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Active Tectonics: Studies in Geophysics
FIGURE 3.3 Geomorphic features related to active faulting.
cations for ground investigations. Low-Sun-angle techniques for reconnaissance or photography have been extremely useful in detecting subtle geomorphic features that would have otherwise been missed (Glass and Slemmons, 1978), such as in cities where geomorphic expression of scarps may have been smoothed out or altered, but general elevation differences still exist.
Key areas found through geomorphic investigation are often used as sites for further geologic investigations as exploratory trenching. Geomorphic investigations form a large part of the data base used in paleoseismic investigations. Freshness and continuity of geomorphic expression in space strongly suggest a surface rupture created during one event or over multiple events closely spaced in time.
Recently there have been numerous efforts to quantify the degree of degradation of fault scarps relative to age (see Chapters 7, 8, and 12, this volume).
Recognition of activity along some faults is possible by repeated geodetic surveys. Geodetic methods are capable of detecting and measuring tectonic strain of regions or across active faults. Reduction of the geodetic data permits determination of rate and direction of ground movement. The data provide another measure of fault displacement, both seismic or aseismic, and can assist in locating active branch faults or focus on areas of current movement within complex zones of faulting. Sylvester (Chapter 11, this volume) presents several methods of near-field geodetics including level lines, alignment surveys, trilateration, triangulation, and creepmeters. Regional geodetic leveling and trilateration surveys are made to monitor regional strain accumulation and release (Prescott et al., 1979; Vanicek et al., 1980).
Advances using satellite geodesy, e.g., the Navstar Global Positioning System (GPS), offer surveying techniques with a precision superior to classic surveying at one-twentieth the cost (Kerr, 1985).
Detailed studies of earthquake epicentral and hypocentral distributions of many fault zones can indicate the activity, continuity, location, dip and strike, seismogenic depth, and possible stress regime of the fault zone. However, this is often a difficult task. The quality of seismic data must be scrutinized and understood. The best-quality data come from dense seismometer networks that are limited to a few areas and are often temporary. Quarry blasts and possible geothermal, volcanic, and reservoir-induced seismicity must be separated from fault-related seismicity and can be analyzed as an additional seismic hazard. The remainder may be a well-defined zone of activity or a diffuse pattern of distributed activity. Well-defined zones of activity are common in aftershock areas and along creeping sections of faults. Diffuse patterns are harder to interpret but at least indicate that some strain is taking place in the area. A diffuse pattern of historical