How much does a high-velocity slab contribute to the apparent non-double-couple components in deep-focus earthquakes?

Abstract

Abstract It is not established yet whether the presence of the subducting slab as an unmodeled high-velocity zone is a partial contributor to the non-double-couple behavior observed in intermediate-depth and deep earthquakes. In order to shed light on this question, we carried out simple simulations of momenttensor inversion, locating a seismic source of pure double-couple type in a disk-shaped high-velocity zone surrounded by an infinite homogeneous three-dimensional space and inverting the far-field P seismograms for the moment tensor without taking into account the presence of the high-velocity zone. The results imply that the unmodeled high-velocity slab can give rise to such spurious non-double-couple behavior as is observed in natural earthquakes when the characteristic wavelength of the seismic waves analyzed is comparable to the thickness of the slab. When the characteristic wavelength exceeds the thickness of the slab by far, however, the apparent non-double-couple behavior is influenced not only by the slab but also by all the large-scale features in the subduction zone, and the effects of the high-velocity anomaly in the slab tend to be neutralized by the surrounding mantle, especially by the low-velocity anomaly in the mantle wedge. Thus, it is less likely that the presence of the high-velocity slab is a significant contributor to the apparent non-double-couple components observed in lower frequency bands, e.g., in the Harvard CMT solutions.