Three-dimensional wavefield imaging of data from the USArray: New constraints on the geometry of the Farallon slab

Abstract

This paper presents preliminary results from direct imaging of P to S conversion data from the Earthscope Transportable Array (TA). Input data are receiver function estimates from the Earthscope Automated Receiver Function Survey (EARS). The waveforms from EARS were first stacked to produce composite events from 60 different source regions defined by a radial grid with a center in the western United States (U.S.). The composite waveforms were imaged with a three-dimensional, prestack migration method using a plane wave decomposition and an inverse generalized Radon transform algorithm to yield estimates of radial and transverse component scattering potential from each composite event. Data deficiencies limited this processing to 50 of the composite events. The migrated results from these 50 events were stacked to produce a final threedimensional image volume examined in this paper through three-dimensional visualization techniques. The crust-mantle boundary, the 410 km, and 660 km discontinuities are imaged. An erratic negative conversion at the depth that has been suggested as the seismic lithosphere-asthenosphere boundary is observed in these results, but I avoid interpreting this feature as it may be biased by interference with first-order, free-surface multiples from the Moho. The most significant new feature imaged by this technique is a series of east-dipping conversions directly above the 410 km discontinuity. This feature is imaged in the region immediately above the transition zone across the entire western U.S., although it becomes partly obscured in Arizona by interference with what I interpret as sediment reverberations that contaminate some stations in southern California and the Rio Grande Rift. Overlaying these results with published tomography models suggests that this feature is linked to the Juan de Fuca-Farallon slab. I follow this hypothesis to build a three-dimensional model of the top of the Farallon slab that honors the new data and results from published tomography models. The model clarifies that these dipping features are consistent with a bending plate model of the slab. Using a simple kinematic model of the geometry of the slab window linked to the widening of the San Andreas system, I argue that the same dipping features seen under Nevada are consistent with those seen to the north, suggesting that there is a continuous, relatively smooth slab-related boundary layer under most of the western U.S. © 2011 Geological Society of America.