Isotropic Crustal Velocity beneath Central Idaho/ Eastern Oregon using Ambient Seismic Noise

We present a new, high resolution isotropic crustal velocity model beneath central Idaho and eastern Oregon. We produced the velocity model from vertical component Rayleigh wave group velocity measurements on data from the IDaho/ORegon (IDOR) Passive seismic network, 85 3-component broadband seismic stations, using ambient noise tomography and the method of Gallego et. al (2010). We calculated inter-station group velocities in narrow frequency bands from travel-time measurements of the stacked cross-correlations (bandpass filtered between 2 and 30 seconds), which we used to invert for velocity structure beneath the network. Goals of our work include refining models of crustal structure in the accreted Blue Mountain terranes in the western study area; determining the depth extent of the Salmon River Suture/ West Idaho Shear Zone (WISZ), which crosses north-south through the middle of the network; determining the architecture of the Idaho batholith, an extensive largely crustal derived pluton; and examining the nature of the autochthonous (?) North American crust and lithosphere beneath the batholith. We cross-correlated seismograms for each IDOR Passive station pair in 24 hr segments, and then band-passed, removed mean and trend, whitened, and progressively stacked these cross-correlated seismograms for the number of days of available data for each station pair. We made travel-time measurements in relative width, narrow frequency bands by picking the peak of the envelope of the stacked seismograms. To overcome the loss of temporal resolution in the narrow bands, we measured 1-sided cross-correlated seismograms made by adding the negative side to the positive side of the stacked seismograms, and multiplying by a step function. We derived Rayleigh wave group velocity models for each frequency band using the least-squares inversion method of Tarantola (2005). We determined depth sensitivity of the various frequency bands from group velocity dispersion curves. Similarly, we created the 1-D starting model by constructing the dispersion curve of the average velocity in each period band, and then inverting for velocity with depth. The IDOR Passive seismic network spanned a roughly rectangular region about 450 km east-west by 170 km north-south. Basin and range style extension dominates the tectonics of the eastern half of the network in eastern Idaho just north of the Snake River Plain. Stations in the eastern portion of the network were located primarily on Paleozoic shelf sequences of North America and on extensive Challis volcanics; the central portion of the network lies on the Atlanta lobe of the Idaho Batholith. The western half of the network crossed the WISZ, which is thought to be a lithospheric boundary between Precambrian North America to the east and accreted island arc terranes to the west. The western IDOR Passive network lies on the Blue Mountain terranes and their Columbia River Basalt cover in northeastern Oregon. Average station spacing was around 10 km between a dense E-W central line of 29 stations, and ~30 km between other stations.