David M. Tralli

Stochastic estimation of tropospheric path delays in global positioning system geodetic measurements

Water vapor radiometric (WVR) and surface meteorological (SM) measurements taken during three Global Positioning System (GPS) geodetic experiments are used to calculate process noise levels for random walk and first-order Gauss-Markov temporal models of tropospheric path delays. Entire wet and combined wet and dry zenith delays at each network site then are estimated simultaneously with the geodetic parameters without prior calibration. The path delays and corresponding baseline estimates are compared to those obtained with calibrated data and stochastic residual delays. In this manner, the marginal utility of a priori tropospheric calibration is assessed given the ability to estimate the path delays directly using only theGPS data. Estimation of total zenith path delays with appropriate random walk or Gauss-Markov models yields baseline repeatabilities of a few parts in 108. This level of geodetic precision, and accuracy as suggested by analyses on collocated baselines estimated independently by very long baseline interferometry, is comparable to or better than that obtained after path delay calibration usingWVR and/orSM measurements. Results suggest thatGPS data alone have sufficient strength to resolve centimeter-level zenith path delay fluctuations over periods of a few minutes.

A Prediction of Mars Seismicity from Surface Faulting

The shallow seismicity of Mars has been estimated by measurement of the total slip on faults visible on the surface of the planet throughout geologic time. Seismicity was calibrated with estimates based on surface structures on the moon and measured lunar seismicity that includes the entire seismogenic lithosphere. Results indicate that Mars is seismically active today, with a sufficient number of detectable marsquakes to allow seismic investigations of its interior.

Preliminary determination of Pacfic‐North America relative motion in the southern Gulf of Calfornia using the Global Positioning System

Global Positioning System (GPS) data from experiments conducted in 1985 and 1989 in the southern Gulf of California, Mexico, allow a determination of relative motion between the Pacific and North American plates. The data indicate motion of Cabo San Lucas on the Pacific plate relative to North America at a rate of 47±7 mrn/yr and azimuth of 57±6° west of north (1σ errors), equivalent within uncertainties to the NUVEL-1 global plate motion model.

Comparison of Kalman filter estimates of zenith atmospheric path delays using the Global Positioning System and very long baseline interferometry

Kalman filter estimates of zenith nondispersive atmospheric path delays at Westford, Massachusetts, Fort Davis, Texas, and Mojave, California, were obtained from independent analyses of data collected during January and February 1988 using the Global Positioning System (GPS) and very long baseline interferometry (VLBI). The apparent accuracy of the path delays is inferred by examining the estimates and covariances from both sets of data. The ability of the geodetic data to resolve zenith path delay fluctuations is determined by comparing further the GPS Kalman filter estimates with corresponding wet path delays derived from water vapor radiometric (WVR) data available at Mojave over two 8-hour data spans within the comparison period. GPS and VLBI zenith path delay estimates agree well within one standard deviation formal uncertainties (from 10–20 mm for GPS and 3–15 mm for VLBI) in four out of the five possible comparisons, with maximum differences of 5 and 21 mm over 8- to 12-hour data spans. For one comparison, the maximum difference between GPS and VLBI is 50 ± 20 mm and clearly shows an unexplained systematic difference which is probably related to poor elevation angle coverage in the VLBI data at the time, GPS fiducial network sensitivity, and poor azimuthal coverage. The root-mean-square differences between GPS estimates of the total path delays and WVR measurements added to the hydrostatic delay component determined from surface barometric pressure data are between 9 and 15 mm, however, with biases of 10–15 mm.

Programmatic risk balancing

_- Abstract. A lifecycle risk management decision- support tool developed for NASA space systems is used to assess and visualize risk in a research and applications program. This paper presents the first tool demonstration and assessment for balancing programmatic risk. The NASA program for case study has a principal goal development of geospatial information products for decision-support systems. Risk balancing is performed by selecting optimal combinations of risk controls, such as planning activities, assessments, research and applications projects. Marginal benefits are measured in terms of residual risk. Relative differences in risk impact and control effectiveness provide an indication of program integration across science, technology and applications. Results are preliminary. Sensitivity analysis suggests that at implementation, prioritization and coordination with government agencies are mitigations yielding the greatest marginal benefit towards requirements attainment. At introduction of projects, establishing performance characteristics yields the greatest marginal benefit. The framework helps strategy design, execution, integration and prioritization. -

Conceptual Case for Assimilating Interferometric Synthetic Aperture Radar Data Into the HAZUS-MH Earthquake Module

The study of the Earth as a system is being adopted widely by geoscientists. Numerical models and simulations are providing the capability to rapidly test hypotheses and make forecasts of complex geophysical behavior. International efforts are seeking to integrate existing and emerging Earth observation systems into a global network, with enhanced data distribution, models, and decision support tools. Remote sensing is poised to fulfil the increasing need for a synoptic framework. However, the desire to improve the connection between scientific research and societal benefits has not been matched with resources and tools required to bridge the gap between research and applications. Natural hazards research and disaster management are a prime example. Here, we present a conceptual case for how interferometric synthetic aperture radar (InSAR) data could make a definitive contribution to understanding earthquake processes while simultaneously supporting policy- and decision-making. InSAR measurements derived from time series of radar observations from Earth orbit uniquely can provide geographically comprehensive maps of surface deformation. Observing system simulations are suggested to evaluate the potential contributions of a future system. Simulations would adopt an open seismic hazard analysis (SHA) framework, OpenSHA, recognizing the need for more physics-based modeling and computational infrastructure. SHA is employed by the HAZUS-MH earthquake module to estimate losses. InSAR measurements of strain accumulation would provide event magnitude recurrence bounds for probabilistic SHA, while coseismic InSAR measurements would add constraints on fault rupture models for deterministic approaches. Moreover, interferograms would be incorporated graphically as proxy seismic risk maps for planning and mitigation

