中文摘要：

针对2014年8月24日世界时10∶20发生在美国加利福尼亚州地区（38.21°,-122.32°）的M 6.0地震,利用美国地区全球定位系统（GPS）双频观测台网和欧洲定轨中心（CODE）绘制的全球电离层地图（GIM）,分析震前上空电离层电子浓度总含量（TEC）的时间和空间变化。结果表明,震前2 d震中附近区域TEC相对于前25 d,TEC下边界限出现明显负异常;通过对全球范围电离层TEC变化的空间分析,震前2 d震中附近区域异常减小的时间持续约40 h,且TEC减小幅度和相对变化较为明显。此次地震前电离层TEC异常减少,与以往亚洲地区地震的统计分析结果较为一致,但对北美地区地震空间前兆规律还有待于积累更多资料及分析。

英文摘要：

On 24 August 2014, a magnitude 6.0 earthquake struck north central California near the coast. This paper first examined temporal and spatial variation of the ionospheric total electron content（TEC） observed by a network of the global positioning system receivers in the US. It was found that the TEC value significantly decreased 2 d before the earthquake around the epicenter compared with that of reference days（previous 25 d low bound or median）. Further, spatial analysis of global TEC variation was performed with the global ionosphere map developed by Center for Orbit Determination in Europe. The results showed that extreme decrease of TEC from a relatively large area of the epicenter to a small area around the epicenter was up to 40 h, and the amplitude and relative variation of extreme decrease of TEC were significant. The extreme decrease of ionospheric TEC observed before the California earthquake agreed with the conclusion from the statistic analysis of earthquakes occurring in Asia. However, more data need to be accumulated to explore the spatial precursor of earthquakes in North America.