Gravity measurements and their variations before the 2008 Wenchuan earthquake

It was recognized 50 years ago that coseismic gravity changes take place during a large earthquake. In the past 30 years, there have been research efforts devoted to investigating possible associations between gravity variations and earthquakes in China. Before the Wenchuan (M (sub w) 7.9) earthquake, a group of researchers at the Second Crust Monitoring and Application Center of China Earthquake Administration noticed significant gravity changes in a region covering the well-known south-north earthquake belt in China and suggested in 2006 that the possibility for a major earthquake to occur near Wenchuan in either 2007 or 2008 was high. These researchers used gravity changes as the primary earthquake precursor to make the suggestion. In this article, we report the method used for repeated regional gravity survey, the procedures for gravity survey data analysis, and the characteristics of gravity variations before the Wenchuan earthquake that ruptured on 12 May 2008. Although gravity changes at a number of locations in the region surrounding Wenchuan were significant, more research is needed to investigate whether these gravity variations could be viewed as a precursor of the Wenchuan earthquake. Uncertainties in the reported gravity variations include inevitable measurement errors related to ground gravity surveys covering a large region, hydrologic effects on gravity, and the effects of vertical crustal movements on gravity. Other limitations of the data are that the density of the gravity observation stations is coarse and that the time intervals of the surveys were two to three years long. Based on these observations, we make several recommendations about data collection and data analysis procedures that would enhance future earthquake research using gravity monitoring data in China.