Geodesy in the Year 2000 (1990) / Chapter Skim
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Implications of Precise Positioning
Implications of Precise Positioning
Pages 23-52
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From page 23... ...
Nevertheless, in the past decade, space geodesy has begun to provide useful constraints on the solution of difficult geological problems of immediate importance, such as the distribution of crustal deformation both east and west of the San Andreas Fault in central California (e.g., Jordan and Minster, 1988b)
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From page 24... ...
It is convenient to discriminate secular motions persisting on geological time scales of thousands to millions of years from transients associated with, for example, seismic and volcanic events. Practical research is more concerned with the transients, because they tend to disturb human activities.
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From page 25... ...
Because the geological data sets are large and the inverse problem is strongly overdeter~nined, the formal uncertainties in the angular velocity components are already quite small, and correspond to formal uncertainties of 1 or 2 mm/yr in the predicted rates of relative motions. More importantly, the fourth generation NOVEL- 1 model listed in Table 3 is consistent, at the 1- 2 m~7/yr level, with the hypothesis that maj or plates behave rigidly over a million-year time scale.
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From page 26... ...
In other words, the most important and interesting geodetic signals with characteristic time scales ranging from 1 hour to 100 years are detected as departures from predictions of million-year average rates based on geological rigid-plate models. However, there exist examples for which the reliability of currently available global models is difficult to assess, and some improvement could be made from space-geodetic data.
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From page 27... ...
large scale and regional scale non-rigid behavior (plate deformation and plate boundary zones of deformation)
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From page 28... ...
Although California's San Andreas Fault can be identified as a major locus of movement on the Pacific-North America plate boundary, the likelihood that significant crustal deformation is occurring both east and west of the San Andreas has long been recognized by geologists. Geological and geodetic observations of the present rate of slip along the fault in central California (about 34 mm/yr)
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From page 29... ...
motions along plate boundaries and within plate boundary deformation zones is insufficient at the present time to map the time and spatial scales involved. A space-geodetic system capable of high sampling rate and dense spatial coverage over large areas will make possible a nearly unprecedented exploration of how crustal deformation varies with time.
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From page 30... ...
An initial sudden displacement of a fault diffuses outward with a diffusivity ~ = he/, where ~ is the Maxwell time of the system. Repeated jerky offsets on plate boundary faults result in very smooth motion some distance away, corresponding to the steady velocities of plate interiors.
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From page 31... ...
The first large event was the Elmore Ranch earthquake (Ms~6.2) , followed 12 hours later by the main Superstition Hills event (Ms~6.6)
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From page 32... ...
It is hard to believe that the Superstition Hills earthquake was not triggered by the Elmore Ranch earthquake; but it is clear that no simple model of elastic strain and brittle failure can explain the 12-hour delay between events: if the Superstition Hills fault were close to failure, it should have ruptured during the dynamic strains generated by the Elmore Ranch earthquake, or at least soon after it, when the elastic stress changes had been fully imposed. We can think of several models, all speculative, to account for this.
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From page 33... ...
, but the ductile zone then begins to flow, interacting with the overlying elastic layer to cause a diffusion of stress outward from the fault, and hence increasing the stress on the Superstition Hills fault. For the usual estimates of crustal viscosity, we would expect little change in stress over 12 hours.
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From page 34... ...
time series, suggesting that there is time variation on time scales of a month or less that is missed by the rather infrequent measurements, i.e., that aliasing is a serious problem. Jachens et al.
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From page 35... ...
Only with such systems will we be in a position to characterize and understand the spatial distribution and the time dependence of deformation within tectonic regions, from which constraints on the physics of the deformation process can be inferred. TYPES OF GEODETIC SIGNALS AND MEASUREMENT TECHNIQUES The arguments presented so far lead us to conclude that the most significant departures from rigid-plate motions occur in zones of 100 to 1000 km width, within which differential motions are accommodated by a combination of seismic slip and aseismic deformation.
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From page 36... ...
The events considered here correspond to geodetic signals, expressed in terms of the displacement of a monument by a fixed fraction ~ of the spatial scale A, over the time scale r. The model for a allows us to construct a detection map in the (N,~)
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From page 37... ...
