Defining the Earthquake Potential of the Puente Hills Blind-Thrust Fault, Los Angeles, California with Deep Borehole Data

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This project seeks to understand the fundamental mechanism of blind-thrust faulting and folding, as well as to better assess regional earthquake hazards. One of the key goals is the characterization of the constancy of fold growth (and therefore fault slip) over time intervals longer than those provided by shallow borehole data from earlier studies, and shorter than those provided by analysis of the deep, petroleum industry seismic reflection data. A 220-m-deep borehole across the locus of active folding above the western, Los Angeles segment of the Puente Hills thrust will be drilled directly into features identified in a high-resolution seismic reflection profile. By directly sampling and dating key reflectors, the geometry of the growth strata together with the known geometry of the thrust ramp can be used to determine slip rates over a wide range of time scales as well as the uplift rate of the fold. The uplift rate, in turn, will be used to constrain the fault slip rate from inception of this structure to the most recent earthquake.Constraints on fault slip rate are vital to interpreting the level of activity of the fault, and to quantifying earthquake hazards by defining strain accumulation and release rates. Combined with the Holocene record of paleo-earthquakes on the adjacent segment of the fault, this will define the size and frequency of earthquakes on the fault system, which will serve as the basis for deterministic seismic hazard assessment. This fault segment has heretofore been largely unstudied, but due to its location immediately beneath the high-rise district of metropolitan Los Angeles the Puente Hills thrust represents one of the largest deterministic hazards in the United States.