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Group B, Poster #106, Earthquake Geology

On the interaction of active faults and bedrock landslides

Nicolas C. Barth
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Poster Presentation

2022 SCEC Annual Meeting, Poster #106, SCEC Contribution #12470 VIEW PDF
Active faults often create topography, reduce rock strength, and generate ground shaking that contribute to an abundance of co-located bedrock landslides. Landslides, particularly bedrock slumps and rock avalanches, can bury the surface expression of an active fault beneath a thick and laterally-extensive deposit. Landslide transport reworks the existing bedrock mass containing organized planes of weakness (joints, foliations, faults, etc.) into a landslide deposit with new textures and rheology that may be more resistant to through-going fault rupture. How do active faults regain their surface expression following burial by a bedrock landslide deposit? This study presents observations of do...zens of case studies where landslide deposits cover active faults in southern California. Southern California was chosen for study due to the range of active fault kinematics represented, quality of slip rate data, availability of high-resolution topographic data, and legal importance in classifying surficial fault zone complexity and activity. The general observations seen here are likely to be applicable to active faults worldwide with adjacent topography suitable to produce bedrock landslides. A wide range of interactions are observed owing to the variations in fault properties (kinematics, geometry), deposit properties (thickness, width, lithology, type), accumulated slip since landslide event, substrate material, and preexisting fault surface expression. Generally, fault trace complexity is increased as earthquakes propagate upward and laterally through the landslide deposit to reestablish offset of Earth’s free surface. A simplified model is proposed in which fault kinematics and orientation play a central role in expected behavior: (1) Normal fault zones widen when they propagate through landslide deposits. (2) Thrust fault zones may exploit the low-angle interface between sediment and overlying deposit to daylight at the landslide deposit’s toe. (3) Strike-slip fault zones can manifest as en echelon traces or surficial step-overs. Thus, an improved understanding of the interactions between active faults and landslide deposits may require reinterpretation of surface rupture hazard widths, recency of fault activity, and expected earthquake rupture lengths.