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.
The research carried out under this grant used seismic recordings from EarthScope's USArray network of seismometers. The focus was to use data from distant earthquakes which have long paths the energy takes from the earthquakes to the USArray sensors located in the United States. These paths travel through Earth's deep interior and thus afford us the opportunity to image the deep planet in higher resolution that previously possible. We developed a software suite that allowed efficient processing of the huge volume of seismic data recorded by the USArray. Our studies focused on the very deep planet, for example Earth's core mantle boundary, below it in the outer core, above it in the lowermost mantle, as well as some focus more shallow in Earth's upper mantle. The USArray data, when used in a network (array) fashion, provided information about small scale heterogeneity, fine scale layering, and ultimately inference on dynamical motions in the interior. The USArray data afforded our detailed analysis of several different seismic waves in the Earth, which in turn permit detailed investigation of specific regions or boundaries in the planet. We summarize key seismic waves, their region of study/sampling, and results below: ScS: this phase reflects of the core-mantle boundary (CMB). We developed an algorithm that removes ScS from the seismogram to be able to study any small amplitude reflections off of fine scale layering (if present) near the CMB, such as ultra-low velocity zones. This work is about to be submitted to JGR. S: S waves in the deep mantle can split into two waves if they refract along sharp boundaries. We used multipathed S waves to image a sharp top to the large low shear velocity province beneath the Pacific Ocean, and related this thermochemical structure to a plume feeding Hawaii. This work was submitted to Science, but now is in preparation for EPSL. P'P': This is a P wave that goes all the wave through the center of the planet, then hits the underside of the surface, and goes all the way back through the center. It is an important probe for layering in the Earth near the surface. We have completed a study of this with USArray data to study the sharpness and/or irregularity of upper mantle discontinuities associated with the phase changes in the olivine mineral system. These seismic waves (and others), combined with the abundance of the high-quality USArray data have allowed us to make several high resolution studies of the deep planet.
