Multi-mode phase speed distribution in North America using eikonal tomography

Abstract:

The North American continent encompasses a variety of complex structural features, including tectonically active regions and stable cratons. The deployment of the high-density Transportable Array (USArray) has facilitated seismological studies to unravel the crust and mantle structure in North America. To delineate the deep root of cratonic lithosphere and asthenosphere, the use of higher-mode surface waves is essential.

In this study, we employ the single-station method of multi-mode dispersion measurements based on a fully nonlinear waveform fitting (Yoshizawa & Ekström, 2010) to extract the multi-mode phase speeds for each source-receiver path for permanent and temporary stations in North America. These measurements are then applied to the method of eikonal tomography (Lin et al., 2009). In this hybrid approach, at first, multi-mode phase speeds for all stations from a seismic event are used to reconstruct travel-time fields for each mode and period. Then, the phase speed distributions for each event derived from the lateral gradient of travel time fields are stacked for all events to reconstruct final phase speed models.

Our hybrid method is applied to teleseismic events (M ≥ 6.0) from 2007 to 2015, including over 300 events for Rayleigh waves and over 400 events for Love waves. The contiguous United States can be covered with a large number of ray paths (over 75000 paths for the fundamental-mode Rayleigh wave at 100.0 s), which allows us to reconstruct phase speed models with 3.0° x 3.0° grids.

We could successfully retrieved phase speed maps of Love and Rayleigh waves for the fundamental-mode and up to the 4th higher modes. The images of small-scale features, such as the Snake River Plains, Colorado Plateau and Rio Grande Rift, are well imaged in eikonal models for the fundamental mode. In the higher-mode models, some large-scale tectonic features are imaged, such as the fast anomalies related to deep cratonic root and/or the subducted Farallon plate, but the interpretation of higher-mode phase speed maps are not straightforward, due to their complicated vertical sensitivities. These multi-mode phase speed maps will be used to map a new 3-D anisotropic S wave model in a wide depth range, including the continental lithosphere and asthenosphere, and underlying transition zone.