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At least 163 records · Page 9

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NASA NTRS1985

Studies of the marine crustal magnetization at intermediate wavelengths

The data can be filtered at intermediate wavelengths to provde a data set which complements the satellite fields of MAGSAT, TSS and GRM. The filtered marine data set provides a high resolution data set which is closer to the source bodies than satellite survey data. However, the GRM and TSS could provide the necessary resolution to match the filtered sea surface field. The added resolution determines the nature of crustal magnetizations which give rise to the intermediate wavelength field. It is found that remanent magnetization is an important component over the oceans. Crustal deformation and plate motions result in magnetization vectors which differ significantly from the present day field directions. Induced magnetization or GRM are important components over the oceanic plateaus and spreading centers.

Labrecque, J. L.↗

NASA NTRS1985

A comparison between Lageos laser ranging and very long baseline interferometry determined baseline lengths

NASA's Crustal Dynamics Project (CDP) has the objective to improve the understanding of geodynamics by measuring crustal deformation, tectonic motion, and polar motion and earth rotation. Three different approaches are utilized for obtaining these measurements. One is based on satellite laser ranging (SLR), while another makes use of very long baseline interferometry (VLBI) which uses reception of radio signals from quasars. The third approach involves laser ranging to the moon. An important part of the CDP is to compare baselines periodically or the straight-line distance between two points on the earth's surface as determined by either SLR or VLBI. Attention is given to the SLR analysis, the VLBI analysis, a local survey, and error sources. A table is presented with the baselines between SLR survey markers as measured by VLBI and SLR.

Kolenkiewicz, R.↗

NASA NTRS1986

The Geoscience Laser Altimetry/Ranging System (GLARS)

The Geoscience Laser Altimetry Ranging System (GLARS) is a highly precise distance measurement system to be used for making extremely accurate geodetic observations from a space platform. It combines the attributes of a pointable laser ranging system making observations to cube corner retroreflectors placed on the ground with those of a nadir looking laser altimeter making height observations to ground, ice sheet, and oceanic surfaces. In the ranging mode, centimeter-level precise baseline and station coordinate determinations will be made on grids consisting of 100 to 200 targets separated by distances from a few tens of kilometers to about 1000 km. These measurements will be used for studies of seismic zone crustal deformations and tectonic plate motions. Ranging measurements will also be made to a coarser, but globally distributed array of retroreflectors for both precise geodetic and orbit determination applications. In the altimetric mode, relative height determinations will be obtained with approximately decimeter vertical precision and 70 to 100 meter horizontal resolution. The height data will be used to study surface topography and roughness, ice sheet and lava flow thickness, and ocean dynamics. Waveform digitization will provide a measure of the vertical extent of topography within each footprint. The planned Earth Observing System is an attractive candidate platform for GLARS since the GLAR data can be used both for direct analyses and for highly precise orbit determination needed in the reduction of data from other sensors on the multi-instrument platform. (1064, 532, and 355 nm)Nd:YAG laser meets the performance specifications for the system.

Cohen, S. C.↗

NASA NTRS1986

Basic research for the geodynamics program

Further development of utility program software for analyzing final results of Earth rotation parameter determination from different space geodetic systems was completed. Main simulation experiments were performed. Results and conclusions were compiled. The utilization of range-difference observations in geodynamics is also examined. A method based on the Bayesian philosophy and entropy measure of information is given for the elucidation of time-dependent models of crustal motions as part of a proposed algorithm. The strategy of model discrimination and design of measurements is illustrated in an example for the case of crustal deformation models.

Source record↗

NASA NTRS1987

Tectonics of Tharsis Dorsa on Mars

The tectonics of the Tharsis and adjoining areas is considered to be associated with the convection in the Martian mantle. Convection and mantle plume have been responsible for the primary uplift and volcanism of the Tharsis area. The radial compressional forces generated by the tendency for downslope movement of surface strata, vertical volcanic intrusions, and traction of mantle spreading beneath Tharsis were transmitted through the lithosphere to form peripheral mare ridge zones. The locations of mare ridges were thus mainly controlled by the Tharsis-radial compression. The load-induced stresses then contributed to further ridge formation over an extended period of time by the isostatic readjustment, which was responsible for long-term stresses in the adjoining areas. Extrusions, changes in internal temperature, and possible phase changes may also have caused changes in mantle volume, giving rise to additional compressional forces and crustal deformations.

