Active Tectonics Impact on Society (1986) / Chapter Skim
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10. Geodetic Measurement of Active-Tectonic Processes
10. Geodetic Measurement of Active-Tectonic Processes
Pages 155-163
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From page 155... ...
Since about 1970, annual laser-ranging surveys in the western United States and Alaska have delineated the pattern and.current rates of deformation in these seismically active regions and have begun to provide accurate fault-slip rates to compare with late Holocene geologic estimates. The imperfect balance between interseismic strain buildup and coseismic strain release introduces a component of permanent deformation into the earthquake cycle that under favorable conditions can be estimated geodetically, providing another link between present-day movements and those preserved in the recent geologic record.
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From page 156... ...
Historical surveys, which typically have repeat times of decades or longer, sample long portions of the cycle, record coseismic and postseismic movements related to past great earthquakes, and provide estimates of the permanent deformation component of the cycle. Modern observations have been most useful in determining interseismic movement rates with high accuracy and refined temporal resolution and are beginning to provide precise estimates of present-day fault slip rates and evidence for hitherto unsuspected short-term irregularities in deformation rate.
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From page 157... ...
Although direct evidence is lacking on occurrence times and offsets of past events, measures of late Holocene slip rate confirm the value obtained from geodetic measurements. Dated offsets of late Holocene geomorphic features that cross the San Andreas Fault near Crystal Springs Reservoir, 40 km northwest of the geodetic network shown in Figure 10.2(A)
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From page 158... ...
IRREGULARITIES IN DEFORMATION RATE The largest variations in movement rate occur during the postseismic phase of the seismic deformation cycle, and once these transients have died out the measured strain rates are, as a rule, at least roughly constant. This constancy is demonstrated by comparisons between historical and modern data, and precise measurements of the past decade also show that year-to-year variations in rate are generally small.
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From page 159... ...
Comparing changes in gravity with changes in areal strain at all localities establishes a common linear scale r EXPLANATION O GRAVITY STATION · RE F E R E NCE GRAVITY STAT ION LEVEL LINE TRILATERAT10N NETWORK (LOCAL NETWORK SHADED) _ FAULT 120 ~, TEJON PASS ~:~ =61 ~nALE a NN ~ ~ ~ ~ ~ PA S 5 VIA L E O o o o 0 50 100 1 1 FIGURE 10.4 Index map showing locations of gravity stations, leveling lines, and trilateration networks in southern California that have been surveyed repeatedly during the past 5 to 10 yr.
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From page 160... ...
Frequently this permanent deformation is crudely similar to the pattern of coseismic movements observed in historical earthquakes, indicating that the two deformation processes are related and showing that at least in some regions strain accumulation and release are not in perfect balance. Here, two examples are used to demonstrate this imbalance and to illustrate the several links that connect the geologic and geodetic records.
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From page 161... ...
An extensive leveling network on the island of Shikoku and adjacent Honshu has been surveyed five times or more since about 1890, and numerous tidal gauge stations provide independent constraints on the vertical movement history of the region. In all, the geodetic record is about 90 years long, samples all parts of the deformation cycle, and has a duration comparable with the time interval between the past two events.
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From page 162... ...
and clearly exhibits the main elements of the cycle: the postseismic transient, the relatively steady interseismic phase of the cycle, and the significant component of permanent deformation. FUTURE DIRECTIONS During the next decade, full implementation of highly accurate extraterrestrial geodetic surveying methods will have an important impact on crustal deformation measurements in active regions.
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From page 163... ...
VLBI or SLR measurements over intercontinental baselines will then be capable of resolving relative movements of the Earth's major tectonic plates, and GPS networks with station separations of about 100 km can be used to outline the broad-scale deformation patterns in intracontinental active regions like the western United States and central Asia. Depending on their ultimately achievable accuracy and measurement costs, extraterrestrial methods may also be competitive with land-based surveying over shorter ranges as well.
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