EVIDENCE FOR REPEATED LATERAL SPREADING ALONG THE LOWER PAJARO RIVER, WATSONVILLE, CALIFORNIA

This investigation, designed to evaluate whether lateral spreads occur repeatedly in the same location, documents evidence of recurring liquefaction-induced sand injection and lateral spreading along a stratigraphic unconformity within the Pajaro River floodplain near Watsonville, California. We excavated two trenches across a lateral spread formed by the 1989 Loma Prieta earthquake and located south of the Pajaro River in floodplain sediments at the Miller Farms site that was originally identified by the US Geological Survey during post-earthquake studies. In addition to liquefaction-related features produced in 1989, the trench walls revealed evidence for at least two to three prior lateral spread failures and associated liquefied sand bodies. The site likely records evidence for failure from the 1906 M 7.8 San Francisco earthquake and earlier events on the San Andreas fault. The spreading repeatedly occurred along a ~1-m-wide zone that coincides with a buttress unconformity between middle to late Holocene floodplain deposits (south of the unconformity) and late Holocene to historic fluvial deposits of an aggraded inset river terrace (north of the unconformity) of the Pajaro River. Trench walls exposed a secondary zone of discontinuous normal faults with small (< 1 cm) vertical displacements, located toward the river and several meters north of the primary lateral spread zone. Minor normal faults generally coincide with ground cracks caused by the 1989 earthquake, although it is permissible that an earlier lateral spread produced some of the faults. The minor displacements on faults in the secondary zone relative to the massive failure along the primary lateral spread zone indicates that the primary mode of deformation at the Miller Farms site has been repeated failure localized along a buttress unconformity. Detrital charcoal collected from within and above a structurally tilted sand layer suggests that the ante-penultimate event happened after A.D. 1400.