Existing ALSM Data Along the Northern San Andreas Fault

Existing ALSM data along the northern San Andreas Fault

The U.S. Geological Survey in cooperation with NASA acquired ALSM data along the northern San Andreas Fault (NSAF) in 2003 (Figure 1) (Prentice et al., 2003, Prentice et al., 2004, This is the first time that LiDAR data had been collected for a study of the San Andreas Fault. This data set provides an excellent example of the great utility of ALSM data in densely vegetated areas along the fault (Figure 2). The project area encompasses a region of 418 km2 and includes a 70-km-long section of the NSAF as well as coastal marine terraces along approximately 100 km of the coastline in Sonoma and Mendocino Counties (Figure 1). Because this area is so densely vegetated (Figure 2), even basic mapping of the active fault traces has been difficult using traditional tools such as aerial photography (Figure 3A). The ALSM data have given us the first detailed and comprehensive look at the geomorphology of the fault beneath the forest canopy (Figures 3B and 3C), and will provide a significant advance in our understanding of this section of the fault. Currently available mapping is shown superimposed on the ALSM bare-earth image in Figure 3D, and in many places the mapped fault traces do not coincide with the geomorphic features that delineate the fault and are clearly visible on the ALSM bare-earthimage.

The ALSM data were collected by a commercial company according to particular contract specifications. The average data density is two pulses per m2, with up to four returns per pulse. The resulting bare-earth DEMs were constructed with 1.8m grid spacing (available for downloading at Vertical accuracy is better than 20 cm RMSE, confirmed by a network of ground-control points established by USGS using high-precision differential GPS surveying.

We have been using the ALSM data for two projects: mapping the active traces of the NSAF and mapping and analysis of the marine terraces. We are using shaded relief images generated from bare-earth DEMs to conduct detailed mapping of fault-related geomorphic features (e.g. scarps, offset streams, linear valleys, shutter ridges, and sag ponds) between Fort Ross and Point Arena. Initially, we map fault traces digitally, on-screen, based only on the geomorphology interpreted from bare-earth LiDAR images (Figure 4A). We then conduct field reconnaissance using the initial computer-based maps in order to verify and further refine our mapping (Figure 4B). We found that field reconnaissance is of utmost importance in producing an accurate and detailed map of fault traces. Many lineaments identified as faults from the on-screen images were determined in the field to be old logging roads or other features unrelated to faulting. Also, in areas where the resolution of LiDAR data is poor, field reconnaissance permits a more accurate location of fault-related geomorphic features. LiDAR images are extremely valuable as a base for field mapping in this heavily forested area, and the use of LiDAR is far superior to traditional mapping techniques relying only on aerial photography and 7.5 minute USGS quadrangle topographic maps. Comparison with earlier mapping of the northern San Andreas fault (Brown and Wolfe, 1972) shows that in some areas the LiDAR data allow a correction of the fault trace location of up to several hundred meters (Figure 4C). Geomorphic features observed in the field are noted on a separate layer in the GIS compilation (Figure 4C).TheGIS compilation will also include photographs taken of the surface rupture in 1906, modern photographs of fault features, and all exisiting paleoseismic data. The ALSM bare-earth images are also extremely valuable in identifying sites with potential for future paleoseismic investigation. The North Coast segment of the SAF has few paleoseismic sites due to the difficulty of finding such sites in the heavily vegetated terrain. The existing ALSM data has allowedresearchers to identify multiple potential paleoseismic sites, and our field reconnaissancehas confirmed the usefulnessof ALSM data for this purpose (Figure 4D).

References

Brown, R.D., and Wolfe, E.W., 1972, Map showing recently active breaks along the San Andreas fault between Point Delgada and Bolinas Bay, California: U.S.G.S. Misc. Geol. Investigations Map I-692.

Prentice, C.S., Crosby, C.J., Harding D.J., Haugerud, R.A., Merritts, D.J., Gardner, T., Koehler, R.D., Baldwin, J.N., 2003, Northern California LiDAR data: A tool for mapping the San Andreas fault and Pleistocene marine terraces in heavily vegetated terrain [abs.]: EOS, Transactions of the American Geophysical Union, v. 84(46), Fall Meeting Supplement, Abstract G12A-06.

Prentice, Carol S., Koehler, Rich D. III, Baldwin, John N., Harding, David J., 2004, Evaluation of LiDAR Imagery as a tool for mapping the northern San Andreas Fault in heavily forested areas of Mendocino and Sonoma Counties, California [abs.]: EOS, Transactions of the American Geophysical Union, v. 85(47), Fall Meeting Supplement, Abstract G11B-07.

Figure 1: location and project area

Figure 2: aerial view of SAF & forest

Figure 3A: air photo project area 2

Figure 3B: full feature

Figure 3C: bare earth

Figure 3D: bare earth with Brown traces

Figure 4A: bare earth with on-screen mapping

Figure 4B: bare earth with field-checked mapping

Figure 4A: Bare earth with potential trench sites

Figure 4B: Bare earth with geomorphic features

User Community

John Baldwin (WLA)

Rich Koehler (UNR)

Tim Hall

Bob Wright

George Hilley

Dorothy Merritts

Kevin Furlong

Mark Hemphill-Haley