Michael Bevis

GPS meteorology: Remote sensing of atmospheric water vapor using the global positioning system

We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground-based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time-varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground-based GPS networks could be analyzed in concert with observations of GPS satellite occultations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary scale.

GPS meteorology: mapping zenith wet delays onto precipitable water

Abstract Emerging networks of Global Positioning System (GPS) receivers can be used in the remote sensing of atmospheric water vapor. The time-varying zenith wet delay observed at each GPS receiver in a network can be transformed into an estimate of the precipitable water overlying that receiver. This transformation is achieved by multiplying the zenith wet delay by a factor whose magnitude is a function of certain constants related to the refractivity of moist air and of the weighted mean temperature of the atmosphere. The mean temperature varies in space and time and must be estimated a priori in order to transform an observed zenith wet delay into an estimate of precipitable water. We show that the relative error introduced during this transformation closely approximates the relative error in the predicted mean temperature. Numerical weather models can be used to predict the mean temperature with an rms relative error of less than 1%.

GPS Meteorology: Direct Estimation of the Absolute Value of Precipitable Water

Abstract A simple approach to estimating vertically integrated atmospheric water vapor, or precipitable water, from Global Positioning System (GPS) radio signals collected by a regional network of ground-based geodetic GPS receiver is illustrated and validated. Standard space geodetic methods are used to estimate the zenith delay caused by the neutral atmosphere, and surface pressure measurements are used to compute the hydrostatic (or “dry”) component of this delay. The zenith hydrostatic delay is subtracted from the zenith neutral delay to determine the zenith wet delay, which is then transformed into an estimate of precipitable water. By incorporating a few remote global tracking stations (and thus long baselines) into the geodetic analysis of a regional GPS network, it is possible to resolve the absolute (not merely the relative) value of the zenith neutral delay at each station in the augmented network. This approach eliminates any need for external comparisons with water vapor radiometer observation...

Sensing atmospheric water vapor with the global positioning system

Global Positioning System (GPS) receivers, water vapor radiometers (WVRs), and surface meteorological equip- ment were operated at both ends of a 50-kin baseline in Colorado to measure the precipitable water vapor (PWV) and wet delay in the line-of-sight to GPS satellites. Using high pre- cision orbits, WVR-measured and GPS-inferred PWV differences between the two sites usually agreed to better than 1 min. Using less precise on-line broadcast orbits increased the discrepancy by 30%. Data simulations show that GPS mea- surements can provide ram-level separate PWV estimates for the two sites, as opposed to just their difference, if baselines exceed 500 km and the highest accuracy GPS orbits are used.

GPS/STORM—GPS Sensing of Atmospheric Water Vapor for Meteorology

Abstract Atmospheric water vapor was measured with six Global Positioning System (GPS) receivers for 1 month at sites in Colorado, Kansas, and Oklahoma. During the time of the experiment from 7 May to 2 June 1993, the area experienced severe weather. The experiment, called “GPS/STORM,” used GPS signals to sense water vapor and tested the accuracy of the method for meteorological applications. Zenith wet delay and precipitable water (PW) were estimated, relative to Platteville, Colorado, every 30 min at five sites. At three of these five sites the authors compared GPS estimates of PW to water vapor radiometer (WVR) measurements. GPS and WVR estimates agree to 1–2 mm rms. For GPS/STORM site spacing of 500–900 km, high-accuracy GPS satellite orbits are required to estimate 1–2-mm-level PW. Broadcast orbits do not have sufficient accuracy. It is possible, however, to estimate orbit improvements simultaneously with PW. Therefore, it is feasible that future meteorological GPS networks provide near-real-time hig...

Instantaneous geodetic positioning at medium distances with the Global Positioning System

We evaluate a new method of Global Positioning System (GPS) data analysis, called instantaneous positioning, at spatial scale lengths typical of interstation spacings in a modern crustal motion network. This method is more precise and versatile than traditional GPS static and kinematic processing of multi-epoch batches of data. The key to instantaneous positioning is the ability to resolve integer-cycle phase ambiguities with only a single epoch of dual-frequency phase and pseudorange data, rendering receiver cycle slips irrelevant. We estimate three-dimensional relative coordinates and atmospheric zenith delay parameters independently every 30 s over a 12-week period for baseline distances of 50 m, 14 km, and 37 km. Horizontal precision of a single-epoch coordinate solution is about 15 mm and vertical precision is about 7–8 times worse. Removing that component of each time series which repeats with a period of exactly 1 sidereal day, and thus manifests signal multipath, reduces the scatter by about 50% in all components. Solution averaging of the high-frequency time series can be performed using any number of measurement epochs to further improve coordinate precision. We demonstrate that the daily coordinates estimated with instantaneous positioning are more precise (by 20–50% per coordinate component) than those estimated with 24-hour batch processing. Spectral analysis of the single-epoch solutions indicates that the flicker noise characteristic of GPS time series observed in lower-frequency bands also affects GPS solutions in the frequency band 0.01 mHz to 10 mHz. We argue that the flicker noise is induced by tropospheric effects. Since modern GPS receivers are capable of observing at frequencies as high as 10 Hz, our technique significantly overlaps and complements the frequency band of broadband seismology and benefits other research areas such as earthquake geodesy, volcanology, and GPS meteorology.

