HR: 0800h
AN: S41B-0561    [Abstracts]
TI: Mapping Lateral Heterogeneity with Wavefront Modeling of Rayleigh Waves in the High Lava Plains, Oregon
AU: Snoke, J A
EM: snoke@vt.edu
AF: Virginia Tech, Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall (0420), Blacksburg, VA 24061,
AU: * Warren, L M
EM: lmwarren@email.arizona.edu
AF: University of Arizona, Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721-0077,
AU: James, D E
EM: james@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd., Washington, DC 20015,
AB: We have developed an analytical procedure that takes advantage of high-density broadband seismic networks to test directly for lateral homogeneity, the results from which can be used to map boundaries of laterally homogeneous subregions. This procedure, termed wavefront modeling, is a variant of the first phase of Forsyth's two-plane-wave modeling. In both methods, for a well-recorded teleseism, corrections for wavefront perturbations caused by heterogeneities along the path between the epicenter and the study area are calculated at a fixed period. The two methods differ as to how the perturbed wavefront is modeled. In our wavefront modeling, relative phase delays are calculated among all instrumentally-corrected vertical-component waveforms recorded in the study region. We do a nonlinear inversion of the phase delays to determine the azimuth of the best-fit single-plane-wave wavefront and the average phase velocity across the station array. Contour plots of calculated-minus-predicted wavefront phase delays provide a simple visual display of the degree of lateral homogeneity within the recording network. A high density of stations allows us to subdivide the study area into smaller regions and solve for the wavefront perturbations in each of these, with the objective of identifying laterally homogeneous subregions. 2-D phase-velocity maps derived by this procedure provide a robust input dataset for inversion to obtain the 3-D S-wave velocity structure. We are currently applying this analysis procedure to data recorded on and around the High Lava Plains (HLP) of central and eastern Oregon, southwestern Idaho, and northern Nevada. The HLP "hotspot" track is a prominent volcanic lineament that trends oblique to plate motion from the southeast corner of Oregon in the northern Great Basin to Newberry volcano in the eastern Cascades. The HLP region is instrumented by the 2004--2007 deployment of the USArray Transportable Array, with its ~70 km station spacing on a regular grid, and the 2006--2009 high-density HLP seismic deployment with ~25 km spacing. Preliminary results find that phase velocities in the HLP region are distinct from and slower than the Proterozoic Blue Mountain block to the north at a period of 68 s. Based on the sensitivity kernels, this corresponds to a similar relationship between the S-wave velocities at a depth of about 90 km.
DE: 3260 Inverse theory
DE: 7218 Lithosphere (1236)
DE: 7255 Surface waves and free oscillations
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Seismology [S]
MN: 2007 Fall Meeting