Shallow seismic reflection profiling in the Los Angeles basin
Chih-Yo Chien
Abstract
There are Santa Monica Fault Zone and Puente Hills blind-thrust fault system respectively in northwest and central Los Angeles basin. The 1994 Mw 6.7 Northridge earthquake and the deep 1987 Mw 6.0 Whittier Narrow earthquake demonstrated the hazards within the Los Angeles metropolitan region. The Santa Monica fault in west Los Angeles reveals the near-surface geometry of this active, oblique-reverse-left-lateral fault in high-resolution seismic reflection data. We interpret the fault to dip northward at 30o to 35o in the upper few hundred meters, steepening to ?65o at 1 to 2 km depth. High-resolution seismic reflection profiles of the Puente Hills blind-thrust fault system image discrete folds in the shallow subsurface (<600m) above two segments, the Santa Fe Springs segment and the Coyote Hills segment. The profiles demonstrate late Quaternary activity at the fault tip, precisely locate the axial surfaces of folds within the upper 100 m, and constrain the geometry and kinematics of recant folding. The Santa Fe Springs segment shows an upward-narrowing kink band with active anticlinal axial surface, consistent with fault-band folding above an active thrust ramp. The Coyote Hills segment shows an active synclinal axial surface that coincides with the base of 9-m-high scarp, consistent with tip-line folding or the presence of a backthrust. We describe high-resolution reflection data, acquired from the 15-to-300 m depth range of the Santa Monica fault, that provide useful constrains on the geometry, deformation rate, kinematics of recant fault motion, and use high-resolution reflection profiling to determine whether the shallowest strata above the tip of the PHT (Puente Hills blind-thrust fault system) are deformed by fault-related folding, and if so, to accurately locate the axial surfaces in the shallow subsurface.
Reference
Pratt, T. L., J. H. Shaw, J. F. Dolan, S. A. Christofferson, R. A. Williams, J. K. Odum, and A. Plesch (2002), Shallow seismic imaging of folds above the Puente Hills blind-thrust fault, Los Angeles, California, Geophys. Res. Lett., 29(9), 1304, doi:10.1029/2001GL014313
(Abstract) (Full text)
Dolan, J. F., T. L. Pratt(1997), High-resolution seismic reflection profiling of the Santa Monica Fault Zone, West Los Angeles, California, Geophys. Res. Lett., 24(16), 2051-2054
Pratt, T.L., J. F. Dolan, J. K. Odum, W. J. Stephenson, R. A. Williams, and M. E. Templeton, Multiscale seismic imaging of active fault zones for hazard assessment: A case study of the Santa Monica fault zone, Los Angeles, California, Geophysics, 63(2), 479-489, 1998
(Abstract) (Full text)
Shaw, J. H., and P. Shearer, An elusive blind-thrust fault beneath metropolitan Los Angeles, Science, 283, 1516-1518, 1999
(Abstract) (Full text)