Seismic Observations from the Random Wavefield: A New Tool for High-Resolution Seismology in the Context of EarthScope

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0450082RitzwollerThe proposed research will advance and apply a new method of surface-wave tomography based on measurements of surface wave dispersion that are independent of earthquake occurrence. The method is based on the extraction of surface-wave Green functions by cross-correlating sequences of ambient or background seismic noise that form part of the random seismic wavefield. Proof-of-concept experiments for stations separated by various distances within the US demonstrate that dispersion measurements made with this method are repeatable and reproduce information provided by traditional measurements obtained on ballistic waves following earthquakes. Noise-based surface- wave measurements allow us to construct tomographic images of the principal geological units beneath California, with low-speed anomalies corresponding to the main sedimentary basins and high-speed anomalies corresponding to the igneous cores of the major mountain ranges. This method can improve the resolution and fidelity of crustal images obtained from surface-wave analyses, in particular, in the context of a continental scale array such as the USARRAY component of EarthScope, where numerous inter-station surface-wave measurements promise to focus seismic images and models of the crust and upper mantle beneath the US. The proposed work has two principal objectives. The first is to improve understanding of background seismic noise and to continue to develop a robust method to measure surface-wave dispersion from two-station cross-correlation. The second objective is to apply the method to analog and emerging USARRAY data. Analysis will concentrate on three spatial scales: (1) continental-scale, analogous to the permanent component of USARRAY, using data from the US National Seismic Network (USNSN) and the Global Seismic Network (GSN), as well as the broad-band component of the emerging Advanced National Seismic System (ANSS); (2) regional-scale, analogous to the transportable component of USARRAY, using data from broad-band arrays in California and the Rocky Mountains; and (3) local-scale, analogous to deployments of the flexible component of USARRAY and PASSCAL experiments, using data from arrays in Southern California and the Yellowstone areas.