Three-Dimensional Surface Rupture Maps of the 3 November 2002 M7.9 Denali Fault Earthquake Produced Using Digital Photogrammetry

The M7.9 Denali Fault earthquake produced about 340 km of surface rupture along the Susitna Glacier thrust fault and the Denali and Totschunda right-lateral strike-slip faults. Digital photogrammetric methods were used to create a surface rupture map superior to traditional approaches. The process involved: 1) collecting vertical-incidence aerial photographs at a scale of 1:6000 along with aircraft orientation information and GPS positions of each photograph, 2) high-resolution scanning of the photographs, 3) aerotriangulation of the orientation data, 4) 3-D digitization in ArcGIS using Leica's StereoAnalyst plug in. The resulting digital database is high precision with X, Y, Z datapoints collected along the fault trace at intervals from 2-10 m for the entire surface rupture. The absolute accuracy of points along the fault traces is a few meters, but the relative location accuracy can be less than 0.5 m horizontal and 1.0 m vertical. The fault trace map made from this approach is superior to traditional methods in terms of relative and absolute accuracy, completeness, detail, and as a basis for 3-D visualization of the fault trace in three dimensions. Fieldwork complements the air photo observations in locations of dense vegetation, on bedrock, or where the surface trace is weakly developed. The new fault trace map reveals aspects of the geology of the surface rupture. The overall dip of the Susitna Glacier thrust is about 45° north, based on earthquake focal mechanisms. In the near surface, at one location where the fault intersects a stream, the fault dips about 16° northward between 10-30 m depth, but within 10 m of the surface, it rolls over to almost a flat dip. At several other locations the fault rolls over to southward dips and slid downhill at the toe of the thrust. Normal faults demonstrate extension in the hanging wall of the thrust in at least two locations. The Denali Fault strike-slip rupture is typically left stepping with segments 20- to 50-m long. The fault dips north at about 80° near its western extent, where south-side-up offsets were observed, which suggests a component of extension. The fault rupture through glacier ice is commonly expressed as a wide shatter zone 50-500 m across. The transfer zone from the Denali to the Totschunda Fault has anastamozing segments or normal fault segments as part of the main fault trace. The Totschunda fault is remarkably linear and generally lacks the left-steps of the Denali fault. The completed rupture maps show the details of a large surface rupture in unprecedented detail.