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  • 1
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    Online Resource
    A scheme for reducing the effect of selective availability on precise geodetic measurements from the Global Positioning System
    American Geophysical Union (AGU) ; 1991
    In:  Geophysical Research Letters Vol. 18, No. 7 ( 1991-07), p. 1289-1292
    Details
    In: Geophysical Research Letters, American Geophysical Union (AGU), Vol. 18, No. 7 ( 1991-07), p. 1289-1292
    Abstract: From March to August 1990, the signals transmitted by the Block II satellites of the Global Positioning System (GPS) were dithered under a policy of “Selective Availability” (SA). The dithering appears as a ∼10 −10 deviation of the satellite oscillator frequency, which, when accumulated over several minutes, can produce an error of ∼100 cycles (∼20 m) in the model for carrier beat phase. Differencing between simultaneously sampling receivers minimizes the error. If, however, the receivers do not sample simultaneously, it is necessary to model the frequency deviation, which we estimate from the phase observed at a station with a stable local oscillator. We apply such a model to data collected in March 1990 by TI4100 and Minimac receivers sampling at times separated by 0.92 s. Applying the algorithm significantly improves the root mean square (RMS) scatter of the estimated relative position vectors. The RMS scatter from a data set including dithered satellites is similar for both simultaneously and non‐simultaneously sampling receivers, a result which indicates that SA can be adequately modeled.
    Type of Medium: Online Resource
    ISSN: 0094-8276 , 1944-8007
    URL: Article
    DOI: 10.1029/91GL01559
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1991
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  • 2
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    Online Resource
    Geodetic measurement of tectonic deformation in the Santa Maria Fold and Thrust Belt, California
    American Geophysical Union (AGU) ; 1990
    In:  Journal of Geophysical Research: Solid Earth Vol. 95, No. B3 ( 1990-03-10), p. 2679-2699
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 95, No. B3 ( 1990-03-10), p. 2679-2699
    Abstract: We have analyzed geodetic observations to resolve tectonic deformation across the Santa Maria Fold and Thrust Belt, northwest of Santa Barbara, California. The geodetic network forms a braced quadrilateral with ∼40 km sides whose southwest corner is the Vandenberg very long baseline interferometry station. The data include (1) historical triangulation from the 1880s, 1920s and 1950s; (2) electronic distance measurement trilateration from 1971 and 1985; and (3) Global Positioning System observations from 1986 and 1987. We combine the three different types of data to estimate two‐dimensional station positions and strain rate parameters simultaneously. Using a model which constrains the relative velocity field to be linear in space and constant in time, we find significant strain rates. The maximum compressive strain is oriented N17°E±5°E, and the compressive strain rate in this direction is 0.13±0.03 μstrain/yr. Under the assumption that the unresolved rotational component of the velocity field is zero, we estimate that the integrated rate of deformation across the basin is 7±1 mm/yr oriented at N03°E±13°. This vector can be decomposed into 6±2 mm/yr of crustal shortening on the general structural trend of N30°E and 3±1 mm/yr of right‐lateral shear across this axis. On the basis of these values and earthquake focal mechanisms in the area, we infer that the deformation occurs on northwest trending folds and thrusts within the belt. These results are consistent with the rate and direction of deformation across the central California Coast Ranges inferred by balancing Pacific‐North America plate motion against San Andreas slip and Basin and Range extension. They imply that the Santa Maria Fold and Thrust Belt is the primary active element in transforming motion from the Coast Ranges to the western Transverse Ranges and the Santa Barbara Channel.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/JB095iB03p02679
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1990
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  • 3
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    Online Resource
    Analysis of coseismic surface displacement gradients using radar interferometry: New insights into the Landers earthquake
    American Geophysical Union (AGU) ; 1994
    In:  Journal of Geophysical Research: Solid Earth Vol. 99, No. B11 ( 1994-11-10), p. 21971-21981
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 99, No. B11 ( 1994-11-10), p. 21971-21981
