EFFECTS OF DATA VARIANCE AND PARAMETERIZATION ON A NEW NORTH AMERICAN 3D UPPER-MANTLE MODEL

With the advent and implementation of continental-scale geologic initiatives such as EarthScope, a plethora of new data is daily being introduced to the scientific community. We have analyzed and fit 5549 regional S and Rayleigh wave trains generated by events from the years 2000 through 2006, including the surge of USArray data. We have combined the new fit constraints with those used for the 3D S-velocity model NA04 to create a new North American model.

However, our new model comes with alternates, resulting from our investigation of the effects of model parameterization (ie. node spacing, flattening, damping) and data heterogeneity. Each alternate provides a fit to our data that is comparable to that of the preferred model. Although the alternates do not cover the entire null space, they do provide a useful guide for assessing the variance of the preferred model, 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 mantle heterogeneity.

To create these alternate models, we remove varying sets of waveforms from our data set before inversion. We identify robust velocity features as those features consistently present with similar properties in all models in the suite. As expected, one of the most prominent robust features is the North American Craton. Our seismic-model suites define the craton with an average velocity of 4.71 km/s at a depth of 70 km. At 70 km, the suite has an average standard deviation of 20 m/s in the craton. At 200 km the craton averages 4.58 km/s with the model suite varying by ~10 m/s. The fast cratonic velocities reach depths around 250 km, with the exception of the Archean Wyoming Province, which does not exhibit high velocities beneath 75 km.

The western United States is 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.