The influence of the Moho in local and teleseismic wavefield simulations

Abstract:

Earth's lithospheric structure exhibits variations in geometry---such as surface topography, Moho topography, discontinuities across faults, and subducting slabs---as well as variations in elastic properties---such as differences in composition between a sedimentary basin and an exhumed batholith. Three-dimensional models of Earth's structure need to account for these known geometric and volumetric variations. The Moho is a globally identifiable discontinuity between the lower crust and uppermost mantle. We review recent efforts on (1) constructing the Moho surface from existing data sets, (2) forward-modeling the local and teleseismic wavefield and its interactions with the Moho, and (3) prospects for simultaneously iteratively improving the Moho surface and surrounding crust and mantle velocity models, by using adjoint-based imaging techniques. Estimating the Moho is particularly challenging in regions spanning oceanic and continental crust such as southern California or Alaska, where the Moho depth varies from 10 km to 50 km (below sea level). The Moho surfaces in southern California and Alaska have been estimated from receiver functions, gravity data, and wide-angle active-source data (PmP and Pn). The influence of the Moho surface on the regional seismic wavefield can be quantified with 3D wavefield simulations using local earthquakes, with slab earthquake (if present) enhancing coverage. Recent efforts have combined frequency-wavenumber methods for teleseismic body wave propagation with 3D wavefield simulation methods for near-station body wave propagation and reverberations within the crust. These hybrid methods allow us to model crustal interactions with teleseismic body waves. Using a 2D synthetic problem we demonstrate how the Moho topography can be treated as unknown parameters within the same simulation-based framework that is used to iteratively improve tomographic models of the crust and upper mantle.