HR: 1330h
AN: S12B-0389    [PDF]
TI: Three-dimensional Geometry of Buried Fold Scarps Associated With Ancient Earthquakes on the Puente Hills Blind Thrust Fault
AU: * Leon, L A
EM: lorrainl@usc.edu
AF: Dept. of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AU: Dolan, J F
EM: dolan@usc.edu
AF: Dept. of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AU: Hoeft, J S
EM: hoeft@usc.edu
AF: Dept. of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AU: Shaw, J H
EM: shaw@eps.harvard.edu
AF: Dept. of Earth & Planetary Sciences, Harvard University, Cambridge, MA 02138 United States
AU: Hartleb, R D
EM: hartleb@usc.edu
AF: Dept. of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 United States
AB: The Puente Hills thrust fault (PHT) is a large blind thrust fault that extends east-west beneath the heart of the metropolitan Los Angeles region (Shaw and Shearer, 1999; Shaw et al., 2003). Christofferson (2002; in prep.) and Dolan et al. (2003) identified four buried fold scarps associated with large (Mw greater than or equal to 7), ancient earthquakes on the PHT beneath the City of Bellflower, in northern Orange County. One of the major outstanding questions regarding this research concerns the subsurface, three-dimensional geometry of these buried scarps. Specifically, we want to determine the extent to which the subsurface geometry of these scarps is controlled by tectonic versus fluvial processes. In order to begin addressing these questions, we drilled a north-south transect of hollow-stem, continuously cored boreholes across the buried fold scarps. This new borehole transect, which comprises six, 20-m-deep boreholes, was drilled parallel to, and $\sim$ 100 m west of, the original Carfax Avenue transect of Christofferson (2002) and Dolan et al. (2003). The overall pattern of progressive southward thickening of sedimentary units observed in the Carfax borehole transect extends westward to the new transect. Moreover, several key sedimentary contacts that are traceable laterally between the two transects occur at approximately the same depths at all locations along both transects. This three-dimensional data set thus defines several buried fold scarps that extend east-west beneath the study site. These observations confirm that the buried scarps are primarily tectonic, rather than fluvial features.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8010 Fractures and faults
SC: Seismology [S]
MN: 2003 Fall Meeting