Active Tectonics Impact on Society (1986) / Chapter Skim
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14. Seismic Hazards: New Trends in Analysis Using Geologic Data
14. Seismic Hazards: New Trends in Analysis Using Geologic Data
Pages 215-230
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From page 215... ...
These are the most frequently asked questions in evaluating seismic hazards. The ability to answer these, whether estimating a maximum earthquake, the amount of potential surface displacement on an active fault, or the probability of exceeding a particular level of ground motion, rests on the ability to recognize and characterize seismic sources Seismic source characterization is the quantification of the sizers)
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From page 216... ...
Because of this they are now being used to estimate earthquake recurrence on individual faults, especially in probabilistic seismic hazard analyses. r FAULT ZONE :~ | SEGMENTATION | Rupture Lengthy 1 / \ Maximum Earthquake G EO LOG I C DATA Slip Rate Recurrence Interval Elapsed Time .',~-, ~.
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From page 217... ...
The recognition of geomorphic features such as tectonic terraces and individual stream offsets, morphometric analysis of fault scarps, and evidence of past liquefaction also provide direct information on the number of past events for many faults. Where datable material is found, the actual intervals between successive events can be determined, although in many cases only average recurrence intervals can be estimated.
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Examples of Segmentation Wasatch Fault Zone, Utah The Wasatch is a 370km-long normal-slip fault that has not had a historical surface-faulting earthquake. Based on historical surface ruptures on normal faults in the Great Basin, which have ranged in length from about 35 to 65 km, only a part of the Wasatch Fault zone will be expected to rupture in future earthquakes with lengths comparable to the historical examples.
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concluded that the 1980 displacement pattern was similar to past surface ruptures and that features of fault geometry and barriers that control the nucleation and propagation of rupture on this fault have persisted through geologic time. Lost River Fault Zone, Idaho Surface faulting associated with the October 28, 1983, Borah Peak, Idaho, earthquake (Ms = 7.3)
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From page 220... ...
Allen (1968) recognized differences in the historical behavior of various parts of the San Andreas Fault zone and identified four segments (Figure 14.5~: a northern segment that was the location of the 1906 rupture, a central segment that is currently creeping and has been the location of repeated moderate earthquakes during this century, a south-central segment that was the location of the 1857 rupture, and a southern segment that has not generated large earthquakes during the historical period.
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showed that Wasatch Fault segments defined on the basis of paleoseismicity data are also reflected by systematic changes- in the elevation of the Wasatch Range. The elevation of the range is highest where late Quaternary slip rates are fastest and recurrence intervals are shortest, the elevation of the range decreases at segment boundaries and where Holocene scarps die out, and the elevation of the range is lowest at each end where paleoseismicity data reveal the lowest Holocene slip rates and the longest recurrence intervals along the fault zone.
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The independent behavior of fault segments has important implications for seismic hazard evaluation. Segment identification provides a physical basis for the selection of rupture lengths used in the calculation of maximum earthquakes.
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223 auk .4~ew _ _ _ ¢_ ~ i..
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From page 224... ...
The evaluation of geologic recurrence rests on the ability to recognize past events, date the interval between events, and evaluate the size of each event. By combining the recurrence intervals for large-magnitude earthquakes developed from geologic data with the recurrence for smaller-magnitude events developed from seismicity data, a characteristic earthquake recurrence model is derived that has the general form shown in Figure 14.7.
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Unfortunately, these types of paleoseismicity data are not currently available for most faults. The late Quaternary geologic slip rate, however, can frequently be obtained and is being used to constrain fault-specific earthquake recurrence relationships for seismic hazard analysis.
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This difference can have a significant impact on seismic hazard assessment at a site, depending on whether the moderate-magnitude events or the large events contribute most to the hazard. One final consideration that is important in assessing earthquake recurrence from fault slip rate (moment rate)
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Along plate boundaries, such as major transform fault zones like the San Andreas, or subduction zones, where the rate and source of stress are relatively constant and the rate of strain accumulation is high, major faults and fault segments may approach a generally uniform behavior. In these cases, a time-predictable model may provide a reasonable approach to quantifying hazard, even if there is some uncertainty in the precision regarding the regularity of recurrence.
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SOME FINAL THOUGHTS In recent years there has been an evolution in the approach toward the evaluation of seismic hazards. Deterministic estimates of maximum earthquake size and associated ground motion that are based on a restricted data base are gradually being replaced by probabilistic assessments of future earthquake potential that incorporate information on earthquake recurrence intervals, displacement per event, fault slip rate, fault segmentation, and the uncertainties in these parameters.
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(1970~. Earthquake recurrence intervals on the San Andreas Fault, California, Geol.
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From page 230... ...
Implications of fault slip rates and earthquakes recurrence models to probabilistic seismic hazard estimates, Bull. Se~smol.
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