中文摘要：

美国地质调查局（USGS）与各有关大学,在美国西部、特别在圣安德列斯断层及其临近地区布设了包括地磁在内的地球物理观测网,用以监测该地区的地震活动.地磁观测资料的分析结果显示：5.2～7.3级地震,在离震中3～50 km的观测点与台站上,得到地磁总强度的变化异常幅度为0.3 ～6 nT.震磁研究结果表明：某些地震存在震磁前兆信息,因此震磁效应的观测研究是探索地震预测的一种手段.深入分析了2004年9月28日帕克菲尔德6.0级地震前后7个台站的地磁数据,结果表明：该地震的同震震磁效应为-0.4～0.3nT,这是由压磁机制引起的;而该地震较长时间的震磁异常为-5.0～1.0nT,该异常与局部地质构造及其活动、应力变化状况、地下介质的电磁性质等因素有关.

英文摘要：

The U.S.Geological Survey （USGS） and the relative universities have set up the geophysical observation networks,including geomagnetic observation in the western America,particularly in the San Andreas Fault and its adjacent area in order to monitor the seismic activities.For the M5.2 ～ 7.3 earthquakes,the analytical results of the geomagnetic data show that the change anomalies amplitude of the geomagnetic total intensity were 0.3 ～6 nT observed at the sites and stations which were apart 3 ～50 km from these earthquakes epicenters.The seismomagnetic research results show that there are seismomagnetic precursors for some earthquakes.Therefore,the observation and research on the seismomagnetic effects is a method to study earthquake prediction.The results through carefully analyzing the geomagnetic data at seven stations before and after Parkfield M6.0 earthquake on Sep.28,2004 show that the co-seismomagnetic effects are-0.4 ～ 0.3 nT,which are caused by piezomagnetic mechanism.The seismomagnetic anomalies with longer time for this earthquake are-5.0 ～ 1.0 nT,which are related to the factors such as the local geological structure and its activity,the state of stress changes,the electromagnetic properties of the underground media.