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  • GEOPHYSICS  (7)
  • 1
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    Seismic velocity, attenuation and rheology of the upper mantle (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Seismic and rheological properties of the upper mantle in the vicinity of the low-velocity zone are expressed in terms of relaxation by dislocation glide. Dislocation bowing in the glide plane explains seismic velocities and attenuation. Climbing at higher stresses for longer periods of time give the observed viscosity, and explain the low velocity and high temperature attenuation found at seismic frequencies. Due to differing parameters, separate terms for thermal, seismic and rheological lithospheres are proposed. All three lithospheres, however, are related and are functions of temperature, and must be specified by parameters such as period, stress, and stress duration.
    Keywords: GEOPHYSICS
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  • 2
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    Numerical modelling of instantaneous plate tectonics (1974)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Assuming lithospheric plates to be rigid, 68 spreading rates, 62 fracture zones trends, and 106 earthquake slip vectors are systematically inverted to obtain a self-consistent model of instantaneous relative motions for eleven major plates. The inverse problem is linearized and solved iteratively by a maximum-likelihood procedure. Because the uncertainties in the data are small, Gaussian statistics are shown to be adequate. The use of a linear theory permits (1) the calculation of the uncertainties in the various angular velocity vectors caused by uncertainties in the data, and (2) quantitative examination of the distribution of information within the data set. The existence of a self-consistent model satisfying all the data is strong justification of the rigid plate assumption. Slow movement between North and South America is shown to be resolvable.
    Keywords: GEOPHYSICS
    Type: Geophysical Journal; 36; 3, Ma; Mar. 197
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  • 3
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    Present-day plate motions (1977)
    In:  CASI
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    Publication Date: 2019-06-27
    Description: A data set comprising 110 spreading rates, 78 transform fault azimuths and 142 earthquake slip vectors was inverted to yield a new instantaneous plate motion model, designated RM2. The mean averaging interval for the relative motion data was reduced to less than 3 My. A detailed comparison of RM2 with angular velocity vectors which best fit the data along individual plate boundaries indicates that RM2 performs close to optimally in most regions, with several notable exceptions. On the other hand, a previous estimate (RM1) failed to satisfy an extensive set of new data collected in the South Atlantic Ocean. It is shown that RM1 incorrectly predicts the plate kinematics in the South Atlantic because the presently available data are inconsistent with the plate geometry assumed in deriving RM1. It is demonstrated that this inconsistency can be remedied by postulating the existence of internal deformation with the Indian plate, although alternate explanations are possible.
    Keywords: GEOPHYSICS
    Type: NASA-CR-156761
    Format: application/pdf
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  • 4
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    Relative array analysis of upper mantle lateral velocity variations in Southern California (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The many densely spaced stations of the Southern California Seismic Network, SCARLET, provide a unique opportunity to examine local variations in upper mantle velocities. By averaging the incoming teleseismic wavefronts over 100-km aperture subarrays, ray parameter dT/dDelta, azimuth, and average arrival time estimates were obtained for each event. These values are compared to whole array plane fit parameters with relative array diagrams and representations of net subarray delays. The conducted study confirms the existence of a high-velocity body at depth beneath the Transverse Ranges and a low-velocity region under the Salton Trough. An uppermost mantle anomaly beneath the San Bernardino Mountains is well established both by this study and from local data. The relation of this body to the deeper structure is investigated.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 87; Mar. 10
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  • 5
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    The physics of creep and attenuation in the mantle (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The objective is to investigate the role of dislocations in both creep and attenuation. Attenuation in the mantle at seismic frequencies is thought to be caused by the glide of dislocations in the subgrains. Both kink and impurity drag can contribute to the glide time constant. The kink-formation, or Peierls barrier, model for dislocation glide is seen as a low-temperature, high-frequency mechanism most appropriate for pure systems. It is noted that a small amount of impurity drag brings the dislocation glide characteristic time into the seismic band at upper-mantle temperatures. The attenuation and creep behavior of the mantle are related by way of the dislocation structure. Analysis of the various possible mechanisms is facilitated by casting them and the geophysical data in terms of a pre-exponential characteristic time and an activation energy.
    Keywords: GEOPHYSICS
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  • 6
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    Present-day plate motions (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: An instantaneous plate-motion model, Relative Motion 2 (RM2), is obtained by inverting a data set comprising 110 spreading rates, 78 transform fault azimuths, and 142 earthquake slip vectors. RM2 is compared with angular velocity vectors which best fit the data along individual plate boundaries and, while the model performs close to optimally in most regions, attention is directed to those regions which are not suitably described by the model. Reasons for the discrepancies between RM2 and observations for the India-Antarctica plate boundary, the Pacific-India plate boundary, and the east-west trending transform fault azimuths observed in the French-American Mid-Ocean Undersea Study area are discussed.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 83; Nov. 10
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  • 7
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    Detection of crustal deformation from the Landers earthquake sequence using continuous geodetic measurements (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The first measurements are reported for a major earthquake by a continuously operating GPS network, the permanent GPS Genetic ARRY (PGGA) in southern California. The Landers and Big Bear earthquakes of June 28, 1992 were monitored by daily observations. Ten weeks of measurements indicate significant coseismic motion at all PGGA sites, significant postseismic motion at one site for two weeks after the earthquakes, and no significant preseismic motion. These measurements demonstrate the potential of GPS monitoring for precise detection of precursory and aftershock seismic deformation in the near and far field.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 361; 6410; p. 337-340.
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