Imaging Co-Seismic Fault Zone Deformation from Air Photos: the Kickapoo Stepover along the Surface Ruptures of the 1992 Landers Earthquake

Co-seismic deformation of the ground can be measured from aerial views taken before and after an earthquake. As a test example we chose the area of the Kickapoo-Landers stepover along the 1992 Landers earthquake zone, using air photos in a scale of 1:40,000 scanned at 0.4 m in resolution. A pair of photos acquired after the earthquake is used to assess the accuracy and evaluate various sources of noise. Optical distortions, deformation of films, scanning errors or errors on viewing parameters can yield metric bias at spatial frequencies lower than 1km-1. Offset field at shorter wavelength is more reliable and mainly affected by temporal decorrelation. This factor and resulting uncertainty on offsets are assessed from correlation score. Surface relative displacements are measured with independent measurements every about 15 m and an uncertainty typically below 10cm (rms). The offset field reveals most of the surface ruptures mapped in the field, and the fault slip is measured with accuracy of about 7cm (rms) and independent measurement every 200 m from stacked profiles. Slip distribution compare well with field measurements at the kilometric scale but reveal local metric discrepancies suggesting that deformation is generally, though not systematically, localized on the major fault zone measured in the field. This type of data can provide useful insight on fault zone’s mechanical properties. Our measurements indicate that elastic co-seismic strain in the near fault zone can be as large as 0.5 10 -3 , while anelastic yielding was attained for strain in excess of about 1-2 10 -3 .