TROPICS

We will supplement the RF analysis with observations of crustal-refracted and
Moho-reflected body waves from earthquake sources (“passive source refraction”, PSR).
This method is not limited to probing the immediate vicinity of the station, and can thus
be used to fill in gaps between stations. Being a higher-frequency technique than RF, it
may also be able to provide information on “Moho character”, which may help in
understanding whether mechanical deformation processes have been active there. PSR
also provides a better control on Vp than does RF. The comobination of these two
tectniques and will yield superior maps of
variations in seismic wave speed and
Poisson’s ratio.
Figure 15. A likely arrangement of seismic
sources to be recorded in 3 years. Local
earthquakes of M>4 and global sources of M>6
are shown.
Love/Rayleigh dispersion (LRD)
(especially in the 20-120 second period
range), is our main tool for measuring
upper mantle shear velocity and
anisotropy. Geographically, it is a
relatively low-resolution technique,
limited to features of ~100 km or greater in
size. But its strength is in being able to
determine mantle shear velocity, especially in the 75-150 km depth range, with great
precision. Surface-wave derived mantle velocity models will provide an estimate of
mantle temperature (since shear velocity strongly varies with temperature) and hence of
mantle density. Surface wave measurements are also likely to detect upper mantle
seismic wave speed inversions associated, say, with lithosphere underthrusting events.
Finally, seismic anisotropy may give insight into mantle flow directions, and hence
the modes of deformation and their regionalization. Anisotropy will be inferred on the
basis of splitting of shear body wave phases (SKS and similar, and local S from slab
sources), which are useful in determining the azimuth of horizontal anisotropy, and also
from differences in Love and Rayleigh wave velocities, which are useful in
discriminating "vertical" from "horizontal" anisotropy. Gaherty (2001), for example, uses
this later technique to test models of active and passive convection on Reykjanes Ridge.
Additional constraints on anisotropic properties within the lithosphere are available
from RF analysis (e.g., Levin et al., 2002ab). Most methods aimed at resolving seismic
anisotropy depend for their success on obtaining observations from as many directions
as possible. Our interest in resolving anisotropic properties largely motivates a 3 yearlong
deployment, to facilitate numerous observations from all major subduction zones.
At least one member of the proposal team is well-versed in each of these techniques.
Levin has previously used RF methodology to map out crustal structure variations in
Kamchatka and Italy (Levin et al., 2002ab, Piana et al. 2008). Menke has previously
employed the PSR technique in Iceland to measure P, S, PmP and SmS traveltimes and
produce “whole-crust” models of that basaltic plateau (Menke et. al., 1998). He also has
developed 3D traveltime inversion codes for modeling such data (Menke, 2005). Both
Menke and Levin have experience in measuring LRD on regional networks of
seismographs, and in using these data to infer mantle velocity structure (Menke and
Levin, 2002). They also both have experience in measuring splitting parameters and
interpreting them in a variety of tectonic settings (Levin et al., 1999, 2004, 2006, 2008),
and have developed special tools that help in cases of complex mantle patterns, as
expected here (Menke and Levin, 2003).
C-24
TPI 6838742