Variation of seismic slip in the Gulf of California and the possible effect on geodetic measurements of Pacific-North American Plate motion

The Gulf of California is a unique tectonic transition zone which progresses from an oceanic ridge-transform system at the mouth of the gulf to a predominantly continental transform system in southern California. Slip rates estimated from seismic moment data amount to about 17 to 30% of the accumulated tectonic slip in the gulf predicted by the NUVEL-1 plate motion model, even after accounting for the transition in crustal structure. The seismic to tectonic slip ratios also appear to increase slightly from south to north. This trend in the seismic moment release may reflect changes in the accommodation of Pacific-North American plate motion along the gulf which can be examined geodetically. On the other hand, geodetic measurements of the relative plate motion can be disturbed by short-term effects associated with episodic seismicity. To assess these effects, the surface displacements due to typical transform events in the gulf are estimated using a simple dislocation model. The results of this numerical calculation suggest that if a large transform event (M0 ∼ 1.5 × 1026 dyne cm) were to occur within 100 to 200 km of a geodetic baseline, for example one comprised in the GEOMEX Global Positioning System network across the southern gulf, the relative distance measurements could be affected by up to 15 mm. This is marginally at the error level of a few millimeters plus 2 parts in 108 of baseline length for GEOMEX measurements, which thus are sensitive only to the far-field displacement along the plate boundary (over baselines of a few hundred kilometers in length).

Concept for a High MEO InSAR Seismic Monitoring System

Demonstration of a spaceborne system to image seismic surface waves dynamically (i.e. coseismically) would be the early steps of a future operational capability for monitoring earthquakes and discriminating clandestine underground nuclear tests. Complementing the global network of seismic instruments, such system would enable unprecedented global mapping of the velocity structure of the Earths crust, thereby improving hypocentral location, understanding of rupture dynamics and wave propagation effects, and source characterization. Seismic wave measurement requirements include lower bounds on detectability of events and wave amplitude accuracy for different levels of analysis, such as source characterization and crustal tomography, with 10-100 mum wave amplitude resolution for waves nominally traveling 5 km/s, an upper frequency bound based on earthquake surface displacement spectra, and minimum horizontal resolution (1-5 km) and areal coverage. Advanced radar technologies are keys to demonstrating a pre-operational system leading to a high MEO (10,400 km orbit altitude) constellation for continuous surveillance.

Regionalized temperature variations in the upper 400 km of the Earth's mantle

Abstract Tectonically regionalized variations in the temperature of the upper 400 km of the Earths mantle are estimated from analysis of global seismic travel-time data cataloged by the International Seismological Centre (ISC). Seismic parameter profiles are determined from estimates of P and S velocities obtained by tau inversion. Summary phase diagrams for the olivine and pyroxene-garnet subsystems are constructed in conjunction with a thermodynamic potential formulation that allows self-consistent determination of density, bulk modulus and adiabats throughout the pressure and temperature regimes of the mantle. Perturbations in estimated seismic parameters are expressed in terms of variations in temperature using the model temperature derivatives of the bulk modulus and density at a given temperature and pressure. Confidence bounds on the velocity estimates are used to place corresponding bounds on the constructed seismic parameters. A simple differential relationship is solved iteratively to obtain a temperature variation for a given variation in seismic parameter. This approach allows the estimation of a range of seismically determined temperature variations by employing a given compositional model. Results indicate that whereas the P and S velocity variations in the upper mantle are consistent with the tectonic regionalization, variations in V p V s ratios are irregular. This leads to unstable estimates of the seismic parameters and thus estimates of mean temperature anomalies, typically within 600°C of the weighted mean, that are inconsistent with the regionalized seismic data. A comparison of two compositional models is used to show the trade-off with estimated temperature variations. A refined regionalization and analysis of a larger ISC data set are suggested to stabilize the S velocity inversion, reduce statistical uncertainties on the seismic parameters, and thus improve constraints on estimated temperature variations.

Spectral comparison of continuous Global Positioning System and strainmeter measurements of crustal deformation

Temporal power spectral density models of noise in continuous crustal deformation measurements obtained with the Global Positioning System (GPS) and high-quality strainmeters are compared. The crossover frequency at which GPS measurement noise is less than that of strainmeters is determined. Assuming GPS precision of 0.1 to 1 cm in horizontal components for baselines up to 100 km in length, local deformation monitoring with GPS may be preferable to strainmeters for observations of short-term deformation in under 6 months of continuous (at least daily) measurements. Short-period tropospheric path delays and multipath effects, which may obscure GPS-determined strain signals in local network measurements, also are discussed.