摘要:
2008年5月12日在中国四川省境内龙门山断裂带发生了8级巨大地震．在其主震前后随着震源区附近应力状态的改变，地壳介质的物性是否也存在相应的异常变化过程？该问题成为汶川地震研究的热点科学问题之一．本研究搜集和整理了中国地震局四川数字地震台网2001年1月1日-2010年5月31日产出的震相观测报告，和中国地震局地球物理研究所流动数字地震台网2008年5月12日——9月30日产出的震相观测报告，采用多台和达法和多地震联合测定法，根据入选的直达波Pg和Sg的震相数据，重点研究了龙门山断裂及其附近地区的P波和S波视速度的时间变化特征，并与波速比的时间变化特征进行了对比分析．为保证计算结果的可靠性和稳定性，对所收集到的数据进行了较为严格的筛选和限定．研究发现，龙门山断裂及其附近地区，在汶川地震前P波和S波视速度存在4年左右的明显降低过程，到震前约半年出现快速恢复．在震前P波和S波的视速度明显降低的过程中，波速比的低值异常过程仅为2年多，且幅度小持续时间较短．在波速比和视速度异常发展的进程中，进入发震中短期异常快速恢复是两者的共同特征．
Abstract:
On May 12, 2008, a great earthquake of MS8.0 occurred on Longmenshan fault, Sichuan, China. With the stress change near-source region before and after the mainshock, would there have been corresponding physical property change of crust media? It became one of the hot questions in the research of Wenchuan earthquake. In this study we collected seismic phase data from January 1, 2001, to May 31, 2010, reported by Sichuan Digital Station Network of China Earthquake Administration (CEA), and the phase data from May 12 to September 30, 2008, reported by the Mobile Digital Seismic Network of Institute of Geophysics, CEA. We processed these data using the Wadati method. Based on the phase arrival times of selected direct wave Pg and Sg we paid special attention to the study of temporal variation of wave apparent velocity, and compared the velocity variation with the temporal variation of wave velocity ratio. To ensure the accuracy and stability of the calculated result, we carefully winnowed the data. Our research result shows that, in Longmenshan fault zone and its surrounding area, there was a 4 years' obvious decrease process of P-and S-wave apparent velocity before the Wenchuan earthquake, and at the time of half year before the earthquake an abrupt return appeared. Different from apparent velocity, the anomalous variation of wave velocity ratio showed a low amplitude decrease and lasted only for more than 2 years. But both wave velocity ratio and apparent velocity decrease returned fast at the time of half year before the mainshock.
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