Seismic Anisotropy and Hydrous Phases in Subduction Zones

A significant portion of my research thus far has focused on using teleseismic receiver functions as a tool for interrogating seismic anisotropy in the subduction zone mantle wedge (also see Receiver Function Forward Modeling, below). Constraints on the orientation of the anisotropic symmetry axes at depth can allow us to infer the presence of anisotropic hydrous phases in the mantle wedge, such as serpentinites. A particularly exciting result of this work was the inference of a thin layer of serpentinite above the subducting Philippine Sea plate in the Ryukyu subduction zone (see our paper with former IRIS intern, Kimmy McCormack). We seek to better constrain the degree and extent to which the mantle wedge is hydrated and how this manifests itself in the form of hydrous minerals and observable seismic anisotropy. This has important implications for the degree of slab-mantle coupling, wedge thermal structure, and mantle viscosity. (Wirth and Long, 2012; McCormack et al., 2013)

With Maureen Long, I have also looked at seismic anisotropy in the subduction zone mantle wedge (as inferred from shear wave splitting) from a global perspective. (Long and Wirth, 2013)

With Jun Korenaga, I investigated the occurrence of small-scale convection in the mantle wedge above subducting slabs. Although occurring on short length scales, small-scale convection can appreciably influence large-scale flow and have important implications for geophysical observables such as seismic anisotropy and surface heat flow. We developed numerical models to evaluate the likelihood of small-scale convection in the mantle wedge above subducting slabs, using a 3-D single mode approximation. We found that mantle wedge viscosity plays the most significant role in dictating the occurrence and strength of small-scale convective motions in the mantle wedge. Numerical models run with subduction parameters similar to that of northeast Japan, where it has been proposed that small-scale convection may be occurring based on small shear wave splitting delay times and the presence of “hot fingers”, require a mantle wedge viscosity of ~1018 Pa s for significant small-scale convection to occur. (Wirth and Korenaga, 2012)

The Cascadia M9 Project

The M9 Project is a large-scale, multidisciplinary study at the University of Washington that aims to reduce the catastrophic potential of a megathrust earthquake (i.e., of Magnitude 9) in Cascadia. This will be accomplished by integrating modeling of seismic events and subsequent hazards (e.g., tsunamis, liquefaction, and landslides) with advances in earthquake early warning and adaptive community planning. Initially, I will focus on ground motion simulations using a 3-D seismic velocity model that will account for factors such as directivity of the rupture and amplification from sedimentary basins, which are critical in evaluating building response and the potential for liquefaction and landslides. Check out the M9 Project website and our latest progress, here.

Propagation of seismic waves from a hypothetical M9 earthquake scenario in the PNW. (Animation by Nasser Marafi, UW)


Small-Scale Convection in the Mantle Wedge

Cartoon of Earth structure beneath the Ryukyu subduction zone, as inferred from anisotropic receiver function analysis. Arrows indicate the orientation of the anisotropic symmetry axes at depth. (Figure modified from McCormack, Wirth, and Long, 2013.)

(a) Mean temperature and velocity field and (b) out-of-plane perturbation temperature and velocity, at a characteristic wavelength of 200 km. Note small-scale convective motions in the mantle wedge. (Figure modified from Wirth & Korenaga, 2012).