HR: 1340h
AN: T13C-1395    [Abstracts]
TI: Noncharacteristic Slip on the Northern San Andreas Fault at the Vedanta Marsh, Marin County, CA
AU: * Zhang, H
EM: zhanghw@umkc.edu
AF: University of Missouri-Kansas City, 5100 Rockhill Road, Flarsheim Hall 420, Kansas City, MO 64110-2499 United States
AU: Niemi, T M
EM: niemit@umkc.edu
AF: University of Missouri-Kansas City, 5100 Rockhill Road, Flarsheim Hall 420, Kansas City, MO 64110-2499 United States
AU: Allison, A
AF: University of Missouri-Kansas City, 5100 Rockhill Road, Flarsheim Hall 420, Kansas City, MO 64110-2499 United States
AU: Fumal, T E
EM: tfumal@usgs.gov
AF: Earthquake Hazards Team, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Three-dimensional excavations along the 1906 trace of the northern San Andreas fault at the Vedanta marsh paleoseismic site near Olema, CA have yielded new data on the timing and amount of slip during the penultimate earthquake on this fault section. The excavations exposed a 3-m-wide paleochannel that has been offset right-laterally 7.8-8.3 m by coseismic slip during the past two large earthquakes: 1906 and the penultimate earthquake. The paleochannel was eroded into a silty clay marsh deposit and was filled after AD 1400. Both the silty clay layer and the paleochannel deposit are directly overlain by an in situ burn/peat sequence. The penultimate earthquake occurred while the peat was at the ground surface whereas faulting from the 1906 earthquake terminates within an overlying gravel/fill sequence. Preliminary OxCal analyses of radiocarbon dates indicate that the penultimate earthquake occurred in the late 17th to early 18th century. In plan view, two main fault traces were mapped in the excavation. The northwestern portion of the paleochannel is offset across a single fault trace. Just southeast of this portion of the channel the fault splits into two traces. We believe that one of these traces likely slipped only during 1906 and the other trace slipped on during the penultimate earthquake. Unfortunately, the overlying stratigraphic section that could resolve the exact reconstruction of movement on these faults is missing due to the excavation of an artificial drainage ditch at this location in the 1940's. Matching the north margin of the paleochannel to the first exposure of gravel in the zone between the two fault traces gives an offset of 5 m. We have historic records that show the 1906 coseismic slip near the study site was about 5m from field notes of David Starr Jordan (Stanford University Archives) who describes two 16 ft (5m) offsets: one of a tree located about 150m SE of the offset channel and the other of a path to the Shafter barn located about 300m NW. As the locations of these two historical records are so close to the study site, it is reasonable to assume that our excavation site has the same amount of coseismic slip in 1906. Our data indicate that the paleochannel was offset about 2.8 to 3.3 m during the penultimate earthquake which occurred in the late 17th to early 18th century, and that the San Andreas fault at this section is capable of slip in earthquakes smaller than 1906.
DE: 7221 Paleoseismology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting