Exploration of Suspected Surface Ruptures of the M_S8.0 Wenchuan Earthquake at Frontal Areas of Long

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The great M_S8.0 Wenchuan earthquake on May 12,2008 was generated by abrupt faulting in the Yingxiu-Beichuan fault along the Longmenshan fault zone. The earthquake not only produced surface ruptures along the Yingxiu-Beichuan and Guanxian-Jiangyou faults,but also surface ruptures,arching of highway pavement,sand-boils and waterspouts in various degrees in areas such as Shifang and Mianzhu on the Chengdu Plain. To understand the shallow geological structures under the surface rupture zone,a 6350m long high-resolution shallow seismic reflection profile in near-EW direction was performed. This profile is located at Shigu town,Shifang city,where a suspected earthquake surface rupture zone was discovered. In this study,a group interval of 3m,shotpoint interval of 18m,and a 300-channel 25-fold observation system were used. In consideration of both near-surface reflections and dipping interface imaging,we adopted the split-spread geometry and asymmetrical zero-offset receiving technique. To better suppress random-noise and raise the signal-to-noise ratio of seismic data,30 times vertical stacking of vibrator signals was made for each common-shot gather after correlation of individual records. By using the above work method and spread geometry,we obtained high-resolution images of structures in the depth range of 15m~800m after data processing. The result shows the existence of buried thrust faults thrusting to the plain area and back-thrust faults under the surface rupture zone. It also shows that the activity of the buried thrust faults may be the main cause for folding and deformation in near-surface strata and coseismic surface rupturing. The great M_S8.0 Wenchuan earthquake on May 12,2008 was generated by abrupt faulting in the Yingxiu-Beichuan fault along the Longmenshan fault zone. The earthquake not only produced surface ruptures along the Yingxiu-Beichuan and Guanxian-Jiangyou faults, but also surface ruptures, arching of highway pavement, sand-boils and waterspouts in various degrees in areas such as Shifang and Mianzhu on the Chengdu Plain. To understand the shallow geological structures under the surface rupture zone, a 6350m long high-resolution shallow seismic reflection profile in This profile is located at Shigu town, Shifang city, where a suspected earthquake surface rupture zone was discovered. In this study, a group interval of 3m, shotpoint interval of 18m, and a 300-channel 25- fold observation system were used. In consideration of both near-surface reflections and dipping interface imaging, we adopted the split-spread geometry and asymmetrical zero-offset receiving technique. To b etter suppress random-noise and raise the signal-to-noise ratio of seismic data, 30 times vertical stacking of vibrator signals was made for each common-shot gather after correlation of individual records. By using the above work method and spread geometry, we The result shows the existence of buried thrust faults thrusting to the plain area and back-thrust faults under the surface rupture zone. It also shows that the activity of the buried thrust faults may be the main cause for folding and deformation in near-surface strata and coseismic surface rupturing.
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