Abstract: In the western Sichuan(100°～105°E,26°～32°N),a mobile array consisting of 297 broadband seismic stations has been deployed by the State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration since October of 2006.Until June of 2008,a total of 690 teleseismic events(m_b>5.0,30°≤Δ≤90°)have been recorded.The May 12 Wenchuan earthquake(M_S8.0)provides an opportunity to test the western Sichuan array.The preliminary data analysis of the May 12 Wenchuan earthquake and its larger aftershocks has been carried out in this study.Our results show: 1)The event parameters of the May 12 Wenchuan earthquake sequence need to be modified and their location error reaches to 8～24km.A more reasonable estimation of the location of the main shock is possibly at the depth of 19km.2)the wavefield analysis of the Lixian earthquake(M_S5.9)of May 16,2008 manifests that the surface waves of this event are not fully developed,and thus its source depth should not be very shallow.The peak values of the ground-motion velocity on the vertical and horizontal component have an abnormal increase by 4 times and more of the normal attenuation,which is related closely to the faults within the range of 200～250km,when the topography and site effects are not considered.3)The preliminary analysis of the crustal and upper mantle structure beneath the Sichuan basin and the Songpan-Ganzi block manifests that the crust beneath the Sichuan basin thickens along the western direction and its lower crust displays the hard structure.The crustal thickness in the northeast of Chengdu City reaches 46km.The crustal structure beneath the Songpan-Ganzi block has complex lateral variations.The crustal thickness in the Wenchuan earthquake source region reaches 52km.In the depth range of 14～20km,its crust has a complex high-velocity structure with the averaged velocity larger than 4.0km/s.The Wenchuan earthquake should be located within the area with high-velocity medium.In the lower middle crust,a low-velocity layer exists with the S-wave velocity of～3.6km/s,which could provide a relaxed boundary condition for the upper crust movement-deformation.This observation is consistent with the abnormal attenuation of ground motion with the epicenter distance obtained from the wavefield measurements.

Key words: M_S8.0 Wenchuan earthquake, seismic wave attenuation, earthquake location, crustal and upper mantle structure, mobile seismic array observation

摘要： 2006年10月,中国地震局地质研究所地震动力学国家重点实验室,在中国四川西部(100°～105°E,26°～32°N)布设了由297个宽频带地震台组成的密集流动地震观测台阵。截至2008年6月,川西台阵已记录远震事件(m_b>5.0,30°≤Δ≤90°)690个。5.12汶川M_S8.0地震的发生为检验川西台阵观测系统的观测能力和工作状况提供了机会。文中对5.12汶川地震及其强余震序列观测记录进行了初步分析。结果表明:1)已发布的5.12汶川地震及其强余震参数有必要进一步修正,它们的震中位置可能存在8～24km误差,主震震源深度为19km的估计可能更接近实际情况;2)2008年5月16日理县(M_S5.9)地震的波场分析表明:该地震面波不发育,它不可能是一个浅源地震,地震动速度波场垂直及水平分量峰值的异常增大与断层密切有关,在不考虑地形地貌、土质条件等因素的情况下,震中距200～250km范围内的地震动速度峰值异常增大可达正常衰减的4倍以上;3)利用接收函数方法对四川盆地和松潘-甘孜地块地壳结构的初步分析表明:四川成都北侧地壳厚度达到46km,四川盆地地壳从西向东逐渐增厚,下地壳显示了较为坚硬的特征,松潘-甘孜地块的地壳结构存在十分复杂的横向变化,汶川地震震源区的地壳厚度达到了52km,深度14～20km范围地壳为复杂的高速结构,其S波平均速度超过4.0km/s,汶川地震应位于这个高速介质区内,其中下地壳为平均S波速度约3.6km/s的低速结构,它提供了其上部地壳横向运动的松弛边界条件。这个观察与地震波场观测给出的地震动速度振幅衰减异常增大吻合。

关键词: 汶川M_S8.0地震, 地震波衰减, 地震定位, 地壳上地幔结构, 流动地震台阵