Newly collected borehole and high-resolution seismic reflection data from a site ~6 km south of downtown Los Angeles demonstrate that the Compton blind-thrust fault has generated multiple large-magnitude earthquakes during the Holocene. This large blind thrust fault, which was originally identified by Shaw and Suppe (1996) using industry seismic reflection profiles and well data, extends northwest-southeast for 40 km beneath the western edge of the Los Angeles basin. The industry seismic reflection data define a growth fault-bend fold associated with the thrust ramp. The industry data, however, do not image deformation in the uppermost few hundred meters, which is of the most interest for seismic hazard analysis. In order to bridge this gap, we acquired high-resolution seismic reflection profiles at two scales across the back limb active axial surface of the fault-bend fold above the Compton thrust ramp, which reveal an upward-narrowing zone of folding that extends to <100 m of the surface. These data, in turn, allowed us to accurately and efficiently site a fault-perpendicular transect of ten continuously cored boreholes across the axial surface of the fold observed in both industry and high-resolution seismic reflection data. The borehole data reveal folding within a discrete kink band that is <~60 m wide in the shallow subsurface, with most deformation occurring within a zone <30 m wide. Analysis of the deformed, shallow growth strata reveals evidence for stratigraphically discrete uplift events, which we interpret as having occurred during large-magnitude (M >7) earthquakes on the Compton fault. The total uplift across the Holocene/Pleistocene boundary, coupled with the dip of the Compton thrust ramp indicates a minimum slip rate on the blind thrust of ~1.5 mm/yr. These observations clearly indicate that the Compton fault is active and capable of producing damaging, large-magnitude earthquakes directly beneath metropolitan Los Angeles.

Lorraine Leon, Department of Earth Sciences, University of Southern California, lorrainl@usc.edu

Research Interests:
Active tectonics, paleoseismology, structural geology, and seismic hazard analysis.

Education:
Ph.D., in progress, Earth Sciences, University of Southern California, Dr. James F. Dolan - advisor
Research Topic: Paleoseismology of blind-thrust faults: Implications for system-wide earthquakes to occur in active fold and thrust belts.

B.S., 2002, (magna cum laude) Geological Sciences, University of California, Los Angeles

Publications:
Leon, L. A., S. A. Christofferson, J. F. Dolan, J. H. Shaw, and T. L. Pratt (2007), Earthquake-by-earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: Implications for fold kinematics and seismic hazard, J. Geophys. Res., 112, B03S03, doi:10.1029/2006JB004461.

Abstracts:
Leon, L.A., Dolan, J.F.., Shaw, J.H., Pratt, T.L., 2006, Paleoseismology of blind thrust faults in the Los Angeles basin, California: AEG Annual Meeting Abstracts.

Leon, L.A., Dolan, J.F.., Shaw, J.H., Pratt, T.L., 2006, Borehole and high-resolution seismic reflection evidence for Holocene activity on the Compton blind-thrust fault, Los Angeles basin, California: EOS Transactions, AGU Fall Meeting Supplement, Abstract H51C-1143.

Leon, L.A., Dolan, J.F., Shaw, J.H., Pratt, T.L..,2006, Borehole and high-resolution seismic reflection evidence for Holocene activity on the Compton blind-thrust fault, Los Angeles basin, California: SCEC Annual Meeting Proceedings and Abstract, v. XVI, p. 118.

Leon, L.A., Dolan, J.F., Shaw, J.H., Pratt, T.L.,2005, High-resolution seismic reflection data to determine Holocene activity of the Compton blind-thrust fault, Los Angeles basin, California: SCEC Annual Meeting Proceedings and Abstract, v. XV, p. 144.

Dolan, J. F., Leon, L. A., Christofferson, S. A., Shaw, J. H., and Pratt, T. L., 2005, Earthquake-by-earthquake reconstructions of fold development above a major blind thrust fault: The Puente Hills thrust fault beneath metropolitan Los Angeles, California: Proceedings With Abstracts, International Conference on Theory and Application of Fault-related Folding in Foreland Basins, Research Institute of Petroleum exploration and Development (PetroChina), Beijing, China, August, 2005.

Leon, L.A., Dolan, J.F., Shaw, J.H., Christofferson, C. and Pratt, T.L., 2004, Paleoseismology of a major blind thrust fault; the Puente Hills thrust fault, Los Angeles, California: Seismological Society of America Annual Meeting Abstracts.

Leon, L.A., Dolan, J.F., Hoeft, J.S., Shaw, J.H., and Hartleb, R.D., 2003, Three-dimensional geometry of buried fold scarps associated with ancient earthquakes on the Puente Hills blind thrust fault: EOS Transactions: AGU Fall Meeting Supplement, Abstract S12B-0389.

Hoeft, J.S., Leon, L.A., Dolan, J.F., Shaw, J.H., and Hartleb, R.D.,2003, Three-dimensional geometry of buried fold scarps associated with ancient earthquakes on the Puente Hills blind thrust fault: Preliminary observations: SCEC Annual Meeting Proceedings and Abstract, v. XIII.