Ailin Mao

Noise in GPS coordinate time series

We assess the noise characteristics in time series of daily position estimates for 23 globally distributed Global Positioning System (GPS) stations with 3 years of data, using spectral analysis and Maximum Likelihood Estimation. A combination of white noise and flicker noise appears to be the best model for the noise characteristics of all three position components. Both white and flicker noise amplitudes are smallest in the north component and largest in the vertical component. The white noise part of the vertical component is higher for tropical stations (+23 o latitude) compared to midlatitude stations. Velocity error in a GPS coordinate time series may be underestimated by factors of 5-11 if a pure white noise model is assumed.

How rigid is the stable interior of the North American Plate

We analyze data from eight permanent GPS stations broadly distributed through the interior of the North American plate, and use the resulting velocities to estimate an Euler vector describing motion of “stable” North America as a single rigid plate. The site velocities fit the single plate model with a mean residual of 1.3 mm/yr. The residuals do not appear to reflect post-glacial rebound, and tests for differential motion between eastern and western North America at the New Madrid seismic zone show no resolvable motion within uncertainties. The residuals likely reflect observational error, and thus our estimate of the stability of the plate interior is likely an upper bound.

Continuous monitoring of surface deformation at Long Valley Caldera, California, with GPS

Continuous Global Positioning System (GPS) measurements at Long Valley Caldera, an active volcanic region in east central California, have been made on the south side of the resurgent dome since early 1993. A site on the north side of the dome was added in late 1994. Special adaptations for autonomous operation in remote regions and enhanced vertical precision were made. The data record ongoing volcanic deformation consistent with uplift and expansion of the surface above a shallow magma chamber. Measurement precisions (1 standard error) for “absolute” position coordinates, i.e., relative to a global reference frame, are 3–4 mm (north), 5–6 mm (east), and 10–12 mm (vertical) using 24 hour solutions. Corresponding velocity uncertainties for a 12 month period are about 2 mm/yr in the horizontal components and 3–4 mm/yr in the vertical component. High precision can also be achieved for relative position coordinates on short (<10 km) baselines using broadcast ephemerides and observing times as short as 3 hours, even when data are processed rapidly on site. Comparison of baseline length changes across the resurgent dome between the two GPS sites and corresponding two-color electronic distance measurements indicates similar extension rates within error (∼2 mm/yr) once we account for a random walk noise component in both systems that may reflect spurious monument motion. Both data sets suggest a pause in deformation for a 3.5 month period in mid-1995, when the extension rate across the dome decreased essentially to zero. Three dimensional positioning data from the two GPS stations suggest a depth (5.8±1.6 km) and location (west side of the resurgent dome) of a major inflation center, in agreement with other geodetic techniques, near the top of a magma chamber inferred from seismic data. GPS systems similar to those installed at Long Valley can provide a practical method for near real-time monitoring and hazard assessment on many active volcanoes.

A GPS estimate of relative motion between North and South America

GPS velocity data are used to estimate the Euler vector describing rigid body motion of North America relative to South America. Assuming the boundary between the North and South American plates is located near the Fifteen Twenty fracture zone in the equatorial Atlantic, the Euler vector predicts extension across the Royal Trough up to 1 mm/yr, and convergence across the Barracuda Ridge at about 2 mm/yr, in agreement with geological estimates averaged over tens of millions of years. Further west, convergence between North and South America at rates up to 8 mm/yr may contribute to deformation of the Caribbean plate along its southwest boundary with South America.

Constraints on present-day shortening rate across the central eastern Andes from GPS data

Two years of continuous GPS data from several sites in South America indicate that Arequipa in the southern Peruvian Andes has a velocity of 13+3 mm/yr (two standard errors) to the northeast with respect to stable South America. We interpret these data as reflecting a combination of elastic strain accumulation associated with a locked Nazca-South America subduction zone and a small amount of crustal shortening across the fold and thrust belt on the eastern margin of the Andes. Models of elastic strain accumulation for fully locked and partly locked subduction zones constrain shortening in the eastern Andes to 0-3 mm/yr (fully locked) and 0-12 mm/yr (partly locked), slower than some geologic estimates averaged over millions of years.