中文摘要：

2008年5月12日汶川Ms8．0地震前后，距震中320km甘肃陇南汉王地震台的地电阻率、大地电场、ELF电磁等多种观测手段捕捉到这次地震前后的异常信息．本文采用解析方法，着重探讨汶川8．0级地震前后观测到的电磁场变化特征，从电磁现象来探讨汶川Ms8．0地震发生前后，电磁场长、中、短、临期的异常特征和变化规律．自2003年以来甘东南地区及甘川交界发生4次5．0级以上地震，从空间上看，有从南北地震带中段的西北向西南方向移动的特点，强度逐渐增强，其中，文县5．0和两次岷县5．0级以上地震可以视为汶川8．0级地震的前震；从时间上分析，震前陇南汉王地震台观测到电磁场出现了长趋势变化和短临异常变化，其特征：①2003～2008年，ZD8B地电仪观测的N54°W、N85°E和N73°W测道地电阻率均显示出长趋势变化和短期变化；②视电阻率各向异性度‘S＇＇年变化率在有震年（异常年）是无震年（正常年）的两倍；③ELF电磁仪观测到了（0．5～39Hz）频率段的电场、磁场在汶川8．0级地震及余震序列中异常变化分别比正常月份大1～5个数量级，并记录到电磁同震现象；④大地电场仪（ZD9A-Ⅱ）在汶川震前17个小时也记录到高频临震变化信息．

英文摘要：

Before and after the 12 May Ms8.0 Wenchuan earthquake, we have observed anomalies associated with this event by several geophysical methods, such as DC resistivity, geoelectric field and electromagnetic （ELF）, at the Hanwang station, Longnan, Gansu province 320 km away from the epicenter. In this paper, we use the analytical method to study the features and changing regularities on long-, medium-and short-term scales of the electromagnetic field prior to and after this quake based on these data. Since 2003, four earthquakes of M≥5 occurred in the southeastern Gansu and around the border between Gansu and Sichuan province. These events seemed to shift from the middle section of the North-south seismic zone in central China mainland toward southwest with increasing magnitude. Among them, the Wenxian M5.0 and other two M5. 0 events at Minxian were likely the precursory quakes to the Wenchuan Ms8. 0 in 2008. Corresponding to these quakes, the observations at the Hanwang station exhibit long-term tendency and short-time anomalous changes described as follows. During 2003-2008, the resistivity values measured by the instrument ZD8B in directions N54°W, N85°E and N73°W all showed long- and short-term variations. The annual change rates of the anisotropic degree ＂S＇＂ of apparent resistivity in the years with MS. 0 events （anomalous years） are two times that of the years without M5.0 quakes （normal years）. The anomalies of the electric and magnetic fields on the frequency band （0. 5 - 39 Hz） measured by the instrument ELF during the times of the Wenchuan Ms8.0 and its aftershocks are greater than those in normal times by 1 -5 orders of magnitude. Meanwhile this instrument has also recorded electromagnetic coseismic signal. And 17 hours prior to the main shock, the telluric electric field instrument （ZD9A-II） detected highfrequency anomalous information.