The western United States is continuously being deformed by plate tectonic forces. In this study area centered on the southern half of Nevada, the characteristic alternating ranges and valleys of Nevada were formed in the past by stretching of the Earth's crust, and more recently, the northwestward motion of the western US coastal regions relative to the core of the continent. Northwestward plate motion responsible for the San Andreas Fault is partly also accommodated by a fault system in the California/Nevada border area. At 15 to 30 km depth, lower crustal rocks are under high enough temperature and pressure to deform without brittle failure. How faulting and surface motions relate to lower crustal deformation is largely unknown. Our study will use seismic recordings from the University of Nevada, Reno Southern Great Basin Digital Seismic Network to study this process. This network detects signals from worldwide earthquakes after they are modified by the structure of the crust under the study area. By looking at the differences between earthquake signals arriving from different directions, the researchers can find signs of the deformation processes occuring in the lower crust. There are several ways in which the lower crust could behave during stretching and shearing of the crust. mong methods to estimate Moho depth and to detect and measure crustal anisotropy, receiver functions offer one of the best options.
Using a data set collected by the University of Nevada, Reno's short-period, dense, long-running Southern Great Basin Digital Seismic Network, they performed a pilot study showing that high-quality receiver functions can be obtained from the data. The receiver functions show arrivals from the Moho as well as from crustal structure, and indicate the presence of anisotropy, scattering, and 3-dimensional structure. The project plan is to apply multichannel stacking techniques, including ones specific to anisotropy detection, a new bootstrap windowing technique for receiver function calculation, and numerical forward modeling in order to distinguish scattering from anisotropic conversions and to detect crustal structures associated with tectonic processes. Receiver functions will be calibrated against permanent and temporary broadband stations in the area, including a handful of broadband stations colocated with the short-period sensors in the same network. Crustal structure and anisotropy findings will be interpreted in the context of the tectonic history and development of southwest Nevada and eastern California in particular, and lower crustal deformation in general.
Distinguishing which processes are operating in this study area may help other geoscientists who look at deforming continental crust understand similar activity in their field areas. Understanding crustal deformation, including what happens in the lower crust, is foundational for studies of faulting and earthquakes in the US and elsewhere.
