Earthquakes - Geotechnical Engineering Investigation - page 931

TABLE 11.7
Criteria Used for Recognizing an Active Fault a

General Criteria Specific Criteria

Geological Active fault indicated by the following features:

Young geomorphic features : Fault scarps, triangular facets, fault scarplets, fault rifts, fault
slice ridges, shutter ridges, offset streams, enclosed depressions, fault valleys, fault
troughs, side-hill ridges, fault saddles
Ground features : Open fissures, “mole tracks” and furrows, rejuvenated streams
Subsurface features : Stratigraphic offset of Quaternary deposits, folding or warping of
young deposits, en echelon faults in alluvium, groundwater barriers in recent alluvium

Historical Description of past earthquakes, surface faulting, landsliding, fissuring, and other

phenomena from historical manuscripts, news accounts, and other publications. Indications
of fault creep or geodetic monument movements may be indicated in recent reports

Seismological High-magnitude earthquakes and microearthquakes, when instrumentally well-located,

may indicate an active fault. A lack of known earthquakes cannot be used to indicate
that a fault is inactive

a

After Cluff et al. (1972).

TABLE 11.8
Some Methods of Dating the Minimum Age of Last Displacements on Faults a

Determining the age of undisplaced strata overlying the fault through the use of fossils, radiometric dating,
or paleomagnetic studies

●

Determining the age of cross-cutting undisturbed dikes, sills, or other intrusions

●

Determining the rate of development of undisturbed soil profiles across a fault

●

Radiometric dating of minerals caused by the fault movement or of undeformed minerals in the fault zone

●

Dating of geomorphic features along or across the fault

●

Dating techniques in fault investigations — see Appendix A.4

●

a

From Adair, M. J., Reviews in Engineering Geology , Vol. IV, Geological Society of America, 1979, pp. 27-39. With
permission.

where 2 m of displacement were adsorbed by 20 m of weathered rock (Bolt et al.,1975). In
such cases, trenches may prove inconclusive.
Present “dead” faults : Many faults that have not been carefully studied may be consid-
ered to be dead or inactive because they have not been the locus of recorded events or
activity, but may be potentially active. The Kern County event (1952) occurred along the
White Wolf fault ( Figure 11.9), which was little known and considered to be dead fault,
although it was approximately 64 km in length. It may be connected to either the San
Andreas or the Garlock Fault, the two largest in California, which are located only about
24 km apart. In August 1975, an earthquake of M

5.7 had its epicenter near the Oroville
Dam on one of the faults of the Foothills System of the Sierra Nevada range of California,
which was considered to be a dead fault. In 1952, the Ramapo Fault in northeastern New
Jersey (see Figure 6.61) was considered to be long dead. In the intervening years, the
installation of a seismograph station at Lamont, in addition increased area development
and habitation, have revealed that there is a substantial amount of activity along the fault
and today it even has its own recurrence equation (see Equation 11.15).

Fault Displacements
Importance
Correlations have been made among the amount of displacement, the fault length along
which displacement occurs, and the magnitude of the event. Displacement can vary