HR: 0800h
AN: S21A-0244    [Abstracts]
TI: Nature of Active Traces of the Hayward Fault at the University of California, Berkeley
AU: * Wells, D L
EM: dwells@geomatrix.com
AF: Geomatrix Consultants, 2101 Webster Street, 12th Floor, Oakland, CA 94612, United States
AU: Swan, F H
EM: bswan@geomatrix.com
AF: Consulting Geologist, 240 Laidley Street, San Francisco, CA 94131, United States
AU: Thompson, S C
EM: thompson@lettis.com
AF: William Lettis & Associates, 1777 Botelho Drive, Suite 262, Walnut Creek, CA 94596, United States
AU: Baldwin, J N
EM: baldwin@lettis.com
AF: William Lettis & Associates, 1777 Botelho Drive, Suite 262, Walnut Creek, CA 94596, United States
AU: Williams, P L
EM: plw3@earthlink.net
AF: Williams Associates, P.O. Box 1492, West Tisbury, MA 02575, United States
AU: Rubin, R S
EM: rrubin@geomatrix.com
AF: Geomatrix Consultants, 2101 Webster Street, 12th Floor, Oakland, CA 94612, United States
AU: Lavine, A
EM: alavine@geomatrix.com
AF: Geomatrix Consultants, 2101 Webster Street, 12th Floor, Oakland, CA 94612, United States
AU: Hall, N T
EM: thall@geomatrix.com
AF: Geomatrix Consultants, 2101 Webster Street, 12th Floor, Oakland, CA 94612, United States
AB: The location of the Hayward fault zone at the University of California Berkeley Campus is well defined by geomorphic features including offset stream channels, side-hill benches, and the break-in-slope at the base of the Berkeley Hills, as well as by fault-creep related deformation of curbs, buried culverts and utilities, and structures—most notably Memorial Stadium. Based on the mapped fault traces associated with these surficial features, more than 30 trenches have been excavated at various locations on the campus during the past twenty years to assess the exact location and width of the active fault zone near existing and planned structures. These trenches show that the active fault trace(s) range from well expressed to poorly expressed in various surficial materials as a function of (1) the local geometry of the fault, (2) the stability of the near-surface deposits (e.g., it is poorly expressed where it crosses active landslides), and (3) the age of the deposits (i.e., it is better expressed in older deposits). At locations near the Smyth-Fernwald Housing, Prospect Court, the Greek Theater, and Foothill Housing, trenches showed that the fault is characterized by multiple distinct traces that in many places bound alluvial/colluvial-filled depressions up to 6 meters wide, and are in-filled with Holocene deposits. Quaternary deposits and bedrock units are truncated, indicating that significant lateral and vertical displacement has occurred along these fault traces. The creeping trace of the fault generally coincides with these well expressed fault traces. Trenches also revealed that two sub-parallel active fault traces as much as 40 to 60 meters apart extend along the hillslope directly east of the Greek Theater and north and east of Bowles Hall. It remains uncertain as to how fault creep occurs along the two separate branches. Between Memorial Stadium and Bowles Hall, there is a small right bend or stepover in the fault. The location of the creeping trace is well defined by deformation to Memorial Stadium, curbs, and buried culverts in this area, but trenches reveal that the creeping trace is associated only with a weakly expressed "parting fabric" in homogeneous colluvial deposits and as wide, moderately expressed shear zones within landslide-derived bedrock blocks. Stratigraphic markers in trenches north of Memorial Stadium are disrupted in several places by features that have uncertain origin, yet indicate little cumulative lateral or vertical displacement is associated with these features.
DE: 7221 Paleoseismology (8036)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting