Coseismic gravity changes of the 2008 Wenchuan earthquake in China observed by surface gravimetric data

Coseismic gravity changes provide significant information for the study of the mechanisms of large earthquakes and for developing fault models (Sun, 2012). In this research,  coseismic gravity changes of the 2008 Ms8.0 Wenchuan earthquake in China were studied by using gravity observation data and simulation based on a fault model.

Firstly, a fine processing of relative and absolute gravity data from the Longmenshan Gravimetric Network was carried out and observed gravity change of 22 stations near this earthquake were obtained; Secondly ,simulation of coseismic gravity changes was conducted based on half-space dislocation theory using the fault model obtained by Wang et al(2008) through inversion with multiple types of geodetic survey data, including GPS, INSAR, and leveling, and the results were compared with the observations..

It was found that the observed and simulated results are basically consistent, showing that the significant changes are mainly concentrated in the near-rupture zone in the hanging wall of the Yingxiu–Beichuan fault and that the changes decrease rapidly away from the rupture zone. The changes exhibit a positive to negative trend from east to west in the footwall of the Yingxiu–Beichuan fault and have a distribution characterized by alternate positive and negative changes in the hanging wall of the fault. This demonstrates the reliability of the observed results and the reasonableness of the fault model used in this paper.

In the near-rupture zone on the west and east sides of the Yingxiu–Beichuan fault, there are still some differences between the observed and simulated results. The trends in the spatial distribution of these differences exhibit a deviation similar to “phase delay”; in other words, an observed result deviates from the corresponding simulated result in terms of spatial position, which is speculated to be caused by errors in the geometric parameters and in the slip distribution of the fault model. After the slip distribution of  the Pengguan fault model was modified based on the actual surface rupture distribution, the simulated result at the Hongjiawan station near the eastern boundary of the fault model showed greater consistency with the observed result. This indicates that the observed gravity change results in this paper can provide an important reference for further detailed study of the fault model.         

            Fig1.Schematic of the Chengdu Gravimetric Network                        Fig2.Spatial distribution of observed gravity changes and simulated results