The Role of 3D Crust and Mantle Structure in Predicting North-American Seismograms

We compare the predictive capabilities for seismic waveforms of several 3D tomographic models for the North American upper mantle in an attempt to more narrowly identify the data and theory necessary to model upper mantle structure in efficient ways. To this end, we compare recent (1995-2004) North American S and Rayleigh wave trains to synthetic wave trains computed with different velocity models and different modeling strategies. Synthetics computed, using great-circle averaging, for 3D model NA00 match the observed recent waveforms better than those for standard 1D continental and oceanic models. Despite NA00 being significantly less smooth than global models, for most waveforms NA00 does not account for all of the observed positive and negative delays. The improvements in fundamental mode matching provided by NA00 diminish with increasing frequencies. We identify two possible reasons: 1) High-frequency energy cannot be modeled by great-circle averaging, or 2) the model parameterization is not suitable for shallow, crustal depths. We will investigate the role of scattering and of the crustal model in this waveform matching. Data from Earthscope's USArray will be sufficiently dense to model lateral variations in the crust and depth to the Moho discontinuity. We prepare for these data by targeting regional crustal structures, such as the Illinois Basin. To allow for the resolution of crustal structures, we adopt a new grid capable of supporting fine crustal structure, and analyze high-frequency Rayleigh waveforms from regional earthquakes down to magnitudes of 4. Combining regional surface waves with the data that produced model NA00, which include the constraints on crustal structure by Chulick and Mooney (2002), we aim at 1) learning more about the crust in targeted regions (e.g. the Illinois Basin), 2) a better separation of crust and upper-mantle structures, and 3) the importance of the 3D crustal structure in predicting North-American seismograms.