Dating San Andreas fault earthquakes with lichenometry

Regional rockfall events in the Sierra Nevada of California are caused by distant earthquakes on the northern and southern San Andreas fault as well as by local earthquakes. Lichenometric dating of synchronous pulses of rockfalls that are presumed to be caused by five historical earthquakes suggests an accuracy of 2.2–3.5 yr (95% confidence level) for dating young events. Comparison of lichenometric ages for prehistorical rockfalls with precise radiocarbon ages of surface-rupture times at the Pallett Creek and Wrightwood paleoseismology sites indicates that earthquakes on the southern San Andreas fault caused rockfalls 200 to 400 km to the north. A major rockfall event at about A.D. 1690 supports the Fumal et al. (1993) model for a San Andreas fault earthquake that was not detected at Pallett Creek. The A.D. 1690 event in the chronology indicates that the earthquake recurrence interval was shorter and that the degree of clustering of times of earthquakes was less on the Mojave segment of the San Andreas fault than previously thought.