Gravity Changes Before and After the 2008 Mw 7.9 Wenchuan Earthquake at Pixian Absolute Gravity Station in More Than a Decade

Abstract

Absolute gravity repeated measurements at Pixian station, about 35 km away from the epicenter of the 2008 Wenchuan Mw 7.9 earthquake, have been carried out over more than a decade. Four pre-earthquake measurements at the Pixian station show ~ 30 μGals increase from 2002 to 2008, but 13 measurements since 2008 shows that the trend of gravity increase ended after the Wenchuan earthquake. We analyzed the gravity effects from ground vertical motions using data from continuous GPS stations collocated with the Pixian absolute gravimetry station, and surficial and hydrological processes using local hydrological data. We found that these effects are much smaller than the observed gravity increase before the earthquakes, the continuously gravity increase before the 2008 Wenchuan earthquake cannot be explained by the groundwater level change and vertical deformation and related superficial reasons. All absolute gravity was measured by the FG-5 gravimeter. In this study, the high-precision gravity change revealed that the pre-earthquake gravity increase may be caused by strain and mass (fluid) transfer in broad seismogenic source regions. Further studies are needed to validate such pre-earthquake gravity changes, which however are difficult to be resolved from space-based gravity models.