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Bergen, Kristian J.

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Bergen

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Kristian J.

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Bergen, Kristian J.

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    Pleistocene to Modern Deformation of the Central Los Angeles Basin
    (2015-09-02) Bergen, Kristian J.; Shaw, John; Macdonald, Francis; Meade, Brendan; Mitrovica, Jerry
    We combine the principles of sequence stratigraphy and syntectonic (growth) stratigraphy to assess deformation of the central Los Angeles (LA) basin from the late Pleistocene to the present. Sequence stratigraphy provides temporally correlative horizons that define the activity and structural kinematics of folds and underlying blind-thrust faults. Using these insights, we demonstrate that the slip rate on the western segment of the Puente Hills blind-thrust fault system (PHT), which lies directly beneath downtown Los Angeles, has accelerated from the late Pleistocene through the Holocene. This increase in slip rate implies that the magnitudes and/or the frequency of earthquakes on this fault segment have increased over time, challenging the characteristic earthquake model and presenting an evolving and potentially increasing seismic hazard to metropolitan Los Angeles. To assess the slip rate on the LA segment probabilistically, we developed a new method for estimating uncertainty in the true depths of interpreted geologic features from seismic reflection data. We achieved this by simulating the effects of varying the order and proportionality of interval velocities using an autoregression model based on nearby wellbore velocities. We further assessed the impact of resolution uncertainty on the true depth of interpreted geological features. Combined with age uncertainties for geologic horizons, this approach yields robust assessments of the slip rates on blind-thrust faults. Finally, we map temporally correlative sequence boundaries across the Los Angeles basin from the late Pleistocene to present. This enables us to determine changes in accommodation space in time and from them infer changes in deformation. Our results show persistent deepening in the central trough of the Los Angeles basin, activity of the Compton and PHT faults, and lateral growth of the LA segment of the PHT.