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Geohazards Related to the California High Speed Rail: A Case Study of the Garlock Fault Crossing
AdvisorKent, Graham M
Geological Sciences and Engineering
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The California High Speed Rail will traverse 15 major active faults and 3 mountain grades along its route from San Francisco to Los Angeles. Slope stability, seismically induced shaking, subsidence and fault displacement are the main geohazards associated with the project. Lidar, aeromagnetic and aerogravity data were collected to help understand and address these potential hazards. Between the towns of Tehachapi and Mojave, the proposed train alignment will cross the Garlock fault. The distinct geomorphic lineament dividing the Tehachapi Mountains and Mojave Desert measures 250 km in length. Up to 64 km of total left lateral displacement has been measured along the trace, but historically the fault has not produced any significant earthquakes. A Quaternary map of the fault crossing area was created using lidar data to display how the fault interacts with surficial and bedrock units. A 60 m left-laterally deflected stream channel identified in a Holocene-aged unit, could represent a fault offset. Expected displacement for the fault crossing location was calculated via a probabilistic fault displacement hazard analysis. Displacement exceedance results at a return period of 2,475 years range from 144 cm to 593 cm, depending on the rupture scenario and slip rate considered. Dating of the offset terrace riser identified in the Quaternary map, using Cosmogenic and/or Optically Stimulated Luminescence (OSL), holds the potential to provide a maximum age of the offset surface, which can be used to calculate a minimum slip rate for the western segment of the Garlock fault. This, in turn, may reduce the epistemic uncertainty of the slip rate and, by extension, allow for a better understanding of the potential fault displacements.