SIXTEEN YEARS OF REPEATED OBSERVATIONS ALONG THE EMERSON FAULT RUPTURE ZONE OF THE LANDERS, CALIFORNIA EARTHQUAKE, JUNE 1992 - MAY 2008

The Landers earthquake (June 28, 1992) ruptured a 400-m long segment of the Emerson fault about 1 km south of Galway Lake Road. The earthquake produced geomorphic features as simple as a single steep ramp in a planar surface or quite complex ones with the offset distributed on several parallel surfaces bounding displaced fracture blocks.

We photographed and surveyed the scarp and related features three days after the earthquake. A detailed map of the rupture was produced using a plane table and alidade, supplemented by spot total station elevations, and interpolation by geologists in the field. Eight subsequent surveys have at different times included: general photography, kite platform aerial photography, ground level stereo photography of the scarp, photos at four surveyed scarp profiles, at seven gully profiles, and at three knickpoints. Until the most recent survey in May 2008, the photos were recorded on color positive (slide) film and the surveys were conducted with optical total stations. The 2008 survey marked a complete change in the instruments with digital photography replacing film and terrestrial laser scanning (TLS) replacing point-by-point surveying. The first survey was based on 375 data points; the TLS survey produced 9.54 million X, Y, Z coordinate sets.

Repeated photography of the rupture zone allowed us to: 1) document ephemeral features such as offset dirt bike tracks; 2) record the original morphology of the rupture zone; and 3) observe changes that could not have been recorded by other methods. Our most important observations concerned the changes in the complexity of the rupture. Where the total displacement was distributed across multiple surfaces, the maximum offset measurable now varies over short distances because the smaller scale features have been destroyed.