Active Tectonics Impact on Society (1986) / Chapter Skim
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8. Investigation of Active Tectonics: Use of Surficial Earth Processes
8. Investigation of Active Tectonics: Use of Surficial Earth Processes
Pages 136-147
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From page 136... ...
A large num her of faults capable of producing damaging earthquakes have been identified and evaluated to determine rates of movement and potential earthquake hazard. For a few faults, including the San Andreas Fault north of Los Angeles, California, and the Wasatch Fault near Salt Lake City, Utah, recurrence intervals of recent (prehistoric)
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From page 137... ...
Much of the remainder of this paper will emphasize these points: use of geomorphic indices in reconnaissance studies of active tectonism; landform assemblages as indicators of active tectonism; and use of process-response models in establishing relations between landforms, earth materials, geomorphic processes, and tectonic processes for devising rates of active tectonics. Figure 8.1 summarizes the two main ap Geomorphic Indices- Landform Assemblage Process- Response measurements and ob~srvatlons from topographic maps, serial photographs, or field Dota Input | relative tectonic activity Oultut cress where more detailed | | | work might be productive | Use to | regional planning chronology geologic maps 1 / measured tectonic de form at ion geologic and tectonic f ramework rates of faulting recurrence intervals for earthquakes uplift rates site specific landuse 1 .
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From page 138... ...
Thus a regional evaluation of the San Gabriel Mountains suggests that detailed studies along the southern and eastern fronts of the range as well as near San Fernando have the best chance of yielding rates of vertical tectonics (uplift) , slip rates along active faults, and recurrence intervals of damaging earthquakes.
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From page 139... ...
On a more local scale, as for example along a specific mountain front or fault zone, active tectonics often modifies or produces characteristic landform assemblages. For 139 example, alluvial fans have a variable morphology somewhat dependent on tectonic processes, and active strike-slip faulting produces a specific set of tectonic landforms.
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From page 140... ...
i I -I,, ~j~ter ridge and deflected streams 500 m 1 FIGURE 8.5 Sketch map of offset alluvial fan along the San Andreas Fault in the Indio Hills, California. Modified from Keller et al.
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From page 141... ...
, utilizing topographic data from Seabeam surveys, demonstrated that the data base and resolution is now sufficient to begin studying submarine-tectonic geomorphology to improve mapping of active faults and to evaluate long-term earthquake hazard. Figure 8.6 shows topography and tectonic landforms associated with the San Clemente Fault zone (M.
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From page 142... ...
; and (3) rates of uplift, slip rates on active faults, and/or recurrence intervals of assumed earthquakes based on chronology and offset of landforms such as alluvial fans (Keller et al., 1982a)
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From page 143... ...
Bucknam and Anderson (1979) developed relations between scarp height and scarpslope angle for fault scarps in Utah with estimated ages ranging from 1000 to 100,000 yr (E;igure 8.11~.
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From page 144... ...
Faulted Holocene Deposits Erosional and depositional processes produce stream, marsh, lake, and landslide deposits that, when faulted, may produce valuable information concerning slip rates, rates of uplift or subsidence, and paleoseismicity. Two examples from the San Andreas Fault system in southern California are the Coyote Creek Fault part of the San Jacinto Fault zone (a major branch of the San Andreas Fault)
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From page 145... ...
estimated from offset stream channels of Wallace Creek along the south-central part of the San Andreas Fault (Figure 8.13) that the slip rate during the latest Pleistocene and Holocene has been about 30 to 40 mm/yr.
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From page 146... ...
Important considerations in developing rates of active tectonics based on geomorphic evaluation are measurement of deformation associated with past tectonic events (such as offset streams, glacial moraines, alluvial deposits, or other features) , development of the late Pleistocene to Holocene chronology to derive rates (defined as the ratio of measured deformation to the appropriate time interval)
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From page 147... ...
(1982~. Radiocarbon dates for the most recent large prehistoric earthquake and for late Holocene slip rates: San Andreas Fault in part of the Transverse Ranges north of Los Angeles, Geol.
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