Abstract
Ambient noise tomography is a subsurface imaging technique that uses the earth’s background vibrations to extract surface wave velocities. These surface wave velocities can be used to image the subsurface shear wave velocity, identifying low velocity anomalies like magmatic pathways or high velocity plutonic areas, and radial anisotropy, which can distinguish areas of horizontal or vertical structure or mineral alignment. Seismic Tomography has been applied to the central Cascade Volcanic Arc in Washington imaging shallow magma chambers under Mt. Rainier. This study aims to use ambient noise to map the Shasta Cascades' volcanic features in northeastern California. The magma within the area’s major volcanic centers—Lassen Peak, Mt. Shasta, and Medicine Lake Volcano—have been recorded having varying depths and compositions from geophysical, geochemical, and geological data. Ambient noise tomography can refine our understanding of magma chamber structures, tectonic processes, and their linkages to the surrounding geomorphic provinces.
This study utilized free, publicly available data from broadband seismograms recorded between 2006 and 2011 in northern California, southern Oregon, and northwest Nevada. Preprocessing involved filtering, downsampling, and cross-correlation of seismograms to derive dispersion curves for Rayleigh and Love waves. Phase velocity maps were generated using least-squares inversion, with resolution checks performed through checkerboard tests. Depth velocity maps were created by interpolating phase velocities within grid cells and inverted using the neighborhood algorithm to create the horizontal and vertical shear velocity profiles at depths. This enables the calculation of velocity variations and anisotropy changes across the study region.
Shear velocity maps show LVZs beneath Medicine Lake, Lassen Peak, and Mt. Shasta, suggesting magmatic storage in sill-like and dyke structures depending on positive or negative anisotropy, with a potential heat source for hydrothermal activity near Lassen. Additional features, like the extensional strain and a LVZ interpreted as a sedimentary or pyroclastic basin near the Rocky Ledge Fault zone, are also mapped. The findings emphasize the region's multifaceted geological activity, with future studies aimed at refining subsurface dynamics.