Focal mechanisms along inclined earthquake zones, in the Indonesia-Philippine region

Twenty-eight now focal mechanisms were determined for intermediate and deep-focus earthquakes in the Indonesia-Philippine region. At intermediate depths beneath simple planar sections of the Sunda and Philippine arcs the axes of minimum compressive stress have a preferred orientation down the dip of the seismic zone, suggesting that the descending slab of lithosphere is under extension in this depth range. Deep-focus mechanisms beneath the Sunda arc have nearly vertically dipping axes of maximum compressive stress, implying that the descending slab is under compression at great depth. These data are consistent with models in which gravitational body forces are important factors in the dynamics of the lithosphere. Focal mechanisms from events located near regions where the seismic zones deviate strongly from a planar configuration show a striking reorientation of the stress axes. In the Banda Sea and northern Celebes regions the seismicity indicates possible contortions in the underthrust slabs. The orientations of the stress axes in these regions are consistent with such contortion and give additional support to the idea that the lithosphere is a stress guide.