Abstract

At the Vedanta and Olema Ridge paleoseismic trench sites along the San Andreas fault (SAF) in Marin County, we experimented with collecting tripod LiDAR (Light Detection And Ranging) data in order to test its utility in stratigraphic and tectonic geomorphic mapping. To characterized the terrain surface surroundings and within the exposed trench walls, we performed ground-based LiDAR surveys using a portable color sensitive tripod-mounted system. To produce a digital terrain model (DTM) for each site, we used a Riegl Z210i laser-scanner to target the ground and saturate it with point targets at three or more locations around the exposed trench. Local geo-referencing and control points were established using temporary auto reflectors. Using the LiDAR-based terrain model software package, ISite3D, we then merged these scans into a single surface model for each site. The same technique was used to image and process the exposed walls of the trench. We found that using a rotating scanning-laser allows us to very rapidly produce ultra-high resolution and quantitative DTMs for geomorphic analysis of a large (>0.1 km2) area surrounding the trench and that that the DTM can be used to resolve fine scale (<2.5 cm) morphologic features associated with the fault. The ability of the LiDAR to resolve color allows us another tool to investigate subtle variations in the soil structure exposed in the trench wall. By artificially modifying the color with false and enhanced colors, we can visually extract information not readily visible to the eye. At the Olema Ridge trench site, the 1906 trace of the SAF lies at the base of an east-facing scarp that formed as a slice of the ridge has been translated northwestward along the fault. As a means to compare technologies, we collected detailed geomorphic data from the site using both Total Station and LiDAR surveys. The superior coverage of the geospatial data recovered from the LiDaR allows for a more accurate rendering of the microtopography. Using the DTM for the Olema Ridge site, we can determine the probable age of the SAF at this location by palinspastic reconstruction of the surface topography and restoration of the ridge sliver back to its slope position. In addition, through comparison of archival photographs taken after the 1906 earthquake, our detailed tectonic morphologic mapping shows how the 1906 rupture has changed geologically over the past 100 years. We also compare the historical photographs to the subsurface stratigraphic section exposed in a trench. At the Vedanta marsh site, we have evidence for approximately ten ground-rupturing earthquakes over the past ~3000 years within a nearly continuous section that contains ten organic-rich layers interbedded with marsh clay and silt, and fluvial to colluvial gravel deposits. We tested the LiDAR system on the 4-m-thick vertical exposure at the Vedanta site to see if high resolution geospatial data can capture individual stratigraphic layers, faults, and event horizons that can be interpreted directly from the remotely sensed data. In this presentation we compare the LiDAR to trench logs mapped by photomosaic and graphical techniques.

Abstract | Vedanta Ranch Trench | Color Channel & Intensity Models of the Trench Wall | Olema Ridge Trench
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