Expression of San Andreas Fault on Seasat Radar Image

On a Seasat image (23.5-cm wavelength) of the Durmid Hills in southern California, the San Andreas fault is expressed as a prominent southeast-trending tonal lineament that is bright on the southwest side and dark on the northeast side. Field investigation established that the bright signature corresponds to outcrops of the Borrego Formation, which weathers to a rough surface. The dark signature corresponds to sand and silt deposits of Lake Coahuila which are smooth at the wavelength of the Seasat radar. These signatures and field characteristics agree with calculations of the smooth and rough radar criteria. On Landsat and Skylab images of the Durmid Hills, the Borrego and Lake Coahuila surfaces have similar bright tones and the San Andreas fault is not detectable. On a side-looking airborne radar image (0.86-cm wavelength), both the Borrego and Lake Coahuila surfaces appear rough, which results in bright signatures on both sides of the San Andreas fault. Because of this lack of roughness contrast, the fault cannot be distinguished. The wavelength of the Seasat radar system is well suited for mapping geologic features in the Durmid Hills that are obscure on other remote sensing images.