利用宽角反射/折射地震剖面揭示芦山M_S7.0地震震区深部孕震环境

The deep seismogenic environment of Lushan M_S7.0 earthquake zone revealed by a wide-angle reflection/refraction seismic profile

2013年4月芦山地震发生后,中国地震局迅速成立了芦山地震科学考察指挥部,要求查明芦山地震的深部构造环境和孕震背景.为此,中国地震局地球物理勘探中心于2013年9月至11月在芦山震源区布设了一条长约410km的人工地震高分辨宽角反射/折射探测剖面,获得了信噪比较高的人工地震探测数据,采用地震射线走时正演拟合构建了该区的地壳及上地幔二维P波速度结构模型,结果显示：扬子块体和松潘—甘孜块体显示出迥异的速度结构特征,地壳厚度由南向北逐渐加厚.沉积盖层在四川盆地厚达7.8km,而进入松潘—甘孜块体沉积层最薄处只有几百米厚,几乎出露地表;在中上地壳,扬子块体平均速度比松潘—甘孜块体高0.2km·s-1,在盆地与高原耦合部位（构造转换带）以北深度大约20km左右有一厚度为8.0km的软弱层（低速层）,该层内的速度为5.80km·s-1,明显低于周围介质的平均速度6.00～6.10km·s-1;构造转换带内,震相显示紊乱、不清晰、不能连续对比,由地表至上地幔顶部壳内界面不连续、速度结构异常紊乱且呈现低速异常特征;在中下地壳,沿剖面速度呈现正梯度垂向增大变化;壳内界面在扬子块体内部起伏变化不大,但在构造转换带以北呈现急速加深的趋势,特别是Moho界面起伏变化较为明显,界面深度在距离50km范围内由扬子块体的36.2km迅速变化至松潘—甘孜块体下方的45.8km,形成一陡变带.芦山MS7.0级地震震源位置位于二维速度结构异常紊乱和界面起伏变化的地带,研究表明,壳内界面及速度结构差异、起伏变化的特征与该区域的地震活动性关系密切。

To study the deep tectonic environment and the seismogenic background of the Lushan earthquake in November,2013,we completed an about 410km-long high resolution wide anglereflection/refraction profile,which went across the Yangtze block and the Songpan-Garzêblock and stretched over a number of major faults.By the well-desighed and thorough spread geometry,we obtained the artificial seismic recording data with higher SNR along the profile.Through 7groups of clear crustal P wave phase from 8blasting sections in this paper,we applied the seismic ray travel time forward fitting to construct 2DP wave velocity model of the crust and the upper mantle.Combined with the reliable tracking interval and amplitude variations in different interface phases of the upper crust,we analyzed and discussed the characteristics of crust-mantle velocity distribution,crustal interface distribution and the deep structure of fractures along the profile.According to the 2Dcrust-mantle P wave velocity structure model and previous research results of the study area,this research has obtained the following accomplishments：（1）The 2D crustal velocity structure has obvious zoning characteristics in the transverse direction,and the velocity structure shows low velocity anomaly in the tectonic transformation zone where the velocity structure varies significantly on both sides.The average velocity of the Yangtze block is obviously 0.10km·s-1 higher than that of Songpan-Garzêblock,and the crustal thickness increases gradually from south to north,which presents rapid changes with the discontinuous interface near the tectonic transition zone.（2）The basement along the profile in Songpan-Garzêblock is thinner,almost exposed to the surface near Xiaojin,and the thickest part is approximately 2.5km near Yongfu County.The basement of Sichuan Basin is relatively thicker,about 7.8km;clear changes of the upper crustal velocity structure are located between C1 and C2under the Songpan-Garzêblock.There is a negative gradient velocity structure,whose velocity varies from 5.80 to 5.90km·s-1,0.10～0.20km·s-1 lower than the average velocity around it.The depth range is from 13 to 21km and the thickness is about 8km.（3）The Pn wave of the upper mantle in the profile is difficult to identify,only seen when the SP7 shot point reflects velocity structure of the upper mantle beneath the Yangtze block.There is no Pn wave phase in other shot points.The velocity structure of the upper mantle is clearly different beneath the two blocks.It is 8.00～8.05km·s-1 beneath the Yangtze block while it is 7.95～7.90km·s-1beneath the Songpan-Garzêblock.（4）The 2Dcrust-mantle velocity model results show that along the profile the 2D contoured velocity is disordered with obvious interface undulation changes in the vicinity of the profile distance 150 km,260 km,280 km and 300 km.This phenomenon exactly matches with positions of Longquanshan fault,Jiangyou-Dujiangyan fault,Maowen-Tianquan fault and Xiaojin arc fracture in the geological structure map.In the light of the projection results of aftershocks in this section,the main shock and aftershocks are mainly distributed in the high-low speed transition and negative velocity gradient areas.From the perspective of abnormal thickness in the negative gradient velocity structure,this layer appears to act as a detachment surface in the process of eastward movement of material or the stress on the Qinghai Tibet plateau,which leads to local stress accumulation around the block edge and tectonic activity.In terms of the geological tectonic background of Lushan MS7.0earthquake,the regional tectonic structure of modern earthquakes is controlled by nearly NS local stress field and EW regional stress field,and 2D model shows that the velocity structure has a great reversal in the vicinity of Lushan earthquake epicenter（i.e.about 13～14km）,which presents deep medium conditions leading to strong earthquakes.Therefore,Lushan MS7.0earthquake may be a sudden release of the stress on the southwest section in the Longmenshan fault zone.