AN INSAR ANALYSIS OF SURFACE DEFORMATION ASSOCIATED WITH THE CHELAN SEISMIC ZONE, CENTRAL WASHINGTON

The 1872 Washington earthquake is the largest recorded crustal earthquake to have occurred in the Pacific Northwest. Published magnitude estimates for the 1872 earthquake event range from 6.5-7. The location of the fault that caused the earthquake is unknown, but the epicenter is believed to lie within an area contained by Lake Chelan to the north and the city of Wenatchee, WA to the south. The region is near the Chelan Seismic Zone (CSZ), an area of persistent modern earthquake activity. This relatively steady seismicity suggests the possibility of tectonic activity associated with the 1872 earthquake fault plane. Glacial terrace studies and GIS investigations in the region provide detail of the geomorphic history of the CSZ and suggest historical surface deformation, but there has been no research on active deformation (for example, GPS or leveling surveys) in this area.

We use Interferometric Synthetic Aperature Radar (InSAR), in an attempt to investigate and characterize deformation in the area of the CSZ. Our data include ERS-1/2 scenes that span 1992-2002, which were used to create 90 interferograms spanning one month to two years along two tracks covering the study area. Coherent interferograms were subsequently stacked to minimize atmospheric artifacts and other random noise, and emphasize persistent deformation, which would be expected from active faulting. No discernable deformation signal was found within the stacked interferograms, indicating that surface deformation is too small to discern (less than a few mm/yr), or that no active faulting is occurring in the region. Using the Coulomb 2.5 software, we will compare our results to isotropic elastic halfspace models of the possible deformation field around the Chelan Seismic Zone.