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Active Tectonics: Studies in Geophysics
three factors: (1) the time since the last event or recurrence interval, (2) the surface wave magnitude, and (3) fault slip rate. This figure also shows the general geomorphic expression for each interval of fault slip rates, although such factors as climate, time since the last event, variations in fault slip rate, or noncharacteristic activity can affect the landforms. Cluff and Cluff (1984) noted that the common current use of “active” and “inactive” can be a scientific oversimplification that may lead to improper siting or design of engineered structures. Use of quantitative measures for degree of activity, such as fault slip rate and recurrence interval with probabilities and variance, can lead to deterministic values providing meaningful numbers for analysis by probabilistic methods.
One of the most convincing arguments or evidence of fault activity is the cross-cutting or non-cross-cutting relationship with a datable unit. If Holocene activity is the criterion for activity, then a Holocene age unit crossing the fault could be an ideal location for a trench site. If the unit is offset, then the age of the unit and the amount of offset can be used to estimate a slip rate and a recurrence interval if the nature of characteristic earthquake is known. A wide variety of types of Holocene deposits have been used for evaluation of fault activity, most commonly alluvial and volcanic deposits. Deposits are dated by carbon-14 radiometric methods, tephrochronology, soil development, fossil stratigraphy, and many other techniques. Pierce (Chapter 13, this volume) presents a good summary and review of Quaternary dating methods. Exposures of faulted units may be found in stream cuts and landslide scars or in road cuts or other man-made excavations. To prove whether a fault or strand of a fault system is active, a trench may be dug at the proposed site and the geologic units and soils inspected for faults. If no demonstrated fault activity has taken place in these geologic units within the defined “active” fault period, the proposed structure can be considered reasonably safe from damage from surface faulting.
The structural aspects of young geologic units adjacent to faults may also provide information about activity of a fault. Adjacent units may be brecciated and shattered, have open fissures, be tilted or warped, or have secondary effects of faulting and liquefaction effects (e.g., sand boils and sand dikes). In a detailed study of a fault, the youthful geologic units should be described, delineated, and inspected for evidence of young faulting.
The freshness of appearance and type of geomorphic expression of faults is related to the age of faulting (Matsuda, 1975; Slemmons, 1977, 1982a; Wallace, 1977, 1978). Geomorphic investigations into faulting are relatively easy and can yield considerable information. Many landforms such as depressions and sag ponds, open rifts, and prominent high-angle scarps suggest youthfulness and further help to identify the active traces or strands of faults zones (Figure 3.3).
A geomorphic investigation begins with examination of aerial photographs or an aerial reconnaissance. Overview of the geomorphology allows delineation of key lo-
FIGURE 3.2 Relation between time or recurrence interval between earthquakes, earthquake magnitude, and slip rate across the fault zone. This chart assumes that most of the energy is released by seismogenic rather than aseismic activity and that the average displacement is one half the maximum. [Modified from Matsuda (1975) and Slemmons (1977).]