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.

California earthquake  /  ionospheric TEC  /  spatial precursor of earthquake