Estimating SRTM Resolution for Applications of Fault Offset Recovery

Detailed topographic information, such as that acquired from the Shuttle Radar Topography Mission (SRTM), can provide a record of accumulated slip along active plate boundaries. We have developed a simple algorithm that uses topographic gradients derived from high-resolution topography data, such as SRTM, to estimate fault slip. Our derivation follows that vertical fault motion, S_z, maps directly into topography, while horizontal fault motion, S_y, maps into topography through the topographic slope in the along-fault direction. By identifying the fault location and assuming that fault scarps can be step-like, we are able to recover realistic slip estimates from a single observation of topography. Interpretation of these results is straightforward for a single rupture of preexisting topography where offset uncertainty is related to both the topography and the accuracy of the measurement resolution. However, for a mature fault system, the interpretation of offset results will be more complicated. We plan to use SRTM data, with its uniform accuracy and complete spatial coverage, combined with the known slip rate of the San Andreas Fault system, to assess creation and destruction of topography within the fault zone. An investigation of the resolution capabilities of the C-band SRTM DEM is necessary for both sensible application and size management. A cross-spectral analysis of 30 m SRTM, 30 m NED (National Elevation Data) and a high-resolution laser ranging data set acquired along the Hector Mine trace (~2 m) was performed in an area of southern California, near the 1999 Hector Mine earthquake surface rupture. We examine the coherence of SRTM, NED, and Hector Mine ALSM topography as a function of wavelength and find that the coherence falls below 0.5 at wavelengths less than 204 m. We also identify a significant drop in topographic power at wavelengths greater than 500 m, likely due to the adaptive regridding algorithms imposed during the processing of SRTM DEM. These results provide a basis for sensible application of the SRTM DEM in future studies.