基于模拟退火法的青藏高原东缘噪声层析成像研究

Study on ambient seismic noise tomography of eastern Tibetan Plateau based on simulated annealing method

青藏高原是全球造山带研究的热点地区,此前在青藏高原开展的三维层析成像研究大多基于线性反演方法.本文利用青藏高原东缘及邻区布设的127个宽频带固定地震台站记录的连续波形资料,首先通过噪声互相关提取了3~50 s Rayleigh波群速度频散曲线并反演得到群速度分布,再进一步采用模拟退火法反演了研究区的三维S波速度及泊松比结构.结果显示:(1)松潘—甘孜地块的中下地壳低速异常主要分布在龙日坝断裂带、鲜水河断裂带、龙门山断裂带和岷山隆起所围限的区域,而该区域的中下地壳仅具有中等泊松比值,推测松潘—甘孜地块中下地壳的低速物质可能是青藏高原与扬子块体长期相互作用产生的塑性低速滑脱层;上地壳脆性物质在板块作用下沿中地壳低速滑脱层顶界面发生逆冲增厚,造成龙门山的持续抬升和地形起伏,并在构造边界带形成了应变积累和应力集中;而龙门山断裂带的上地壳低速软弱物质为地壳发生破裂提供了有利条件,从而在某种程度上促进了汶川地震和芦山地震的发生.(2)岷山隆起一带中下地壳的高泊松比异常呈“凸起”形态,结合前人研究发现的较高热流和岩石快速抬升现象,推测岷山隆起一带可能存在岩石圈的拆沉,导致地幔热物质上涌而形成下地壳高泊松比物质.(3)川滇地块的北部和南部具有不同的S波速度和泊松比分布特征.30 km深度下川滇地块北部具有明显的低速异常,而该深度下并不具有明显的高泊松比值特征;此外剖面成像结果也显示川滇地块内的低速异常与高泊松比的分布不一致,因此川滇地块的研究结果不支持下地壳流模型.综合其他地震学证据,本文认为川滇地块的变形模式为上地壳纯剪切增厚,块体变形主要受块体内部的走滑断裂及活动边界断裂控制.

The Tibetan Plateau is a hotspot region in the study of global orogenic belts,previous studies on ambient seismic noise tomography in the Tibetan Plateau were mostly based on linear inversion.In this study,one-year continuous waveform data recorded by 127 broadband seismic stations in the eastern Tibet were collected.Firstly,the Rayleigh wave group velocity dispersion curves at periods 3~50 s were extracted by cross-correlation and then the group velocity distributions were inverted by tomography.Subsequently,the simulated annealing method was employed to invert the 3-D S-wave velocity and Poisson′s ratio.Our results indicate:(1)The low-velocity anomaly in lower-to-middle crust of the Songpan-Garzêblock is mainly confined by the Longriba fault,Xianshuihe fault,Longmenshan fault and Minshan uplift,however,this region just has medium Poisson′s ratio at this depth,so it is speculated that the low-velocity materials in the middle-to-lower crust of the Songpan-Garzêblock may be the plastic low-velocity detachment layer which produced by long-term interaction between the Tibetan Plateau and the Yangtze block,and the brittle upper crust thrusts and thickens along the top interface of the low-velocity detachment layer in the middle crust under the interaction of plates,causing the continuous uplift and topographic fluctuation of the Longmenshan as well as the strain accumulation and stress concentration in the tectonic boundaries.Meanwhile,the local low-velocity materials in the upper crust of the Longmenshan fault provide favorable condition for the rupture of crust,which promote the occurrence of Wenchuan and Lushan earthquakes to some extent.(2)The middle-to-lower crust of the Minshan uplift features as high Poisson′s ratio,which presents a“bulge”shape.Combined with the phenomena of higher heat flow and rapid uplift of rocks founded by previous studies,the Minshan uplift may suffers the lithosphere delamination,resulting in the regional upwelling of hot mantle materials as well as forming the high Poisson′s ratio structure in the lower crust.(3)The northern and southern Sichuan-Yunnan diamond blocks have different S-wave velocities and Poisson′s ratio characteristics.At 30 km depth,low-velocity anomaly is significant in the northern Sichuan-Yunnan diamond block,while there is no obvious high Poisson′s ratio structure at this depth.In addition,the profile imaging results also show that the distributions of low-velocity anomalies and high Poisson′s ratio in the Sichuan-Yunnan diamond block are inconsistent,as a result,the lower crustal flow model cannot be supported with our results.Based on other seismological evidences,our study supports that the deformation model of Sichuan-Yunnan diamond block is pure shearing,the deformations of the block are mainly controlled by the strike-slip faults within the block and active boundary faults.