Detecting co-seismic displacements in glaciated regions : an example from the great November 2002 Denali earthquake using SPOT horizontal offsets

Abstract

We use SPOT image pairs to determine horizontal offsets associated with the Mw 7.9 November 2002 Denali

earthquake in the vicinity of Slate Creek, AK. Field measurements and aerial photographs are used to further

characterize the geometry of the surface rupture. Aerial photographs show that shear localization occurs

where the rupture trace is linear, and distributed off-fault deformation is common at fault bends and stepovers,

or at geologic contacts between rock, glacial sediments, and ice. The displacement field is generated

using a sub-pixel cross correlation technique between SPOT images taken before and after the earthquake.

We identify the effects of glacier motion in order to isolate the tectonic displacements associated with the

Denali earthquake. The resulting horizontal displacement field shows an along-strike variation in dextral

shear, with a maximum of approximately 7.5 m in the east near 144° 52′W, which decreases to about 5 m to

the west near 145° 45′W. If the November 2002 earthquake represents the long-term behavior of the Denali

fault, it implies a westward decrease in the long-term dextral slip rate. A possible mechanism to

accommodate the westward decreasing slip on the Denali fault is to transfer fault slip to adjacent easttrending

contractional structures in the western region of the central Alaskan Range.