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

TLDR

Paleoseismological studies of the Wasatch and San Andreas fault zones support the characteristic‑earthquake model, in which individual fault segments repeatedly produce earthquakes of similar size near their maximum magnitude. The authors analyzed scarp‑derived colluvium in trench exposures along the Wasatch fault to estimate the timing and displacement of individual surface‑faulting events. The study found that each event displaces 1.6–2.6 m (average ~2 m), identifies six major segments, estimates a 400–666 year recurrence interval (≈444 yr), confirms the 1857 M8 earthquake as characteristic for the San Andreas south‑central segment, and shows that only a low b‑value reconciles small‑earthquake statistics with large‑earthquake geology, underscoring the fundamental nature of characteristic earthquakes on these faults.

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 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.