Ground Motions in the Near Field of the November 3, 2002 Denali Fault, Alaska, Earthquake

A free-field strong-motion recording of the Denali Fault, Alaska, Earthquake was obtained by Alyeska Pipeline Service Company just 3 km from where the Denali Fault slipped over 5 m horizontally and 1 m vertically in the earthquake. The instrument was part of the monitoring and control system for the Trans-Alaska Pipeline and was located at Pump Station 10, approximately 84 km east of the epicenter. After correction for a 0.1 Hz high-pass filter, we recover a fault-parallel permanent displacement of the instrument of 2.3 m. Dynamic ground motions during the earthquake have relatively low acceleration (0.39 g) and very high velocity (1.86 m/s). The most intense motions occurred during a 1.5 s interval generated by the propagation of the rupture front past the site. Growth of the fault-parallel displacement is nearly monotonic, with over half of the permanent displacement occurring during this 1.5 s interval. Preliminary modeling suggests that the rupture velocity exceeded the shear wave velocity near the instrument, and that the peak slip velocity on the fault exceeds several m/s. The low accelerations and high velocities observed near the fault in this earthquake agree with observations from other recent large-magnitude earthquakes. Following the earthquake, the permanent displacement of the support structure for the pipeline and other geodetic reference points was determined by GPS survey along more than 50 miles of the pipeline route. These permanent displacement data display a clear signature of elastic rebound, with displacement amplitudes decreasing with increasing distance from the fault trace. The best-fitting model consisting of a uniform dislocation in an elastic half-space has 6 m of right-lateral fault slip from the surface to a depth of 11 km. This model predicts 2.4 m of displacement at Pump Station 10, in good agreement with the strong motion displacement measurement. At the fault crossing, additional displacements were determined from orthographically corrected aerial photographs of slider beams that were part of the system that successfully accommodated the movement of the Denali Fault beneath the pipeline. The principal surface trace of the rupture cuts through glacial fluvial outwash sands and gravels. Survey data show that the total right-lateral displacement across the 500 m-wide deformation zone is 5.8 m. About 4.5 m of the displacement occurs within a 100 m-wide zone, and the maximum displacement between survey marks spaced 15 m apart is slightly more than 2.4 m. The distributed slip zone at the pipeline crossing most likely reflects the local surficial geology, as nearby offsets of up to 5.5 m on single traces were observed. Collectively, the dynamic and static ground motions in the near-field of the Denali Fault paint a simple picture of a rupture that lasted but a few seconds at any point on the fault and displaced the fault with relatively uniform slip amplitude of about 6 m from the surface to a depth of 11 km.