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

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