HR: 0800h
AN: S41B-0562    [Abstracts]
TI: Upper mantle S-velocity variance beneath North America
AU: * Bedle, H
EM: heather@earth.northwestern.edu
AF: Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr, Evanston, IL 60208,
AU: van der Lee, S
EM: suzan@earth.northwestern.edu
AF: Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Dr, Evanston, IL 60208,
AB: With the implementation of continental-scale geologic initiatives such as EarthScope, increasing amounts of seismic data are continuously becoming available. We have analyzed and fit regional S and Rayleigh wave trains generated by events from the years 2000 through 2006, including waveforms from Transportable Array stations of USArray. We have combined the constraints provided by the new fits with those used for the 3D S-velocity model NA04 to create a new North American model, NA07. Additionally, we investigate the effects of data heterogeneity and model parameterization (ie. node spacing, flattening, damping) on the tomographic model. This allows for an appreciation and quantification of reasonable variances for NA07. Alternate models within NA07's estimated variance do not cover the entire null space, but they do provide a useful guide for model uncertainty, allowing for improved interpretations of the compositional, mineralogical and thermal state of the crust and mantle, which will ultimately provide more accurate insights concerning the causes and effects of upper mantle heterogeneity. Here we present model NA07, as well as the alternate models, which are created by removing varying sets of waveforms (such as USArray data) from our data set before inversion. Robust velocity features are identified as those features consistently modeled with similar properties in all models in the suite. The most prominently robust feature modeled is the North American Craton. Our current seismic-model suites define the craton with an average velocity of 4.71 km/s at a depth of 90 km. At 90 km, the suite has an average standard deviation of 17 m/s in the craton. At 200 km the craton averages 4.59 km/s with the model suite varying by 10 m/s. This significant decrease with depth within the lithosphere in the cratonic average for the S-velocity is consistent with an average steady-state geotherm inferred for regions with low heat flow around 40 mW/m². The North American craton appears to be no thicker than about 250 km. The western United States exhibits a very thin lithosphere, underlain by a very low S-velocity asthenosphere that appears to extend down to depths around 150 km. Its average velocities range from 4.12 km/s at a depth of 80 km to 4.25 km/s at 120 km, but the lateral variations have standard deviations of 150 m/s at 80 km and 90 m/s at 120 km. In this region, the slowest velocities are modeled beneath the Northern Basin and Range and Gulf of California.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 8103 Continental cratons
DE: 9350 North America
SC: Seismology [S]
MN: 2007 Fall Meeting