Research Projects

Quaternary investigations to evaluate seismic source characteristics of the frontal thrust belt, Palo Alto region, California

PI: Tom Bullard
Co-PI: K. Hanson, H. AbramsonWard, M. Angell, J. Wesling

Project Period: 2001 - 2004

Funding: US Geological Survey, National Earthquake Hazards Reduction Program

Keywords: Palo Alto, Seismic force, frontal thrust belt, San Andreas fault

Project Description

Active blind and emergent faults that occur in a deformation zone adjacent to and east of the San Andreas Fault pose a seismic hazard to communities and infrastructure in the Palo Alto and regions of the San Francisco Peninsula. Tectonic activity expressed in the geomorphology of this region led to geomorphic analysis that helped target areas for detailed mapping of Quaternary deposits and deformed surfaces. Resulting information was used to evaluate the location and activity of individual faults and the expected style of deformation that might occur in future earthquakes. During our previous studies in the region (Angell et al., 1997), a kinematics model and geometry of faulting was developed based on preliminary Quaternary investigations combined with analysis of bedrock structure and construction of retrodeformable cross sections. The 1997 study showed that the structures in this area were active during the latest Pleistocene and possibly the Holocene. Preliminary age estimates and vertical separations of the terraces suggested uplift rates of 0.15 m/kyr for the Hermit fault, 0.15 to 0.2 m/kyr for the Pulgas fault, and 0.4 to 0.6 m/kyr for the Stanford fault zone. Our most recent study focused on the latest Pleistocene and Holocene terraces of San Francisquito Creek that cross the Pulgas fault and Stanford fault zone. The study incorporated available radiocarbon analyses, detailed topographic survey data, fluvial stratigraphy, archaeological information, and geomorphic analysis. Expression of deformation across the active structures includes aligned topographic relief above growing folds, warped terraces, stream profile inflections, and changes in stream pattern. Terrace mapping refinements showed that latest Pleistocene terraces are gently folded across the Stanford fault zone but are not displaced across the Pulgas fault. Detailed total station survey of the vertical deformation of three terraces and terrace correlation previously dated Stanford Man II site indicate that the uplift rate across the Stanford fault zone is approximately 0.6 ± 0.05 mm/yr.

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