Special Session: The Next Big Earthquake: The Usual and the Unusual Suspects
Type: Poster
Day: 5/15/2018
Time: 8:30 AM
Room: Riverfront South
Abstract
The Fairweather Fault accommodates 43–58 mm/yr of right-lateral, strike-slip motion along the boundary between the North American plate and the Yakutat block in southeast Alaska. Near Icy Point, where the Fairweather Fault trends offshore, the Yakutat block translates through a 20° restraining bend and 4-km right step in the fault. Deformation within the fault bend drives surface uplift at Icy Point, where progressively abandoned marine and stream terraces only occur west of the fault. During the 1958, M7.8 Lituya Bay earthquake 2.5–4 m of dextral offset occurred on the fault at Icy Point, without uplift. West of the fault, lidar data reveal raised stream terraces that grade to a flight of ~12 uplifted marine terraces beveled into Icy Point. Marine terrace risers, ranging from 1 to 6 m high, imply repeated episodes of sudden relative sea-level (RSL) fall. Multiple fault strands cut stream terraces and show both right-lateral and west-side-up offsets. We infer that reverse faults, which offset marine terraces, and an inferred offshore thrust, contribute to terrace uplift. We reconstruct the RSL history of Icy Point from marine terrace shorelines (40–43 m high), and age estimates of glacial landforms and offset terrace sediment. Then we compare the Icy Point RSL history to a published, post-glacial RSL curve for Icy Strait on the opposite (eastern) side of the Fairweather Fault, which shows RSL 5–6 m lower than present at 7 ka. Differencing the two RSL histories over the past 6–7 ka, which accounts for the GIA and eustatic components of RSL change, implies 6.4–8.2 mm/yr of tectonic uplift at Icy Point. These rates agree with the thermochronometric rock uplift rate of 5–10 mm/yr at Icy Point averaged over the past 300 ka. The Fairweather Fault accommodates both right-lateral and vertical displacement at Icy Point, but whether or not strain is partitioned into lateral and vertical displacement in separate earthquakes remains unclear.
Author(s):
Witter R. C. USGS - U.S. Geological Survey
Kelsey H. M. Humboldt State University
Bender A. M. USGS
Scharer K. M. USGS
Lease R. O. USGS
Repeated Earthquake Uplift at Icy Point, Glacier Bay National Park, Alaska: Strain Partitioning at a Restraining Bend in the Fairweather Fault?
Session
The Next Big Earthquake: The Usual and the Unusual Suspects