The Promise of GPS in Atmospheric Monitoring

Abstract This paper provides an overview of applications of the Global Positioning System (GPS) for active measurement of the Earths atmosphere. Microwave radio signals transmitted by GPS satellites are delayed (refracted) by the atmosphere as they propagate to Earth-based GPS receivers or GPS receivers carried on low Earth orbit satellites. The delay in GPS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrated along the signal path. Measurement of atmospheric water vapor by Earth-based GPS receivers was demonstrated during the GPS/STORM field project to be comparable and in some respects superior to measurements by ground-based water vapor radiometers. Increased spatial and temporal resolution of the water vapor distribution provided by the GPS/STORM network proved useful in monitoring the moisture-flux convergence along a dryline and the decrease in integrated water vapor associated with the passage of a midtropospheri...

Accretionary tectonics of Burma and the three-dimensional geometry of the Burma subduction zone

The geometry of the Burma Wadati-Benioff zone (WBZ) has been determined by fitting a trend surface parameterized with eight effective degrees of freedom to 184 well-located hypocenters. The dip of this surface, which passes through the middle of the WBZ, varies from about 50° in the north near the eastern Himalayan syntaxis to about 30° in the Bay of Bengal area. The eastern edge of the Indo-Burman ranges closely follows the map projection of the 60 km depth contour of the WBZ. The curvature of the Indo-Burman ranges is controlled by the geometry of the interface between the more steeply dipping part of the Indian plate and the leading edge of the overriding Burma platelet. Shallow earthquakes beneath the Indo-Burman ranges are primarily confined to the underthrusting Indian plate. Their focal mechanisms indicate strike-slip faulting and north-south shortening parallel to the eastern margin of the Indian plate.

Seismic Slip and Down-Dip Strain Rates in Wadati-Benioff Zones

The rate of accumulation of seismic moment in Wadati-Benioff zones is used to estimate strain rates in subducting slabs that are sinking through the asthenosphere. Between depths of 75 and 175 kilometers a typical down-dip strain rate is about 10-15 per second, which implies that slabs in this depth range typically accumulate strain of order 10-1. This result is in accord with geometrical arguments that subducted slabs must experience large membrane strains to deform to their observed shapes. Mantle seismicity (repeated catastrophic shear failure) is apparently a primary mechanism by which large membrane strains accumulate in the cold cores of subducting slabs. Slabs are penetratively deformed, and they have low flexural rigidity compared to oceanic plates at the earths surface.

The March 9, 1994 (M w 7.6), deep Tonga earthquake: Rupture outside the seismically active slab

We investigate the rupture process of the March 9, 1994, M w 7.6 deep Tonga earthquake and its relationship to the background seismicity of the subducted Tonga slab. Variations in observed P and S wave pulse duration indicate that the rupture propagated to the NNE and extended well beyond the background seismicity. We inverted 47 P and SH waveforms, including regional broadband waveforms from the Southwest Pacific Seismic Experiment, using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane. The results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10-20 km beyond the bounds of the previous seismic activity. A significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region. Rupture along the near-vertical NNE striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near-horizontal nodal plane. This result, combined with the planar alignment of aftershocks and the general NNE directivity of the waveforms, provides strong evidence that the rupture occurred on the near-vertical plane. Thermal modeling of the Tonga slab indicates that the rupture terminated in material about 200°C warmer than the temperature that normally limits the occurrence of smaller earthquakes. Additionally, aftershocks seem to be suppressed in the outer regions of the rupture, which contain about half of the moment release but only 1 of the 15 well-located aftershocks. We suggest that slabs may be composed of an inner cold core, where seismic rupture initiates and small earthquakes occur, and a thermal halo of warmer material, which can sustain rupture and only a few aftershocks. The mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core; nucleation may occur through a process limited to the cold core such as transformational faulting, and propagation through the warmer material may occur through ductile faulting or plastic instabilities. Isolated deep earthquakes in other subduction zones, such as the 1994 Bolivia event, may occur almost completely within the warmer zone, accounting for the lack of background seismicity and the dearth of aftershocks.