HR: 08:30h
AN: S11B-01    [PDF]
TI: The Susitna Glacier Thrust Fault-Characteristics of Ruptures That Initiated the Denali Fault Earthquake
AU: * Craw, P A
EM: patty_craw@dnr.state.ak.us
AF: Alaska Div. of Geol. & Geophys. Surv., 794 University Ave., Fairbanks, AK 99709 United States
AU: Crone, A J
EM: crone@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, Denver, CO 80225 United States
AU: Haeussler, P J
EM: pheuslr@usgs.gov
AF: U.S. Geological Survey, 4200 University Dr., Anchorage, AK 99508
AU: Personius, S F
EM: personius@usgs.gov
AF: U.S. Geological Survey, MS 966, Box 25046, Denver, CO 80225 United States
AU: Staft, L A
EM: lauren_staft@dnr.state.ak.us
AF: Alaska Div. of Geol. & Geophys. Surv., 794 University Ave., Fairbanks, AK 99709 United States
AB: Seismograms show that the M$_{w}$ 7.9 earthquake on the Denali fault began as a M$_{w}$ 7.2 thrust-faulting event. Within days of the Nov. 3, 2002 mainshock, we discovered surface ruptures on a previously unknown thrust fault, which we named the Susitna Glacier fault (SGF). The SGF is a north- to northwest-dipping thrust fault that extends southwest from the upper part of the Susitna Glacier where it joins the Denali fault, downglacier to the southern margin of the Alaska Range, and west along the range front to the east edge of the Nenana River Valley. Field studies in 2003 verified that about 48 km of the SGF ruptured in 2002, of which 16-19 km were in glacial ice. Seasonal melting and ablation are rapidly destroying evidence of faulting in the ice. The westernmost 10 km of ruptures are small and discontinuous: they are difficult to trace because they are morphologically similar to abundant non-tectonic glacial and periglacial surficial geologic features. Primary motion on the SGF was south- to southeast-directed thrust faulting, but we found local minor lateral movement on zones that transfer slip between thrust faults. Data from 26 detailed topographic profiles show that the 2002 throw typically ranges between 1-3 m with a maximum of 4.3 m. We observed near-surface dips of $20\deg$-$40\deg$ in a few exposures. Combining typical throw and dip values suggests that the dip-slip movement was generally 3-6 m at the surface. Ground deformation on the SGF is complex in map view. The thrust-fault ruptures are typically comprised of multiple strands that have lobate morphologies and sinuous traces. Displacement on one strand is usually transferred to adjacent strands by ramps or tear faults. Locally, backthrusts form scarps as much as 2 m high. Folding and extensional grabens are common in the hanging wall; locally, downwarps in the footwall result in exaggerated scarp heights of 6-7 m. Empirical relations of earthquake magnitude versus surface-faulting parameters yield magnitude estimates of 6.8-7.2 for a 48-km-long surface rupture and a typical slip of 5-6 m. These values are consistent with seismological data for the initial thrust faulting.
DE: 7215 Earthquake parameters
DE: 8010 Fractures and faults
SC: Seismology [S]
MN: 2003 Fall Meeting