Raitala, J.↗

NASA NTRS1988

Strain accumulation in the Santa Barbara Channel, 1971-1987

Geophysical evidence suggests a significant amount of north-south convergence occurs across the Santa Barbara Channel. Tectonic studies indicate a discrepancy between observed fault slip in California and the North American-Pacific plate motion. Newer plate motion models (NUVEL-1) yield a lower rate of convergence. Global Positioning System (GPS) data collected in the Santa Barbara Channel in 1987, when combined with 1971 trilateration measurements, should be sufficient to resolve the present-day convergence rate. In early 1987. from January 3 to 7, GPS data were collected at 14 sites in California and at 5 additional stations throughout North America. The data can be used to estimate the rate of crustal deformation (convergence) ocurring across the Santa Barbara Channel. The GPS baselines were computed with the Bernese 2nd generation software. A comparison was made between baseline lengths obtained with the Burnese and MIT softwares. Baseline changes from 1971 to January, 1987 (GPS-Bernese) across the Santa Barbara Channel were computed. A uniform strain model was calculated from the baseline changes. The present-day rate of convergence across the Santa Barbara Channel was determined to be 8 to 10 mm/yr. This conclusion is obtained from changes in the baseline length measured with a 1971 trilateration survey and a January, 1987, GPS survey. The rapid convergence rate, in addition to the history of large seismic events, suggests this region is a prime target for future geodetic and geophysical studies.

Larsen, Shawn↗

NASA NTRS1988

The Crustal Dynamics Project

The Crustal Dynamics Project has been developing, deploying, and operating very-long-baseline interferometry (VLBI) systems and satellite laser ranging (SLR) systems for highly accurate geodetic measurements of global plate motion, plate stability, regional crustal deformation, and earth rotation/polar motion. Over the past 10 years, the measurement accuracies of these systems have been improved by a factor of 10 to the cm level. Plans are to continue these developments to reach mm level accuracies. The present deployment of the VLBI systems is primarily in the Northern Hemisphere. This network has produced measurements of the relative plate motion between the North American, Eurasian, and Pacific plates; the stability of the same plates; and the regional deformation at the North American/Pacific plate boundary in California and Alaska.

Coates, Robert J.↗

NASA NTRS1990

Central and South America GPS geodesy - CASA Uno

In January 1988, scientists from over 25 organizations in 13 countries and territories cooperated in the largest GPS campaign in the world to date. A total of 43 GPS receivers collected approximately 590 station-days of data in American Samoa, Australia, Canada, Colombia, Costa Rica, Ecuador, New Zealand, Norway, Panama, Sweden, United States, West Germany, and Venezuela. The experiment was entitled CASA Uno. Scientific goals of the project include measurements of strain in the northern Andes, subduction rates for the Cocos and Nazca plates beneath Central and South America, and relative motion between the Caribbean plate and South America. A second set of measurements are planned in 1991 and should provide preliminary estimates of crustal deformation and plate motion rates in the region.

Kellogg, James N.↗

NASA NTRS1990

First GPS baseline results from the North Andes

The CASA Uno GPS experiment (January-February 1988) has provided the first epoch baseline measurements for the study of plate motions and crustal deformation in and around the North Andes. Two dimensional horizontal baseline repeatabilities are as good as 5 parts in 10 to the 8th for short baselines (100-1000 km), and better than 3 parts in 10 to the 8th for long baselines (greater than 1000 km). Vertical repeatabilities are typically 4-6 cm, with a weak dependence on baseline length. The expected rate of plate convergence across the Colombia Trench is 6-8cm/yr, which should be detectable by the repeat experiment planned for 1991. Expected deformation rates within the North Andes are of the order of 1 cm/yr, which may be detectable with the 1991 experiment.

Kellogg, James N.↗

NASA NTRS1990

Mantle-flow tectonics and the origin of Ishtar Terra, Venus

It is proposed that western Ishtar Terra formed due to compression and crustal thickening above a cylindrical mantle downwelling. A model for crustal deformation due to downwelling successfully reproduces many observed characteristics of western Ishtar. Although axisymmetric downwelling occur in numerical models of constant-viscosity mantle convection, there is no evidence for their existence in earth's mantle, where downwellings are sheet-like. Either modes of downflow in Venus and earth are fundamentally different, or differences in near-surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes.

