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Active Tectonics: Studies in Geophysics
faults, potential earthquake hazard, and recurrence of large earthquakes obtained from quantitative field studies in geomorphology is extremely useful in long-term (tens to hundreds of years) land-use planning, a goal of which is earthquake-hazard reduction. Specifically, studies of active faults are providing critical information necessary for residential and commercial zoning near active faults; establishing building codes; and planning for large dams, nuclear power plants, liquified natural gas facilities, and other critical facilities.
Geomorphic evaluation of active tectonics has taken two approaches depending on whether reconnaissance information or detailed evaluation is desired. Reconnaissance work to identify areas where active tectonics is particularly significant generally involves the use of geomorphic indices (sensitive to rock resistance, climatic change, or tectonic processes) or assemblages of landforms produced or modified by active-tectonic processes. Detailed, site-specific study of active tectonics often involves evaluation of process-response models that attempt to explore relations between landforms, earth materials, geomorphic processes, and active tectonics integrated through time. The concept of time or chronology is introduced here because without establishment of a reliable chronology, process-response models will not yield rates of faulting and recurrence intervals of damaging earthquakes that are necessary in evaluating seismic risk. 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
FIGURE 8.1 Active tectonics and geomorphology: data input, output, and use to society.
proaches to studying geomorphologic indicators of active tectonics and use to society.
GEOMORPHIC INDICES AND ACTIVE TECTONICS
Geomorphic indices are useful tools in evaluating active tectonics because they quickly provide insight concerning specific areas or sites in a region that is adjusting to relatively rapid rates of active-tectonic deformation. Indices that have been most successful are related to erosional and depositional processes associated with fluvial (river) systems. The best known of these are the stream-gradient index (SL index) developed by Hack (1973), the mountain-front sinuosity (Smf index) developed by Bull (1977a, 1978), and the ratio of valley-floor width to valley height (Vf index) also developed by Bull (1977a, 1978).
The stream-gradient index (Hack, 1973), later applied to the San Gabriel Mountains in southern California by Keller (1977), is defined as
where SL is the stream-gradient index, ΔH/ΔL is the local gradient of the stream reach where the index is computed (ΔH is the drop in elevation of the reach and ΔL is the length of the reach), and L is the total channel length from the drainage divide to the center of the reach, measured along the channel.
The SL index is crudely related to the available stream power, defined as the product of water discharge and water-surface slope, and thus reflects the ability of the stream to transport its load. The index is a surrogate for stream power because the upstream channel length is proportional to bankfull discharge and the slope of the water surface is approximated by the slope of the channel bed.
The stream-gradient index is particularly sensitive to changes in slope and thus is a valuable tool in evaluating active tectonics with a strong vertical component of deformation. However, the index is also sensitive to rock resistance (resistant rock produces a steep channel slope), and differentiating between effects of tectonics and rock resistance may be difficult. That is, values of the index are high in areas where the rocks are particularly resistant or where active tectonics has resulted in vertical deformation at the Earth’s surface. Therefore, anomalously high SL indices in rocks of low or uniform resistance is a possible indicator of active tectonics. Figure 8.2 shows stream-gradient indices for the San Ga-