A better understanding of the structure and tectonic history of the lithosphere in the continental US relies heavily in high-resolution images from the surface down to hundreds of kilometers. Traditionally, these images are obtained from records of seismic waves excited by earthquakes. In the last few years, scientists have started using the ambient seismic field (ASF) for high-resolution tomographic imaging. Contrary to earthquake sources, the ASF is an almost imperceptible, ubiquitous signal that is recorded by seismic stations all around the world, and thus provides an ideal source for tomographic imaging. This proposal will take advantage of the ASF to perform seismic tomography in the continental US, but in addition to the typical velocity structure, the attenuation structure will also be estimated. Key constraints on the thermal and chemical structure of the lithosphere can be obtained by combining these two observables (velocity and attenuation).
We propose to use a Virtual Earthquake Approach (VEA) for velocity and attenuation tomography. This method has proved very useful for basin amplification analysis and ground motion prediction of future large earthquakes in southern California and Japan. The VEA treats one distant seismic station as a virtual source, and both the arrival times and relative amplitudes of fundamental surface waves can be estimated by computing the impulse response functions (IRF) with respect to a dense seismic array in a region of interest. The resulting waveforms obtained thru this approach are similar to traditional earthquake-based methods with the advantage of not being limited by the number and location of earthquakes. The resolution of our velocity and attenuation images will depend on station density rather than earthquake location. The choice of the VEA approach provides additional advantages, including a more complete theoretical framework to characterize the amplitudes, the lack of azimuthal averaging and a reduction of the potential bias due to ASF source distribution and multiple phases (body waves and higher mode surface waves). Accounting for elastic focusing and defocusing is similar to what is required for earthquake-based methods. The proposed research will exploit the vast information available from the ASF and take advantage of the distribution of seismic stations along the continental US to constrain the velocity and attenuation structure of the lithosphere, for example comparing attenuation in the Western and Eastern US, or between tectonically active California and the New Madrid Seismic Zone.
