The EarthScope Transportable Array is collecting seismic data on a previously unprecedented continent-wide scale with station locations ~70 km apart. Since the thickness of the crust across the U.S. varies from ~20 - 55 km, such dense station spacing still offers mostly spot illumination of the crust under each station when teleseismic earthquake waves (with steep incidence angles under the station) are used for imaging structure. With such illumination, the crustal structure is typically assumed to be locally one-dimensional (locally horizontal interfaces) when teleseismic body waves and ambient noise and teleseismic surface waves are used. Crustal material is also typically assumed to be seismically isotropic (direction-independent wave propagation).
The receiver function technique illuminates interfaces under a seismic station where an incident compressional wave partly converts to a shear wave. If an interface is not horizontal, or if a contrast in seismic anisotropy is present, the converted waves show a systematic and potentially high-amplitude signal that varies with the azimuth of arrival of the incident wave. Across the Transportable Array, roughly 20% of the radial and 40% of the transverse component total receiver function signal amplitude consists of arrivals with a systematic variation matching dipping or anisotropic interfaces, and the signal strength is well correlated to tectonic provinces. Dipping crustal interfaces as well as crustal anisotropy result from deformation of the crust. The azimuthally varying signal in receiver functions can therefore be used to map crustal deformation. Rather than attempting to fit the waveform exactly by varying interface dip and/or anisotropy, which is a highly non-unique process, this project maps the signal strength, depth, and orientation of interfaces and anisotropic layers (somewhat akin to mapping delay time and fast orientation in split SKS waves, except that the receiver function method also offers depth information). The resulting maps are compared to surface geology and models or geological histories of crustal deformation and thus offer the chance for hypothesis testing related to those models or histories. The project is based on close collaboration between a seismologist and a geologist and engages undergraduate research assistants through RESESS (Research Experiences in Solid Earth Sciences for Students, a program providing research opportunities for minority undergraduate interns by pairing them with mentors).