Bindschadler, D. L.↗

NASA NTRS1991

Absolute gravity measurements in California, 1984-1989

Repeated absolute gravity measurements have been made at 12 sites in California between 1984.3 and 1989.7. As determined in laboratory tests, the instrument used has an estimated accuracy of 10 microgal (approximately 10 to the -8th g). The repeatability of the measurements is consistent with this accuracy assessment. No gravity changes above the limits set by instrumental uncertainty and environmental noise are observed during this period; the field observations provide upper limits on the rates of secular gravity changes which could be attributed to crustal deformation with a resolution corresponding to vertical displacement rates of 1-2 cm/yr.

Sasagawa, Glenn↗

NASA NTRS1992

Rapid resolution of crustal motion at short ranges with the global positioning system

An analysis of GPS alignment array data collected in November 1990 and February 1991 are used to assess the temporal resolution of crustal deformation for GPS receiver systems operating as strain meters, and guidelines for achieving millimeter-level precision with short-range, kinematic-type GPS measurements are provided. Baselines between the array endpoints computed from 24 min of static observations on three consecutive days show an rms scatter of about 0.1 nm in the horizontal components and of about 0.3 mm in the vertical. They agree to within 0.2 mm with a solution calculated from measurements collected 3 mo earlier, indicating that this segment of the San Andreas fault appears to be locked at the surface over this period of time. Epoch-by-epoch solutions of array baselines with a six-satellite constellation show a multipath signature with typical amplitudes of several millimeters and periods of a few minutes.

Genrich, Joachim F.↗

NASA NTRS1992

Coldspots and hotspots - Global tectonics and mantle dynamics of Venus

Based on geologic observations provided by Magellan's first cycle of data collection and recent models of mantle convection in spherical shells and crustal deformation, the major topographic and geologic features of Venus are incorporated into a model of global mantle dynamics. Consideration is given to volcanic rises, such as Beta Regio and Atla Regio, plateau-shaped highlands dominated by complex ridged terrain (e.g., Ovda Regio and Alpha Regio), and circular lowland regions, such as Atalanta Planitia. Each of these features is related to either mantle plumes (hotspots) or mantle downwellings (coldspots).

Bindschadler, Duane L.↗

NASA NTRS1991

A TOPEX/Poseidon study of oceanic effects on observations of the Earth's interior

The primary objective of our research program is the application of TOPEX/POSEIDON oceanic data to the study of various geophysical observations, thereby improving our knowledge of the Earth's interior. Observations of the Earth's solid tides, rotation, time-dependent gravity, and crustal deformation provide potentially useful constraints on properties of the solid Earth and its liquid core, but all these parameters are also sensitive to oceanic effects. Depending on the type of observation, those effects are usually either removed by modeling (often using particularly simple assumptions about the dynamics of the ocean) or ignored entirely. Useful oceanic data are rarely available. In many cases, the uncertainty in the oceanic correction is the main source of error in the geophysical interpretation of the results. We describe, below several of these geophysical observations, how their interpretation in terms of the Earth's interior is affected by the oceans, and how data from the TOPEX/POSEIDON mission can be used to improve this situation.

