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Geodesy in the Year 2000 (1990) / Chapter Skim
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Geodesy in the Year 2000: An Historical Perspective
Pages 9-22

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From page 9...
... Subsequent study determined the approximate magnitude to have been in excess of 8, the event having ruptured more than 400 kilometers of the nearby San Andreas fault. In the words of the Carnegie Commission, which was empaneled to investigate the earthquake and its causes (Lawson et al., 1908~: "The shock was violent in the region about the Bay of San Francisco, and with few exceptions inspired all who felt it with alarm and consternation.
From page 10...
... But the 1908-1910 Carnegie Commission reports on the San Francisco earthquake stand alone, because it was there that the elastic rebound theory of earthquakes was first introduced by Harry Fielding Reid (Reid, 1910~. The primary data supporting the hypothesis that earthquakes represent a rebound from a state of previously stored elastic strain energy were geodetic survey measurements conducted between 1851 and 1906 (Figure 1, taken from Hayford and Baldwin, 1908~.
From page 11...
... The principle sources of error were the accuracy with which the exact transit time could be determined by the observer, and undetermined errors in polar motion and universal time (UTAH. Moreover, since local transit time is supposed to be taken with reference to the geometrical reference ellipsoid, deflection of the vertical induced by anomalous masses implies a pointing error of the transit telescope, and thus errors in the inferred longitude.
From page 12...
... Other landbased instruments have followed, including multiwavelength electrooptical distance measuring apparatus, which measure and apply atmospheric refraction corrections automatically. Although not as important as horizontal triangulation for analyzing motions related to strike-slip faulting, leveling measurements had, by the late nineteenth century, reached a high state of technical accomplishment (Hosmer, 1919~.
From page 13...
... B Airy, then the Astronomer Royal, proposed in 1855 that mountains floated on a substratum capable of deforming inelastically in response to the excess gravitational load.
From page 14...
... As the chapters in this volume describe, a variety of new space-based observational techniques are under development which will allow significantly new approaches to old problems, and provide means for addressing scientific questions which were previously insoluble. As is implied by the foregoing historical discussion, current problems in geodesy revolve principally around positioning and gravity field determination.
From page 15...
... The average rates given by the global rigid plate motion models are defined over the preceding 2-3 million years, and the typical spatial scales represented are on the order of plate dimensions, that is to say, thousands of kilometers. Physical understanding of the processes responsible for these observations will come from increasingly sophisticated global convection models, which even now are yielding considerable insight into the long term evolution of the earth.
From page 16...
... It is typically upon these spatial scales that one sees significant departures from the predictions of the rigid plate motion models. These departures are in the form of space-time variations in average motion, due, for example, to earthquakes, transient motions of the asthenosphere, and aseismic slip, as well as in the existence of complex boundary zones of deformation.
From page 17...
... In addition to these structural questions, repeat measurements of gravity at the microgal level are a ready means of measuring, on a temporally continuous basis, small vertical motions of the crust. Typically, these vertical motions are most important in post-glacial rebound studies, where one expects to observe changes of a few microgals per year, and in association with volcanoes or thrust faults, where changes of tens to hundreds of microgals may be observed over time intervals of hours to years.
From page 18...
... For the continental lithosphere, gravity data allow models of rifting and continental extension to be systematically tested. For example, locations of continental rift zones are often associated with prominent gravity anomalies.
From page 19...
... The rationale lies in the search for an improved understanding of general circulation and dynamic topography in the oceans, which is in turn motivated by questions related to atmosphere-ocean interaction, and problems related to global climate change. As a final note, precise determinations of the earth's gravity field have taken on new importance in light of recent suggestions that classical Newtonian gravitation should be revised to admit shorter range interactions (the popularly termed fifth- and sixth-forces)
From page 20...
... F., The California Earthquake of April 18. 1906, Report of the State Earthquake Investigation Commission, published by the Carnegie Institution of Washington, Washington, DC; Volume II, The Mechanics of the Earthquake, 1910.
From page 21...
... roro Figure 1. Crustal motions, obtained by first order triangulation near San Francisco, California, during the years 1851-1868, and 1868-1906.
From page 22...
... Rates of Baseline length change obtained by Satellite Laser Ranging, compared with rates predicted by the global, rigid plate motion model of Minster and Jordan (1978~. Departures of the observed rates from the predicted rates are due to violation of the assumptions inherent in the rigid plate models.


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