Seismic Anisotropy across the USArray using Surface-wave Arrival Angles

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

The deployment of the USArray transportable array (TA) has allowed seismologists to compile seismic datasets and images with unprecedented detail. Seismic analyses of an extremely rich dataset of some 2000 stations have so far been based on 'standard' seismic data such as travel times. However, other kinds of data such as the particle motion of surface waves provide complementary observations to model smaller details in shallow Earth structure. The short distance between TA stations allows seismologists to trace in detail how surface waves from distant earthquakes bend around as they traverse the TA. This, in turn, can be used to fine-tune models of Earth structure beneath North America and improve scientists' understanding how the continent is made up. Certain mantle rocks exhibit seismic anisotropy where the speed at which seismic waves travel depend on the direction from which they approach. Anisotropy also forms when a plate, such as the North American plate, responds to forces tugging along its edges. Small subsets of TA stations can be used like antennas to measure the properties of this anisotropy. This, in turn, allows scientists to explore mantle processes that form, move and ultimately destroy Earth's plates. This project will collect both types of data. In global tomographic studies, surface-wave arrival angles have proven extremely useful to enhance the imaging of short-wavelength structure. Due to the sparseness of the global seismic networks the interpretation of such data were interpreted using perturbation theory to a great-circle approach. The dense USArray Transportable Array (TA) allows seismologists to observe the structure-induced evolution of the teleseismic wavefield across a large network in unprecedented detail. While Helmholtz tomography utilizes the amplitude and phase of one-component surface wave waveforms, the evolution of surface wave particle motion provides complementary constraints on small-scale heterogeneity beneath North America. While arrival angles support phase-based dispersion analyses of the mantle, observables such as the dip and the ZH ratio are particularly useful to constrain shallow crustal layers. This project will provide a new database of surface-wave particle motion observable. A second aspect of this project focuses on Rayleigh wave azimuthal anisotropy. The dense station spacing of the TA allows a leap-frogging "beam forming" type approach using small subarrays. In comparison to an all-inclusive tomographic joint inversion scheme for anisotropy and heterogeneity, this approach can remove some of the trade-offs between the two. The underlying assumption here is that structure within a subarray is homogeneous, but given the dense station spacing of the TA, this should be a valid assumption. In return, obtained observables will be "in-situ".