Collaborative Research: Spatial and Temporal Evolution of an Active Blind-Thrust Fault from Inception to the Most Recent Earthquake

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This research project is focused on deciphering paleo-earthquake records for thrust faults systems through geological and geophysical investigations of folds that develop above them. In the case of blind-thrust faults, these fault-related folds provide the only discernable record of paleo-earthquake activity on the underlying faults, and surface fold scarps in their own right present a potentially significant surface deformation hazard. Scientists from Harvard University and University of Southern California are examining a series of buried folds scarps above the active Puente Hills blind-thrust fault in the Los Angeles basin, California, to determine how they constrain past earthquake histories. The approach involves analysis of petroleum-industry reflection data, acquisition of multi-scale, high-resolution seismic reflection imagery, and excavation of detailed borehole transects to document and directly sample the youngest folded strata. Seismic reflection and borehole data will be collected above all three segments of this fault system, thus facilitating a comprehensive analysis of fault system behavior. Moreover, to elucidate the relative constancy of fold growth and fault slip rate over the past approximately 100,000 years, a deep (about 220 meters deep) continuously cored borehole will be drilled directly into the well-defined growth strata above the western, Los Angeles segment of the Puente Hills blind-thrust fault. Collectively, these data will help define what components of folding occur seismically, how various segments of a blind-thrust system interact over many rupture cycles, and how slip rates on these faults vary over different time scales, ranging from seismic cycles to millions of years.The motivations for this study are to understand the fundamental mechanism of blind-thrust faulting and folding, as well as to develop better methods of assessing regional earthquake hazards. The Puente Hills blind thrust underlies the heart of metropolitan Los Angeles and, because of its size and location, represents one of the worst deterministic seismic hazards in the United States. This study will help define the likely magnitude and frequency of future earthquakes on this fault, thereby improving the understanding of regional earthquake hazards. Moreover, the methodology developed through this study will prove readily exportable and thus serve as a tool to learn more about potentially dangerous faults around the world.