SEISMOGENIC MODEL

Earthquakes are generally assumed to be elastic waves radiating out from a rupture in the Earth that slips suddenly and generally in a brittle manner. Most evaluations of the potential for surface faulting and earthquakes assume that earthquakes of above magnitude MS=6 are by brittle failure and are represented by fault-rupture parameters. This simple model is effective for many examples of surface faulting, particularly for faults with moderate to steep dips and with good surface exposure. The release of energy is a function of fault rupture parameters and is also affected by the elastic rebound of the strained volume of rock, associated folding (King and Stein, 1983; Hill, 1984; Molinari, 1984), detachment faulting (Hardyman, 1978; Berberian, 1982), fault type and attitude (low-angle thrust faults), and surface exposure.

HAZARDS RELATED TO FAULTS

Earthquake and Ground Motion

Perhaps the best-known hazards of active faulting are the destructive effects of earthquake shaking, often called “strong ground motion.” Sudden movement along a fault or fault zone radiates elastic waves that are generally strongest near the causative fault and taper off or attenuate away from the fault. Strong ground motion is the single largest natural factor in causing earthquake damage, including loss of life and property, failure of structures, disruption of utilities, and numerous secondary effects such as landsliding and liquefaction.

The characteristics and intensity of strong ground motion at different sites usually varies with a number of factors, including earthquake size, attentuation, and local ground response. Variation of strong ground motion with distance from a causative fault has been one of the most discussed factors, based on many analyses of strong-motion records from historical earthquakes and various theoretical considerations. Recent studies by McGarr (1984) show that the intensity of strong ground motion may vary with the type of motion along a fault (e.g., reverse motion versus normal motion). Details of the geometry of fault zones can have a large influence on strong ground motion. Local areas where the fault is not so free to slide concentrate stress (Bakun et al., 1980, 1984; Aki, 1984); when these places are broken and the stress is released a large amount of high-frequency energy is generated resulting in high-frequency ground motion.

Source directivity, a phenomenon involving the propagating fault rupture and its relationship with the elastic waves being radiated, can have a pronounced effect on the frequency and amplitudes of the radiating waves and should be addressed for sites near the causative fault (Singh, 1981). Other factors such as the radiated wave’s travel path characteristics (Singh, 1981), topographic effects at a site (Davis and West, 1973), and site materials and geology (Rogers et al., 1983) can also affect strong ground motion. Many siting and design studies use a sophisticated approach to strong ground motion estimation involving the combined expertise of the seismologist, the geologist, and the engineer for deterministic and probabilistic studies.

The effect of strong ground motion is incorporated into building codes and may influence zoning (Berg, 1983). Geotechnical studies of soils as well as geologic and seismologic fault studies are vital to this ground motion potential assessment.

Surface Rupture

One of the most direct hazards (effects) of active faulting is displacement or offset at the foundation of a structure (Swiger, 1978). Ruptures can occur suddenly during earthquakes or slowly or gradually by creep. Three main types of fault movements associated with a faulting event are primary, secondary, and sympathetic movements. A primary rupture occurs along the main fault responsible for the earthquake and can be estimated from observations and regression analyses (Slemmons, 1982b; Bonilla et al., 1984) of historical earthquakes and fault displacement. These are commonly based on rupture length or maximum displacement; actual observed effects can be reduced by drag and distributed rupture, plastic failure, detachment, and other causes.

The construction of some brittle structures may not tolerate even small fault rupturing. The proposed Auburn Dam in California was in advanced stages of site preparation when the possibility of fault displacement of about 15 cm in the foundation was estimated by Woodward-Clyde Consultants (1977). The consultants and a special review panel essentially concurred that, although the recurrence interval was very long, the maximum expected ground displacement of about 15 cm could occur with an associated earthquake of magnitude 6.5. The concrete double arch dam could not accommodate this large a foundation displacement, and the proposed dam was abandoned as inappropriate for the site even though $200 million was spent for site preparation.

Secondary ruptures are those that occur along branch faults and other faults subordinate to the principal fault trace. These faults locally accommodate deformation



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