Modeling of residual spheres for subduction zone earthquakes: 1. Apparent slab penetration signatures in the NW Pacific caused by deep diffuse mantle anomalies

Abstract

We have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the Pacific using regional and global mantle velocity models from tomographic inversions. The mantle models explain much of the observed residual sphere data and, to a certain extent, suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns. For most deep events considered, the fast slablike residual sphere anomalies are caused by diffuse heterogeneities, mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab. The region immediately below the deepest earthquakes, depths of 650–1500 km, has an effect usually smaller than or comparable to the effect of other regions of the mantle. Without a proper account of the teleseismic effect, attributing the long-wavelength anomalies of the residual sphere to near-source slab effects alone, or even primarily, is not valid. The fast bands in many observed residual spheres agree with seismicity trends. Once the deep mantle and receiver mantle effects are removed, these may give the approximate orientation, but not the depth extent, of near-source fast velocities. For most deep earthquakes under Japan the predominant fast band is subhorizontal rather than near vertical. This type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data.