Scientists from Scripps Institution of Oceanography (SIO), University of California, San Diego (UCSD), are analyzing seismograms from the USArray project (part of NSF's Earthscope program) from earthquakes and explosions recorded at regional distances (200 to 2000 km). These data provide new constraints on crust and upper-mantle structure in the western United States. In particular, researchers are studying the upper-mantle seismic phases Pn and Sn and the crustal phases Pg and Lg. The project involves performing Pn tomography using USArray phase picks, producing maps of crustal thickness and both isotropic and anisotropic variations in uppermost mantle P velocity that agree with previous results in their large-scale features but show more detail in many regions. Preliminary results suggest that Sn data in USArray are of comparable quality, enabling joint Pn/Sn inversions to constrain uppermost mantle structure and anisotropy. In many cases, crustal phases such as Lg can be observed to long distances in the USArray dataset, making possible a comprehensive analysis of Pg and Lg times and amplitudes to resolve lower crustal velocities and attenuation. These results complement shear-wave splitting and surface- and body-wave tomography studies by providing superior resolution for structures at lower crustal and uppermost mantle depths. This research will lead to improved models of crust and upper-mantle structure in the western United States, which will be of interest to the geology, tectonics, and geodynamics communities. The project supports the educational program at SIO and UCSD by providing funds for a graduate student. Results will be widely disseminated through publications, conference presentations, and material provided to SIO's education and outreach programs.
During the last ten years, NSF's USArray Project has deployed thousands of seismometers across the United States to record both local and distant earthquakes. Analyses of these data have provided greatly improved images of the structure of Earth's crust and mantle beneath the United States. This project studied earthquakes and quarry blasts recorded by USArray at distances up to 1600 km away (1000 miles), which are particularly useful for probing crustal thickness and the structure of the uppermost mantle, i.e., immediately below the crust at depths of about 35 to 50 kilometers (about 20 to 30 miles). The main result of this research was improved maps of variations in the velocities of seismic waves in the western United States, which provide clues regarding the history and evolution of mountains, faults, and other geological features, including the Colorado Plateau and Yellowstone volcanism. Another important finding was that seismic shear waves do not propagate very well in the upper mantle around the perimeter of the central Great Basin in Nevada, suggesting partially molten rocks in this region. The project provided support for a graduate student, who received her PhD for this research. Four papers were published in peer-reviewed scientific journals describing these results.