EXTRAPOLATION TO GEOLOGI C CONTEXT Unlike the situation in oceanic lithosphere, plate boundary activity within the continental lithosphere is commonly distributed over broad zones up to several thousand kilometers wide, which consist of complex networks of faults and folds. In these areas, it is not unusual for extensional, thrust and strike slip faulting to occur contemporaneously
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From page 38... ...
What regional strain nets cannot provide are the answers to fundamental questions about how strain is in fact accommodated, in what ways and how quickly the spatial distribution of deformation may change with time within a geologic system, and how mechanical and dynamical connections unify disparate types of deformation into a single coherent tectonic picture. However, in combination with other geological and geophysical information, accurate geodetic measurements using carefully positioned stations will provide an extremely powerful tool with which to address specific well-posed questions about crustal deformation in a particular tectonic setting.
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From page 39... ...
One might, however, infer that such large scale flow would be likely to vary only over the same time scale as the overall plate boundary activity, roughly tens of millions of years (for example, England et al., 19859. Similarly, in the case of the Mediterranean, questions aimed at understanding the interaction between active thrusting and extension, and which are amenable to study with geodetic and geologic data might include: How do rates of active convergence vary along the subduction boundaries?
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From page 40... ...
. Moreover, such systems would lead to lower personnel requirements, would allow easier access to remote areas, and would support both low resolution reconnaissance work, as well as detailed surveys of dense networks.
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From page 41... ...
At the same time, we must improve physical models of time-dependent deformation of the crust and upper mantle, and use these models in the planning of spacegeodetic surveys and in the interpretation of the spatio-temporal strain patterns seen by space-geodetic techniques. Of fundamental importance for interpretation purposes will be the systematic comparison of spacegeodetic observations to other, independent geophysical and geodetic data, such as observatory measurements of strain along short baselines, seismicity patterns, and other repeated geodetic and gravity surveys, in an effort to achieve a quantitative understanding of the phenomena which control volcanic and seismic cycles.
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From page 42... ...
surveyed local geodetic networks of sufficient density, around major geodetic sites in active areas, should receive high priority. 7.: The development and systematic deployment of affordable and easily deployable space-geodetic systems with cm to mm precision and high sampling rates -- that is, "occupation" frequency ranging from hourly to weekly -- will permit investigation of geophysical phenomena, particularly the earthquake cycle, in a range of spatial and temporal scales never explored before.
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From page 43... ...
b . Sus twined cen time ter - 1 evel accuracy, ex t ens i ve, l argely unexpl ored zones of , repeated measurements of dense networks at to determine the time dependence and spatial distribution of deformation within and across zones of intense tectonic activity.
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From page 44... ...
Royden et al., Tectonics, 2, 63-90, 1983. Royden, in Geodetic Studies and Crustal Dynamics, U.,S.
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From page 45... ...
Amer., 72, 1701-1715, 1982. Rate Amount and StYle of late Cenozoic Deformation of Southern Ningxia Northeastern Margin of Tibetan Plateau Inst.
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From page 46... ...
? 103 104 Figure 1 Spatial scales sampled by various geodetic methods.
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From page 47... ...
In addition, we must account for contributions from crustal deformation both east and west of the fault. The integrated deformation west of the fault, which consists of NW-SE extension across the Great Basin, can be measured directly from the rates-of-change of VLBI baselines monitored by NASA's Crustal Dynamics Project.
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From page 48... ...
Figure 5. Superstition Hills events recorded at The Pinon Flat Observatory (See Figure 4~.
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From page 49... ...
Map of geodetic signals in terms of spatial and temporal scales.
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From page 50... ...
k I y -stem rly 10° 10 1o2 103 104 Spatial scale X, km 2 -10 yr _1 yr _1 d _1 hr Figure 7. Detection capability of various geodetic techniques at the 10 ~ 7 s train level .
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From page 51... ...
Detection capability of various geodetic techniques at the 10-8 strain levels
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From page 52... ...
52 Table 3. _ _ Plate Pair na-pa cope china c~nz nz-pa nz-an nz-sa an-pa pa-au eu-pa co ca nz ca cu-na af-na aft na-sa af-sa an-se na ca ca-sa au-an af-an au-af au-in in-af ar-af ins ar-cu Tutu in-ar ~ 1 ON I^on~de Longitude °E 1 48.7 -78.2 0.78 36.8 -108.6 2.09 27.9 -120.7 1.42 4.8 -124.3 0.95 55.6 -90.
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