利用双差走时成像研究青藏高原东缘地壳速度结构

青藏高原东缘断裂密布, 强震频发, 是研究高原侧向挤出及深部孕震环境的理想实验室.为了解龙门山次级块体及其西界龙日坝断裂带在青藏高原东缘隆升过程中的作用, 我们基于四川地震台网64个宽频带地震台在2008年1月至2015年12月期间记录的震级≥3.0地震事件波形, 利用双差层析成像方法揭示了四川盆地及青藏高原东缘的地壳速度结构.结果表明: 夹持于龙门山断裂带(LMSF)与龙日坝断裂带(LRBF)之间的龙门山次级块体, 相对东侧龙门山断裂带和四川盆地呈现明显的低速特征.结合该区域的低阻、低密度结构特征, 以及块体内部、特别是龙日坝断裂带现今地震活动缺乏, 我们推测这是因为该块体"相对较软", 不易脆性破裂产生地震, 在青藏高原东缘与扬子块体西缘强烈相互作用过程中, 该块体主要通过地壳缩短增厚和地表隆升吸收板块挤压造成的累积应变能.依据本文获得的速度等值线变化特征及已有地球物理剖面探测结果, 推测龙日坝断裂带为深部向南东倾斜且向下切入基底, 该断裂倾角较陡, 主要以走滑运动调节应变能, 而东侧龙门山断裂带倾角较缓, 表现为逆冲运动导致的地壳缩短是其调节应变能的主要形式.此外, 据本文多条速度剖面及已有电性剖面、重力异常的联合约束, 我们推测鲜水河、安宁河断裂带均以较大倾角向南东倾斜, 至少延伸至中下地壳.

Crustal velocity structure beneath the eastern Tibetan Plateau derived from double-difference travel-time tomography

The eastern margin of the Tibetan Plateau is covered by dense faults, with actively destructive earthquakes, which makes it a natural laboratory to study lateral extrusion mechanism of Tibetan Plateau and deep seismogenic environment. In order to understand the role of the Longmenshan subblock and its western boundary, the Longriba fault zone (LRBF), in the uplift of eastern Tibetan Plateau, we use double-difference tomography to reveal the crustal velocity structure beneath the Sichuan Basin and eastern Tibetan Plateau based on the event waveforms with magnitude ≥3.0 recorded by 64 broadband seismic stations between Jan. 2008 and Dec. 2015. The results show that the Longmenshan subblock, sandwiched between the Longmenshan fault zone (LMSF) and the LRBF, has obvious low crustal velocity characteristics compared with the LMSF itself and the Sichuan Basin to the east. Considering the low-resistance and low-density structure, as well as lack of seismicity within this block, especially in the LRBF, we speculate that this block is "relatively soft" to hardly produce brittle earthquake. During the strong interaction between the eastern edge of the Tibetan Plateau and the western edge of the Yangtze Block, this block absorbs the accumulated strain energy caused by plate extrusion mainly through crustal shortening and thickening and surface uplift. According to the velocity contour changes, we infer that the LRBF steeply dips southeastward in the deep and cuts into the basement, which probably releases its strain energy mainly by strike-slip movement. In contrast, the LMSF has a lower inclination angle leading to crustal shortening mainly by thrusting. In addition, according to the joint constraints of multiple velocity structures in this study, electrical and gravity anomalies revealed by other studies, we speculate that the Xianshuihe and Anninghe fault zones are also inclined to the southeast with a relatively large inclination angle, both of which extend at least into the middle-lower crust.