Wahr, John↗

NASA NTRS1992

Evolution of attached and detached slabs and their associated mantle dynamics

Over the two years of the NASA grant, this project has produced a significant amount of research results related to the plate subduction process and the surface crustal deformation at convergent boundaries (i.e., above subduction zones). While some research objectives are completely accomplished, other research tasks remain active and continue to be investigated at present. A steady state analytic thermal model for subducting slabs was used to examine the torques acting on a descending slab. It is found that gravitational torque vanishes when a slab is dipping either vertically or horizontally, unlike previous studies indicating that the magnitude of gravitational torque decreases as dip angle increases. Subsequently, a new time-dependent, analytic thermal model for a subducting slab was developed. The new model enables us to study transient phenomena associated with plate subduction analytically. On the basis of this model, the nature of slab dip angles was evaluated. Slab dip angles are found to be transient features. As they penetrate into the mantle and increase their lengths, the associated gravitational torque also increases resulting in a downward pulling of the slab to the steeper dip angle. This is especially true once a slab penetrates the olivine-spinel phase boundary at about 400 km depth. However, if the phase transformation does not follow the equilibrium condition, the gravitational torque may have a different behavior. This problem was investigated. Except for fast descending slabs, non-equilibrium phase transformation can only slow down the transient increase of slab dip angles discussed earlier. Its effect is not sufficiently strong to reverse the downward pulling for most of the slabs. However, when slabs subducting at 10 cm/yr or faster, a sufficient amount of metastable olivine can exist beneath 400 km. Because of its low density compared with the surrounding spinel, an upward buoyancy is produced resulting in an upward bending of the slab and possibly an upward rotation of the slab such that smaller dip angles are formed. Seismic studies of the Japanese Slab seem to support this interpretation. The development of oroclinal geometries at convergent boundaries was also examined to study plate obduction which is an important ingredient to the initiation of plate subduction. Although the study suggests that surface features are better modeled by block models, the large scale deformation can be adequately studied by viscous models. Such a model is now under development to complete our original objective to study the initiation of plate subduction. Finally, a three-dimensional, finite element, spherical convective model is developed to study dynamic plate subductions. The model development is now complete and it is being tested to ensure its proper operation. The model is able to generate convection results with a viscosity contrast of about 100. Our research continues to push the viscosity contrast to a level that is appropriate for a subducting slab.

Hsui, Albert T.↗

NASA NTRS1992

Orbital, rotational, and climatic interactions: Lessons from Earth and Mars

Though variations in orbital and rotational parameters of the Earth and Mars are widely recognized as plausible sources of significant climatic variation on 10(exp 3) to 10(exp 8) yr timescales, many aspects of the connection between orbital, rotational, and climatic variations remain poorly understood. In general, the orbital histories are very well known, the rotational histories (especially for Mars) are very poorly known. A brief review is given of recent progress in computing orbital and rotational secular variations, and in connecting them to climatic change. The emphasis is on highlighting those areas that limit the present understanding. It is obvious that mass redistributions associated with climatic change (glaciation) are a source of crustal deformation and geodynamic change on the Earth, and may have played similar roles on Mars. It is much less appreciated, that rates, phases, and amplitudes of deformation of the deep interior of the planet can influence climate. The mantle and core, if decoupled, would precess at different rates, and even with plausible coupling strengths, some degree of differential precession is possible.

Bills, Bruce G.↗

NASA NTRS1992

Results from the Magellan mission to Venus

Magellan's radar-mapping mission to Venus has thus far accomplished the mapping of nearly 95 percent of the planet's surface; the 100-m resolution of these data facilitate studies of Venus' geological evolution. Widespread evidence of horizontal compression and extension at many scales is noted. Crustal deformation appears to be distributed across most of the surface. The density distribution of the interior of Venus will be explored in an upcomimg gravity experiment, in which accurate tracking of spacecraft orbital accelerations is used to identify gravity anomalies.

Plaut, Jeffrey J.↗

NASA NTRS1991

Geoscience Laser Ranging System design and performance predictions

The Geoscience Laser System (GLRS) will be a high-precision distance-measuring instrument planned for deployment on the EOS-B platform. Its primary objectives are to perform ranging measurements to ground targets to monitor crustal deformation and tectonic plate motions, and nadir-looking altimetry to determine ice sheet thicknesses, surface topography, and vertical profiles of clouds and aerosols. The system uses a mode-locked, 3-color Nd:YAG laser source, a Microchannel Plate-PMT for absolute time-of-flight (TOF) measurement (at 532 nm), a streak camera for TOF 2-color dispersion measurement (532 nm and 355 nm), and a Si avalanche photodiode for altimeter waveform detection (1064 nm). The performance goals are to make ranging measurements to ground targets with about 1 cm accuracy, and altimetry height measurements over ice with 10 cm accuracy. This paper presents an overview of the design concept developed during a phase B study. System engineering issues and trade studies are discussed, with particular attention to error budgets and performance predictions.

Anderson, Kent L.↗