Seismic imaging of crust and upper mantle structure in western North America via surface wave inversion and wavefield depropagation.

摘要

Surface wave analysis of both earthquake and ambient noise seismic data from arrays of broadband seismic stations provides new high resolution images of shear wave velocity of the crust and upper mantle in western North America. In the Yellowstone Hotspot region, new constraints are shown on the high velocity midcrustal layer of the eastern Snake River Plain that represents approximately 10 km of magmatic thickening and subsequent forcing of lower crustal outflow. In the Coast Mountain Batholith area of western British Columbia, the lack of a significant region of high velocities in the lower crust indicates that the foundering of negatively buoyant eclogitic lower crust has been efficient. A high resolution shear velocity model of the Sierra Nevada batholith region finds crustal thickening beneath the batholith, sinking material beneath the central Sierras with adjacent upwelling asthenospheric mantle, and the new image suggests that the Isabella (San Joaquin Valley) anomaly has a quasi-planar NW-SE striking geometry perhaps more consistent with being a Monterey plate slab remnant than an eclogite dominated feature. In addition to the surface wave results, new constraints are found on the sharpness of the 410-km velocity discontinuity via the wave-field continuation approach applied to five regional earthquakes in western North America. The 410-km discontinuity gradient ranges from 7-25 km, indicating the presence of water atop the transition zone in the two regions with large 410 km discontinuity gradient widths.