Epicentral confidence regions of nuclear test events at teleseismic distances

The accurate location of seismic events is a basic discriminant for underground nuclear test monitoring (Bolt, 1976; Dahlman and Israelson, 1977; Blandford, 1982). Of particular interest are determining epicentral confidence regions and providing constraints on estimated focal depths. In this study, only routine teleseismic P travel-time data are used, as provided by worldwide stations reporting to the International Seismological Centre (ISC). This lessens the need to model the effects of crustal and shallow-mantle velocity variations, as is necessary with seismographic networks operating at regional distances (Blandford, 1981; Evernden et al., 1986)

A Few Parts in 10\u3csup\u3e8\u3c/sup\u3e Geodetic Baseline Repeatability in the Gulf of California Using the Global Positioning System

Global Positioning System (GPS) geodetic measurements of 350–650 km baselines across the Pacific-North America plate boundary in the Gulf of California are presented. The analysis employs a four-station U.S. fiducial network and combined carrier phase and pseudorange data. Water vapor radiometer (WVR) data at the Gulf sites are used to calibrate the GPS signal for wet tropospheric path delays. Residual tropospheric delays are modeled as first-order exponentially correlated stochastic processes. The measurement precision for horizontal components is a few parts in 108 or better. Comparison of the Gulf data with other geodetic techniques is not yet possible, however the system accuracy appears to be about 1 part in 107 based on simultaneous GPS solutions of a baseline in California collocated with VLBI. The results are encouraging because a high level of precision with GPS is demonstrated even with stations located outside a fiducial network and when wet tropospheric path delays are significant, typically exceeding 20 cm at zenith

Effect of Wet Tropospheric Path Delays on Estimation of Geodetic Baselines in the Gulf of California Using the Global Positioning System

Geodetic baseline measurements using the Global Positioning System (GPS) were acquired in the Gulf of California between Loreto and Cabo San Lucas in Baja California and Mazatlan on the mainland of Mexico. Tropospheric water vapor content was high during the experiment, typically yielding wet path delays in excess of 20 cm at zenith. Surface meteorological (SM) and water vapor radiometer (WVR) measurements were recorded at each site, providing independent means of calibrating the GPS signal for the wet tropospheric path delay. Residual wet delays at zenith are estimated as constants and also as first-order exponentially correlated stochastic processes. In addition, the entire wet zenith delay is estimated stochastically without prior calibration. The results of these approaches are compared in terms of day-to-day baseline repeatability and other system performance discriminants. Calibration with WVR data yields the best repeatabilities, of the order of 1–7.5 parts in 108 in the horizontal components of 350- to 650-km baselines with carrier phase data. Further improvement in these results occurs if combined carrier phase and pseudorange data are used. WVR data are important for direct characterization of the wet tropospheric path delays in humid regions. SM measurements, if used with a simple atmospheric model and estimation of residual zenith delays as constants, can introduce significant errors in baseline solutions. However, SM calibration and stochastic estimation of residual zenith wet delays may be adequate for precise estimation of GPS baselines, with a deterioration in repeatability of less than 1–2 cm compared to WVR calibration. Stochastic estimation of the entire zenith wet delay yields comparable repeatabilities, particularly if both carrier phase and pseudorange data are used. Similar analyses of the Owens Valley Radio Observatory-Mojave baseline in California, where zenith wet delays are factors of 3–5 less, show no significant differences among the various tropospheric calibrations