对低速部分.

Pn VELOCITY VARIATION BENEATH CHINA MAINLAND AND DEEP STRUC TURE BACKGROUND FOR MAJOR EARTHQUAKE GENERATION

Based on a previous work of the authors and abundant updated data are make a new Pn travel time tomography for the China mainland. The supplemented Pn arrival time data are from Annual Bulletin of Chinese Earthquakes for 1997 and 1998, Korean (DPRK) seismological reports and bulletins of Chinese provincial seismic networks. The amount of collected Pn arrival time data reaches up to 140000, from which we have picked up 99440 Pn rays from 8043 earthquakes recorded at 581 stations for the tomography. Spatial resolution reaches 3°×3°. This study makes a comparison study between the Pn velocity lateral variation and delineation of the active tectonic block regions of China continent. The result shows that there is a close relation between Pn velocity lateral variation and delineation of the first order block regions. Roughly demarcated by 105°E, positive velocity anomaly regions dominate in the western part of the China mainland, with the highest positive anomaly in the Xiyu block region and secondary positive anomaly in Qinghai Xizang Plateau region, while the Yunnan Burma block region exceptionally shows negative anomaly. Nevertheless, negative velocity anomaly regions are mainly seen in the eastern part, with the lowest negative anomaly in the North China block region and secondary low anomaly in the Northeastern Asia block region.The mean fast Pn velocity direction in the top mantle of western block regions is generally in NWW SEE, while in eastern block regions it is about in NNW SSE, roughly coinciding with dominant direction of tensional tectonic principal stress there in. Rather significant correlation between Pn velocity lateral variation and location of strong earthquakes can also be seen.Most MS≥7.0 earthquakes took place in the crust above the low Pn velocity anomaly region, with a few occurring in the transition zone between high and low Pn velocity, or in the relative low velocity zone between two high velocity regions.