Coupled-Mode Waveform Tomography: Imaging Upper Mantle Isotropic and Anisotropic Structure

Seismic anisotropy offers a glimpse into upper mantle dynamics that respond to global tectonics. Techniques to measure upper mantle seismic anisotropy include shear wave splitting and Rayleigh wave tomography. Shear wave splitting measurements lack vertical resolution of anisotropic structure, whereas Rayleigh wave tomography is capable of providing depth constraints on structure with analysis at various frequencies but may not always discriminate between isotropic and anisotropic structure. Here we describe a new method, coupled-mode waveform tomography, which employs normal-mode coupling theory and has the potential to measure isotropic and anisotropic structure independently along with providing lateral and vertical constraints. The most prominent manifestation of isotropic structure in long period data are phase shifts and amplitude variations from the reference model and are the result of like-mode coupling. Anisotropic structure manifests itself as polarization anomalies, known as Quasi-Love and Quasi-Rayleigh waves, and are the result of mixed-mode coupling. Using coupled normal-mode summations, we are able to forward model long period data and invert for isotropic and anisotropic structure independently on a model. Under the assumption that any anisotropy present in the model is of hexagonal symmetry and oriented in the horizontal plane, seismic wave speed in the model can be represented via: δ(α2)/α02 = A + B cos2η + C cos4η and δ(β2)/β02 = D + E cos2η, and the model is able to be parameterized with two isotropic parameters: A, D and three anisotropic parameters: B, C, E. We present preliminary results of the method applied in Cascadia using data from USArray and compare them with previous studies done in the region. Coupled-mode waveform tomography may be performed at several frequencies in order to provide depth constraints on structure and has the potential to be a useful tool in the arsenal to measure upper mantle anisotropy.