How tectonically active is the Sierra Nevada today? To what extent is Great Basin extension driven internally by gravitational collapse? How do the Sierra Nevada and Great Basin interact? What is the role of the Sierra Nevada in controlling collapse and extension of the Great Basin? These questions are central to the mission of the EarthScope Program, as they represent key parts to the puzzle of how the Pacific-North America plate boundary is evolving, and what processes are responsible. Yet, despite recent advances in geodesy, we still do not have answers to simple questions that any schoolchild might ask: "How fast is the Sierra Nevada going up today?"; "Is the Great Basin going up or down?"
To address these questions, this 3-year project will accurately characterize uplift and collapse in the Sierra Nevada - Great Basin region by exploiting high precision GPS data from EarthScope's Plate Boundary Observatory to measure vertical motion of the Earth's crust. These measurements will provide crucial first-order evidence on the underlying processes. Measuring vertical motion is important because it indicates the flux of potential energy, a fundamental characteristic of tectonic/mantle dynamic processes, including isostasy, orogeny (mountain building), gravitational collapse, and mantle upwelling. To make the vertical GPS data physically meaningful, methods are being implemented that will accurately reference the changes in surface height to the center of the Earth (the Earth center of mass). In order to reduce the errors in vertical rates to a useful level (sub-millimeter per year), the investigators are making use of several recent advances in GPS observable modeling, including global-scale ambiguity resolution, absolute antenna calibrations, satellite transmitter phase center variations, the modeling of non-gravitational forces on the GPS satellites, and atmospheric refraction. Once an accurate time series of vertical position is derived for each GPS station, the spatial and temporal patterns of vertical variations are then interpreted in terms of both the tectonic processes of interest, as well as non-tectonic effects (such as atmospheric pressure loading and hydrological effects).
The primary data set for this project is being acquired by the specially designed "PBO Transect" of densely-spaced GPS stations, spanning east-west the entire Great Basin and Sierra Nevada range at latitude 39_-40_, together with the more broadly spaced PBO network covering the Pacific-North American plate boundary and beyond. In addition, the University of Nevada, Reno, has since 2004 built and operated the ~300-station "MAGNET" semi-continuous GPS network, which provides data for this project. Although the measurements from MAGNET are not continuous all of the time, pilot studies show that they can accurately track seasonal and secular change in GPS station height; thus MAGNET complements the continuous PBO measurements by providing much more spatial detail to the complex patterns of vertical motions. By the end of this project (mid-2012), pilot studies predict that vertical velocities for most GPS stations have an accuracy of <0.5 mm/yr, at a level that is useful for scientific interpretation, and toward answering the project's research questions.