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The Subaerially Exposed Iceberg Lake Sediments: A ~1000 Yr Long Paleoseismic Record From the Eastern Edge of the Alaska Subduction Zone

Description: 

Paleoseismic records are scarce on and around the Yakutat terrane, at the eastern edge of the Alaskan-Aleutian subduction zone. In this region at the transition between subduction and transform tectonics, the earthquake hazard comes primarily from faults bounding the Yakutat microplate, and from within the subducting Yakutat slab. Several of these fault zones hosted historical earthquakes such as the 1899 Mw 8.1 Yakutat Bay, 1958 Mw 7.9 Lituya Bay and 1979 Mw 7.4 Saint Elias earthquakes. In such a complex setting, which is compounded by extensive ice cover, traditional paleoseismic methods (trenching) do not work well. In contrast, lacustrine paleoseismic shaking records have more potential to provide comprehensive paleoseismic records.

Here, we study the sediments of Iceberg Lake, a glacier-dammed lake in the eastern Chugach mountains, within the Wrangell - St Elias National Park & Preserve. The lake drained during several outburst floods after 1999 CE, which exposed the lake bed and, through active erosion, the lacustrine sediments. In August 2022 we examined 6 outcrops in various sedimentary settings, from sandy delta to deep basin. We described the exposures, created 3D outcrop models using Structure-From-Motion photogrammetry and sampled sediments. Between annually laminated background sediments, we identified sediment gravity flow deposits and in-situ soft-sediment deformation features, such as convoluted bedding, dewatering structures and sand blows, inferred to be related to seismic shaking in prior studies. Cross-correlation of these beds between sites, and with previously studied and dated outcrops and cores, reveal that the most recent deposits can be linked to historical 1899, 1958, 1964 and 1979 CE earthquakes. We further identify an event that caused widespread gravity flows around 1920 CE, as well as similar pre-1899 events, such as a significant one in the second half of the 17^th century. We conclude that Iceberg Lake is an excellent recorder of seismic shaking and has strong potential of preserving a high-quality paleoseismic record for the northern part of the Yakutat terrane.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments - II

Type: Oral

Date: 5/1/2024

Presentation Time: 10:45 AM (local time)

Presenting Author: Maarten

Student Presenter: No

Invited Presentation: 

Authors