    Abstract: The map of the coseismic displacement field generated by interferometric processing of synthetic aperture radar & lpar;SAR & rpar; images taken before and after the June 28, 1992, Landers earthquake sequence brings new insights into the nature of deformation caused by these earthquakes. We use the interferometric map generated by Massonnet et al. & lpar;1993 & rpar; to analyze the surface displacement field in the vicinity of the fault trace. Complexities in the fringe pattern near the fault reflect short & hyphen;wavelength variations of the surface rupture and slip distribution, and attest to large displacement gradients. Along two sections of the fault, characteristic fringe patterns can be recognized, contrasting in density and direction with patterns observed away from the rupture. In order to understand the observed fringe patterns, we compute synthetic interferograms in three simple cases: & lpar;1 & rpar; rigid & hyphen;body rotations about a vertical axis, & lpar;2 & rpar; about a horizontal axis & lpar;tilt & rpar;, and & lpar;3 & rpar; distributed, simple shear. The orientation and spatial separation of interferometric fringes predicted by these models help constrain near & hyphen;field deformation and rupture parameters. Where the Kickapoo fault connects with the Homestead Valley fault, the interferogram shows a clear pattern of parallel N20 & deg;W fringes separated by about 160 m. This pattern and vertical offsets measured along the Kickapoo fault suggest that the block between this fault and the Johnson Valley fault may have been tilted, down to the west. A 5 & hyphen;km block lifted by l m on one side would be tilted by an angle of 0.01 & deg; & lpar;190 & mu;rad & rpar;, producing fringes separated by about 160 m, parallel to the tilt axis. Such a tilt, parallel to a N20 & deg;W direction, would account for the gradual, northward increase of the vertical slip component observed along the Kickapoo fault. This tilt may also explain the 1 m of reverse slip observed along the & ldquo;slip gap & rdquo; section of the Homestead Valley break. Between the southern end of the Johnson Valley fault and the Eureka Peak fault, where no surface rupture has been mapped, the dense pattern of fringes implies distributed shear, probably resulting from fault slip at depth. The density and direction of the fringes in the gap are consistent with a right & hyphen;lateral slip of 1.2 & ndash;3.8 m on a blind fault locked above the depth of 1.5 & ndash;2 km. Such observations of small wavelength features in the SAR interferogram bring new insights into the near & hyphen;field displacement gradient and thus on response of the uppermost crust to seismic rupture.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/94JB01888
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1994
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  • 4
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    Online Resource
    Space geodetic measurement of crustal deformation in central and southern California, 1984–1992
    American Geophysical Union (AGU) ; 1993
    In:  Journal of Geophysical Research: Solid Earth Vol. 98, No. B12 ( 1993-12-10), p. 21677-21712
    Details
    In: Journal of Geophysical Research: Solid Earth, American Geophysical Union (AGU), Vol. 98, No. B12 ( 1993-12-10), p. 21677-21712
    Abstract: We estimate the velocity field in central and southern California using Global Positioning System (GPS) observations from 1986 to 1992 and very long baseline interferometry (VLBI) observations from 1984 to 1991. Our core network includes 12 GPS sites spaced approximately 50 km apart, mostly in the western Transverse Ranges and the coastal Borderlands. The precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95% confidence ellipse with a semiminor axis of approximately 2 mm/yr oriented roughly north‐south, and a semimajor axis of approximately 3 mm/yr oriented east‐west. For other stations, occupied fewer than 5 times, or occupied during experiments with poor tracking geometries, the uncertainty is larger. These uncertainties are calibrated by analyzing the scatter in three types of comparisons: (1) multiple measurements of relative position (“repeatability”), (2) independent velocity estimates from separate analyses of the GPS and VLBI data, and (3) rates of change in baseline length estimated from the joint GPS+VLBI solution and from a comparison of GPS with trilateration. The dominant tectonic signature in the velocity field is shear deformation associated with the San Andreas and Garlock faults, which we model as resulting from slip below a given locking depth. Removing the effects of this simple model from the observed velocity field reveals residual deformation that is not attributable to the San Andreas fault. Baselines spanning the eastern Santa Barbara Channel, the Ventura basin, the Los Angeles basin, and the Santa Maria Fold and Thrust Belt are shortening at rates of up to 5 ± 1, 5 ± 1, 5 ± 1, and 2 ± 1 mm/yr, respectively. North of the Big Bend, some compression normal to the trace of the San Andreas fault can be resolved on both sides of the fault. The rates of rotation about vertical axes in the residual geodetic velocity field differ by up to a factor of 2 from those inferred from paleomagnetic declinations. Our estimates indicate that the “San Andreas discrepancy” can be resolved to within the 3 mm/yr uncertainties by accounting for deformation in California between Vandenberg (near Point Conception) and the westernmost Basin and Range. Strain accumulation of 1–2 mm/yr on structures offshore of Vandenberg is also allowed by the uncertainties. South of the Transverse Ranges, the deformation budget must include 5 mm/yr between the offshore islands and the mainland.
    Type of Medium: Online Resource
    ISSN: 0148-0227
    URL: Article
    DOI: 10.1029/93JB02405
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 1993
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