Application of photogrammetry to produce pre-urbanized DEMs of metropolitan areas: Uncovering a major new dextral fault system in San Diego, CA

Wolfgang H. Lopez, & Karl J. Mueller

Published September 8, 2024, SCEC Contribution #13833, 2024 SCEC Annual Meeting Poster #099

Seismic hazard assessment in densely urbanized regions is inherently more challenging and often more costly than in similar, undisturbed landscapes. This is particularly true for strike-slip faults with little surface expression due to low vertical displacement. Extensive development in San Diego, CA, has significantly altered the landscape, with cut-and-fill construction practices over the past century destroying or obscuring critical geologic evidence used in early regional interpretations. Advancements in photogrammetry and the availability of archival aerial survey photography offer a cost-effective way to recreate the pre-urbanized topography and identify subtle active faults in the landscape. In this study, we employ a refined photogrammetric technique using Agisoft Metashape to generate 3-meter resolution digital elevation models (DEMs) of pre-urbanized metropolitan San Diego. The models are based on a 1953 USDA aerial survey, featuring 2,500 photos captured at a 1:20,000 scale with ~60% overlap. While 600 dpi scans available online produced useful reconstructions, rescanning original prints at 1200 dpi significantly improved DEM quality, particularly in noise and artifact reduction. The resulting DEMs, calibrated with ground control points, allowed us to distinguish fault scarps from channel walls in grabens, locate subtle scarps along nearly pure strike-slip faults, and define folding along small-displacement normal faults (see companion abstract by Mueller and Lopez). We also used ArcGIS and digitized USGS geologic maps to create structure contour maps representing the basal unconformity of an extensive sequence of Pleistocene terrace deposits, supplemented with mapping of stratigraphic contacts in well-mapped Eocene strata. These maps allowed us to identify subtle pressure ridges along strike-slip faults, define small-scale rollover structures in axial grabens, and map detailed fault geometry at their intersections in pull-apart basins. Our findings suggest that these photogrammetric techniques have significant potential for seismic hazard assessments in urban areas over active fault systems. The systematic development of pre-urbanization DEMs for other major California cities could provide similarly valuable insights and should be considered a key component in future research frameworks supported by the Statewide California Earthquake Center.

Key Words
Newly Recognized Fault System, San Diego, Seismic Hazard, Photogrammetry

Citation
Lopez, W. H., & Mueller, K. J. (2024, 09). Application of photogrammetry to produce pre-urbanized DEMs of metropolitan areas: Uncovering a major new dextral fault system in San Diego, CA. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology