Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones

Abstract

Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp-derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400-666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south-central segment of the San Andreas fault suggest that the M 8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constant b value) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes.Only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well. WASATCH FAULT ZONEThe Wasatch fault zone (Figure 1) has not ruptured historically. However, detailed paleoseismologic studies at sites along the zone [Swan et al., 1980; Schwartz et al., 1983] have confirmed that it has generated repeated large-magnitude events in Holocene time and have yielded information on the timing and size of these events. On the basis of data from the first two 5681

Time-predictable model application in probabilistic seismic hazard analysis of faults in Taiwan

“…However, the Gutenberg-Richter relation works well for large regions in general, but in most cases, the relationship underestimates the number of earthquakes with moderate to large magnitudes, such as the characteristic earthquakes of a fault (Schwartz and Coppersmith 1984;Wesnousky 1994). In particular, implied by the elastic-rebound theory, the renewal models are used to quantify the earthquake probabilities.…”

“…The characteristic earthquake model has significant implications for seismic hazard assessments associated with a particular fault. Each fault has an extensive geological and seismological dataset that defines the behavior of certain faults (e.g., Wasatch and San Andreas Fault system; the Chelungpu fault) (Schwartz and Coppersmith 1984). For well-instrumented active faults, the recorded information may possibly be used to estimate the recurrence rates for large earthquake events on an individual fault directly from the paleoearthquake investigation.…”

“…In reality, there is increasing evidence that using the characteristic earthquake model for large earthquake, generated from an individual fault, is more appropriate than using the Gutenberg-Richter relation (Wesnousky and Scholz 1983;Schwartz and Coppersmith 1984;Youngs and Coppersmith 1985;Hong and Guo 1995;Parsons 2005Parsons , 2006Parsons and Geist 2009). Figure 3 shows that the characteristic earthquake model indicates a non-linear earthquake frequency-magnitude relation dominated by the characteristic events (Schwartz and Coppersmith 1984;Youngs and Coppersmith 1985).…”

“…Figure 3 shows that the characteristic earthquake model indicates a non-linear earthquake frequency-magnitude relation dominated by the characteristic events (Schwartz and Coppersmith 1984;Youngs and Coppersmith 1985). In the frequency-magnitude relationship, a very low b-value is required in the moderate-magnitude range to reconcile the data throughout the range of magnitudes (Wesnousky 1994).…”

Factors affecting the recognition of faults exposed in exploratory trenches

“…Under the proper conditions, fault-related rubble and colluvium can leave a distinct record of individual prehistorical events ( fig. 27; Malde, 1971;Bonilla, 1973;Weber and Cotton, 1981;Schwartz and Coppersmith, 1984;Schwartz and Crone, 1985). Fault-related rubble at most sites was developed in unconsolidated materials.…”