USArray Transportable Array (TA) and Backbone Array data will be used in high-resolution seismic analyses of deep mantle structure and processes, building on exciting results obtained using data from the initial 5 years of EarthScope deployment. Arizona State University and University of California, Santa Cruz, researchers will extend their collaboration, analyzing teleseismic earthquake signals that traverse the deep mantle in three sub-regions: beneath the southern U.S.-Gulf of Mexico-Central America-Caribbean, under the eastern and central Pacific, and beneath Alaska and the northern Pacific. Waveform stacking, Born-scattering migration, shear-wave splitting and 2.5D/3D waveform modeling procedures will be applied to broadband P and S arrivals, exploiting the density and spatial aperture of the USArray data coverage to resolve fine-scale structures. As the TA progressively moves eastward, special effort will be focused on the mid- and deep mantle beneath Central America and the Gulf of Mexico, where tomographic models indicate a tabular high velocity structure extending through the lower mantle and spreading broadly in the D" region, putatively associated with the Farallon slab and the long history of subduction beneath North America. The velocity gradients, structural continuity, structural fabric, and spatial extent of this feature will be examined using combined travel time and migration methods, with 2.5D and 3D synthetic seismogram modeling being used to quantify the elastic and anisotropic structure. Corresponding downwelling structure deep beneath the subduction zone under Alaska will be analyzed, and the two convergent zone regions will be compared to the eastern Central Pacific structure where there is an abrupt lateral transition from a chemically-distinct large low shear velocity province to adjacent higher velocity structure in the D" region. Continued collaborations with mineral physics and geodynamic colleagues will allow us to explore connections between shallow and deep structures in the mantle and their dynamic implications for tectonics and evolution of the North American continent.
This project applied the seismic data acquired as part of the EarthScope Transportable Array deployment for analysis of global earthquake rupture process investigations for great earthquakes around the world. While the EarthScope project has a primary focus on studying the structure and processes in the upper mantle beneath North America, the seismic recordings made by the Transportable Array can be applied to parallel investigations of remote structures and processes, with multi-use efficiency. We used the recordings from two great earthquakes that happened during the project period to study the short-period seismic radiation from those earthquakes that was recorded as the P waves swept across the Transportable Array stations. We analyzed recordings from the 27 February 2010 Chile (Mw 8.8), 11 March 2011 Japan (Mw 9.0) and the earlier 2007 Pisco, Peru (Mw 8.0) earthquakes. In all three cases, the EarthScope recordings indicate that short-period seismic radiation from bursts of energy release on the plate boundary thrust faults originated deeper than the areas of large slip during the earthquakes. This new observation indicates that strong short-period shaking comes from below the coast line rather than from offshore, and this is important for seismic hazard analysis. It is also important for our understanding of frictioinal properties and stress heterogeneity on megathrust faults in general. Indeed, following up on this discovery, we have found that other great earthquakes, including the 2004 Sumatra earthquake, have similar rupture character. We interpret this as depth-dependent variations in the scale length of patches of unstable sliding friction and associated high stress along the plate boundary contact. At greater depth the scale-length is smaller and coherent bursts of short-period seismic energy are released up on rupture. This finding demonstrates how the EarthScope project lives up to its name in having Earth-wide scientific contributions to fundamental problems, not just resolving structure under North America.
