Co-seismic deformation following the 2007 Bengkulu earthquake constrained by GRACE and GPS observations

Abstract

Gravity changes caused by giant earthquakes can be detected by Gravity Recovery and Climate Experiment (GRACE), which provide new constraints on earthquake ruptures. However, detailed rupture, seismic moment and density/displacement-induced gravity changes are not clear for less than Mw = 8.5 earthquakes. In this paper, the fault parameters of the 2007 Mw = 8.4 Bengkulu earthquake are retrieved from GRACE and GPS data, and the fault slip distribution is inverted using GPS data. Furthermore, the theoretical coseismic displacements and coseismic gravity changes from different slip models are compared with GPS and GRACE data. The results show that the significant positive and negative gravity anomalies with a peak magnitude of −2.0 to 1.3 μgal are extracted from GRACE data. The GRACE-inverted and joint-inverted seismic moment of the Bengkulu earthquake are 3.27×1021 Nm and 3.30×1021 Nm with the rake angle of 108° and 114°, respectively. The GPS-inverted Mw = 8.4 earthquake is mainly dominated by the thrusting with slight right-lateral strike-slip, which is consistent with the focal mechanism. GRACE-observed coseismic gravity changes agree well with the results from the fault models based on the spherically dislocation theories in spatial pattern, but are larger than model-estimated results in magnitude. The coseismic gravity changes caused by the density change are basically same as those caused by the vertical displacement in the magnitude of order, which are −0.8 to 0.2 μgal and −0.2 to 1.4 μgal for the Caltech model, −0.9 to 0.2 μgal and −0.5 to 1.3 μgal for the USGS model, and −0.9 to 0.2 μgal and −0.3 to 1.3 μgal for the GPS-inverted layered model. In addition, both the near-field and the far-field displacements calculated from the Caltech model and GPS-inverted layered model are in good agreement with the GPS observations, whereas the USGS model has good agreement in the far-field and poor agreement in the near-field with the GPS observations, especially in the Pagai Selatan area.