Gravity anomaly from faulting on a layered spherical earth with application to central Japan

Abstract

The gravity anomaly resulting from mass redistribution from an earthquake source (point dislocation) is re-explored in a spherical Earth geometry. Using a previously developed method of determining the Green's functions for the static displacement field on a layered Earth, I obtain the corresponding Green's functions for the perturbed gravitational potential and gravity anomaly in a spherical harmonic expansion. The synthesized gravity anomaly includes the effects of changes in the intrinsic density distribution due to divergence of the displacement field and vertical displacement along level surfaces in density (stratification displacement). Numerical examples of the Bouguer gravity anomaly produced on a layered crustal model show that the contribution of intrinsic density changes dominates over the contribution of stratification displacement. Application to central Japan yields a new image of a buried thrust structure suspected of having ruptured in the 1891 Nobi earthquake.