青藏高原东北缘地壳三维速度结构

Three-dimensional crustal velocity structure in the northeastern margin of the Qinghai-Tibetan plateau

本文用1980～2000年M≥1．5的2032个天然地震事件的38052个P、S、Pm、Sm、Pn和Sn震相到时及人工地震测深给出的Moho面形态资料，利用地震层析技术反演了32°～40°N，100°～108°E区城内地壳地震波速度结构．从层析成像图象中可以得到，本区的地壳可分成4个层位．第1层（埋深约在0～3km）为沉积层，速度梯度约为0．2s^-1；第2层（埋深约在3～17km）为上地壳，其顶部速度梯度约为0．1s^-1，下部速度横向变化较大且存在低速块体；第3层（埋深约在17～36km）为中地壳，速度梯度约为0．03s^-1；第4层（埋深约在36km-Moho）为下地壳，是一个契形层，总的趋势是西厚东薄，青藏高原较厚逐渐向部尔多斯地块和扬子准地台方向变薄，各处的地震波速度拂度不尽相同．

This paper presents the three-dimensional crustal seismic wave velocity structure in the region 32°～40°N, 100°～108°E reconstructed through seismic tomographic inversion. We use 38 052 arrival time data of P, S, Pm, Sm, Pn and Sn phases recorded from 2 032 earthquakes with M≥1. 5 occurred from 1980 to 2000 and the Moho interface topography data from deep seismic sounding. The 3-D tomographic image clearly reveals that the crust structure of this region consists of four layers. The top layer （about 0～3 km） has a velocity gradient about 0. 2 s-1 that corresponds to the sediments. The second layer （about 3～17 kin） is the upper crust with a velocity gradient about 0.1 s^-1 at top and relative large lateral velocity variation as well as lower velocity blocks at bottom. The third layer （about 17 ～36 kin） is the middle crust with velocity gradient 0. 03 s^-1. The lower crust forms the fourth layer （about 36 km-- Moho）, which is thick in the west and thin in the east. The thickness of this layer gradually reduces from the Qinghai-Tibetan plateau to the Erdos platform and the Yangtze paraplatform with different velocity gradient at different location.