T31A-2820
Preliminary Paleoseismic Results from Excavations across the Surface Rupture Associated with the 2014 South Napa Earthquake
Wednesday, 16 December 2015
Poster Hall (Moscone South)
Carol S Prentice1, Robert R Sickler2, Katherine M Scharer3, Suzanne Hecker4, Stephen B DeLong5, Ron S Rubin5, James J Lienkaemper6, Timothy E Dawson7, Carla M. Rosa4, Alexandra Pickering2, Aaron Page2, Maxime Mareschal5, Daniel John Ponti6 and David P Schwartz4, (1)USGS California Water Science Center Menlo Park, Menlo Park, CA, United States, (2)US Geological Survey, Menlo Park, CA, United States, (3)USGS Pasadena Field Office, Pasadena, CA, United States, (4)USGS, Menlo Park, CA, United States, (5)California Geological Survey Menlo Park, Menlo Park, CA, United States, (6)USGS Western Regional Offices Menlo Park, Menlo Park, CA, United States, (7)California Geological Survey, Sacramento, CA, United States
Abstract:
The 24 August 2014 M6.0 South Napa Earthquake produced a complex surface rupture involving multiple fault traces, which were imaged and mapped in detail during the weeks following the earthquake. We excavated eight trenches across two of the fault strands near the northern termination of mapped surface faulting where the faults traverse fluvial terraces above Redwood Creek. At this location, Fault trace A (the principal rupture trace) and Fault trace C (the largest of the secondary ruptures) are approximately 100 meters apart. Fault slip in 2014 was minor at this location on both traces; linear regression of fence post locations modeled from terrestrial LiDAR scans of the northern property-boundary fence indicates about 5 cm of horizontal displacement on Fault A and 3 cm on Fault C. We excavated eight trenches across the two fault strands. Trenches 1 through 6, excavated on the lower, younger terraces, exposed no faulting in the underlying fluvial deposits. Trenches 7 and 8, sited on the higher, older terrace, exposed shear zones that juxtapose bedrock against Pleistocene fluvial gravel. On this older terrace, faulting extends into the youngest deposits underlying the Pleistocene terrace surface. We collected samples of organic materials for radiocarbon dating in trenches 1 through 6, and samples for luminescence dating from trench 8 to constrain the ages of the deposits. Initial radiocarbon ages of samples collected from unfaulted deposits suggest there has been no significant surface rupture on either fault in more than approximately 6000 years. Additional radiocarbon and luminescence ages will further constrain the timing of faulting. These preliminary results suggest that no large surface rupture has occurred on these faults in the late Holocene at this location, and that the 2014 surface rupture will not be preserved in the stratigraphic record at this site.
