Paleoseismologic data from trenches excavated across the Calico fault in the Eastern California shear zone reveal evidence for four surface ruptures during the past ∼9000 yr. Twelve optically stimulated luminescence dates constrain the timing of these surface ruptures, which are defined by the geometry of growth strata, fissure fills, and upward fault terminations, to 0.6–2.0 ka, 5.0–5.6 ka, 5.6–6.1 (or possibly 7.3) ka, and 6.1 (or 7.3) to 8.4 ka. Geomorphologic mapping of the 8 km section of the fault extending southward from the trenches reveals two sets of displacements that record the slip from the past two or three surface ruptures. The slip caused by the most recent event was ∼2.0 m, while the cumulative slip during the penultimate (and possibly the antepenultimate) event was ∼4.5 m. The ages of the paleoearthquakes coincide with periods of clustered moment release identified previously on other faults in the Eastern California shear zone at 0–1.5 ka, 5–6 ka, and ca. 8–9.5 ka, with two Calico fault surface ruptures occurring during the 5–6 ka Eastern California shear zone cluster. These data strongly reinforce earlier suggestions that earthquake recurrence in the Eastern California shear zone is highly clustered in time and space. Such seismic clustering suggests that at least some regional fault networks undergo distinct periods of systemwide accelerated seismic moment release that may be driven by feedbacks between fault-loading rate and earthquake activity.
- Received 27 October 2009.
- Revision received 20 May 2010.
- Accepted 21 May 2010.
- © 2010 Geological Society of America