Nabesna River Testing

Delta River Testing

Shear wave profiles for 9 sites on the Nabesna and Delta Rivers: Mean grain size and shear wave velocity both fall down-stream (northward) from the Nabesna glacier. On the south side of the fault rupture (nearer to the glacier), the shear wave profiles progressively decrease in velocity toward the fault. Low shear wave velocities in sand and gravel (red zone) indicate potentially liquefiable near-surface strata. On the Delta River, numerous side-entering glacial valleys bring in prodigious loads of coarse material into the mainstem river. Liquefaction observations only occur in areas where ponded sediment is found upstream of (1) major glacial alluvium inputs or (2) the Denali fault.

Asymmetry of the Nabesna River ground failures on either side of the fault rupture are the result of steady northward decline in liquefaction resistance intersecting a fault-normal symmetrical (?) ground motion intensity field. Transition from non-liquefied to fully liquefied zones occurred abruptly along a narrow swath on the south side, with no such transition occurring on the north side.

Delta river liquefaction features are more complex than for the Nabesna. Termination of liquefaction ground failure zones are the result of enormous glacier river inputs of poorly sorted cobble-sand at and below Canwell Glacier. At the fault crossing abundant sand boils are seen along the fault scarp. Fielding and summit lakes are quiet depositional areas of silty sand off Phelan creek.