Accurate seismic velocity models are a necessary parameter for seismic hazard analysis in high hazard regions. Therefore, we present the California-Nevada Adjoint Simulations (CANVAS) model, an adjoint waveform tomography model of California and Nevada that is optimized to fit waveforms. We began by testing different starting models for CANVAS to determine what effect starting model choice plays on final inversion results. Though studies have compared synthetic seismograms to compare starting models (e.g., Zhou et al., 2021), this is the first study to compare results after inversion. We chose CSEM_NA (Krischer et al., 2018), SPiRaL (Simmons et al., 2021), and WUS256 (Rodgers et al., 2022) as starting models and iterated each model to a minimum period of 20 seconds. We then compared the results of the final models using five comparison metrics. All five of the comparison metrics showed that the final models all resolved tectonic structure and fit observed data similarly, regardless of the structure or fit of the starting models. Therefore, we conclude that the choice of starting model has minimal effect on the final model results.

Errors in source mechanisms can produce bad synthetic seismograms, which limit the data available to use in adjoint waveform tomography inversions. Many researchers use source mechanisms from the Global Centroid Moment Tensor (GCMT) catalogue. However, the GCMT catalogue has known issues with resolving shallow crustal earthquakes. Since shallow crustal earthquakes dominate CANVAS’s dataset, we inverted for source mechanisms using MTTime, a time-domain moment tensor inversion code. We calculated 3D Green’s functions using an intermediate version of CANVAS that was iterated to a minimum period of 20 seconds. We showed that the inverted source mechanisms greatly improved waveform fit compared to the GCMT solutions, particularly at distant stations. We also demonstrated that 3D Green’s functions have better and more consistent waveform fits compared to solutions inverted using 1D Green’s functions.

Before introducing shorter period data in CANVAS, we replaced the original GCMT solutions with the inverted solutions. Synthetics calculated using the inverted source mechanisms better matched the amplitude of the observed data, and greatly increased the amount of windowed data that could be included in the inversion of CANVAS. Therefore, we advocate for the importance of regularly updating source mechanisms in adjoint waveform tomography workflows. We conclude by proposing CANVAS, an adjoint waveform tomography model of California and Nevada iterated to a minimum period of 12 seconds. Though CANVAS has coarse resolution compared to other crustal tomography models, it can resolve the geographic extents and velocities of well-known tectonic features throughout California and Nevada. The model particularly excels at fitting the dispersed surface waves, showing significant improvement compared to its starting model, WUS256 (Rodgers et al., 2022). Finally, CANVAS also accurately resolves the depth to basement of large basins in California without a priori basin information. This highlights the inherent information contained in seismic waveforms, which full waveform inversion tomography is exceptional at harnessing. We hope that CANVAS can be used as a starting model for smaller regional tomography studies that could eventually become input velocity models for stress inversions or rupture propagation models.

The Red Sea Rift, an archetype of a newly formed ocean basin, is an ideal environment in which to study the controversial processes associated with continental rifting. Different models have been proposed to explain how rifting in the Red Sea evolved; however, accurate constraints on lithospheric structure have not been available to discriminate rifting models. We use the S-wave receiver function technique to produce the first images of the lithosphere-asthenosphere boundary (LAB) structure along the Red Sea and throughout the Arabian Peninsula. Lithospheric thickness varies considerably, with thin lithosphere centered on the rift axis, thickening toward the Arabian interior. Gravity data are well fit by our structural model and indicate that high surface topography along the rift flank is not in isostatic equilibrium, requiring dynamic compensation for its support. While our derived structure is consistent with active rifting processes, previous studies demonstrated that the Red Sea initiated as a passive rift. Therefore, our results suggest a two-stage rifting history, where extension and erosion by flow in the underlying asthenosphere are responsible for variations in LAB depth. LAB topography guides asthenospheric flow beneath western Arabia and the Red Sea, demonstrating the important role lithospheric variations play in the thermal modification of tectonic environments. (c) 2007 Elsevier B.V. All rights reserved.