Unravelling a 2300 year long sedimentary record of megathrust and intraslab earthquakes in proglacial Skilak Lake, south‐central Alaska

Abstract

Seismic hazards in subduction settings typically arise from megathrust, intraslab and crustal earthquake sources. Despite the frequent occurrence of intraslab earthquakes in subduction zones and their potential threat to communities, their long‐term recurrence behaviour is barely studied. Sedimentary sequences in lakes may register ground shaking from different seismic sources. This study investigates two long sediment cores (13 m and 16 m) from Skilak Lake, a proglacial lake in south‐central Alaska, to evaluate whether different seismic sources leave a distinct imprint. The sedimentary record shows a continuously varved sediment sequence, occasionally interrupted by turbidites, slump deposits and tephra beds. Turbidites and slump deposits were objectively identified using a statistical outlier analysis on varve thickness. The earthquake origin of these deposits was ascertained by resemblance with deposits induced by instrumentally recorded earthquakes (for example, 1964 ce Mw 9.2 megathrust and 1954 ce Mw 6.4 intraslab earthquakes) and correlation with multiple coeval landslide deposits on sub‐bottom profiles. The Skilak Lake record chronicles 19 earthquakes with moderate to very high confidence level in the past 1350 years. The sedimentary evidence of instrumentally‐recorded intraslab and megathrust earthquakes within the past 70 years demonstrates that not only megathrust earthquakes, but also past intraslab events are recorded. Although reported seismic intensities at Skilak Lake are comparable for the 1964 ce megathrust and the 1954 ce intraslab earthquakes, the long duration and low frequency content of seismic ground motion during megathrust earthquakes facilitate the triggering of multiple, voluminous landslides and the generation of megaturbidites. In contrast, the shorter duration and higher frequency source spectrum of intraslab earthquakes may only induce surficial slope remobilization and the generation of thinner turbidites. This study demonstrates that the sedimentary record of Skilak Lake has the potential to decipher multiple seismic sources, which opens possibilities for a comprehensive seismic hazard analysis for south‐central Alaska.

Updating the Crustal Fault Model for the 2023 National Seismic Hazard Model for Alaska

“…This is far more frequent than the recurrence of the largest megathrust earthquakes, which averages to every ~600 years in intertidal records above the Prince William Sound region (Carver and Plafker, 2008;Shennan et al, 2014). Praet et al (2022) also described earthquake recurrence based on sediments in Skilak Lake (Figure 5A), on the Kenai Peninsula, and found 18-20 earthquakes in the last 2300 years, indicating an average recurrence of 115-128 years. Singleton et al (2022) find evidence that Modified Mercalli shaking intensities of at least V½ were needed to induce deposition of a turbidite during the 2018 Anchorage earthquake.…”

A Review of Event Deposits in Lake Sediments

“…This implies that an earthquake source below the lake may produce similar imprints to onshore earthquake sources [102]. Moreover, the type and the size of sedimentary imprint can also be influenced by the duration and frequency content of seismic ground motion [97,103]. In some cases, megathrust earthquakes (long duration and low frequency) facilitate the triggering of multiple, voluminous landslides and the generation of megaturbidites while intraplate earthquakes (short duration and high frequency) may mostly induce onshore landslides, surficial slope remobilization, and the generation of thinner turbidites.…”

Unravelling megaturbidite deposition: Evidence for turbidite stacking/amalgamation and seiche influence during the 1601 ce earthquake at Lake Lucerne, Switzerland

“…To better understand the history of large earthquakes trough time, deposits known as megaturbidites are often used to reconstruct earthquake recurrence in areas where lake or ocean basins offer sedimentary sinks (e.g. Schnellmann et al ., 2006; Fanetti et al ., 2008; Leithold et al ., 2019; Praet et al ., 2022). The term ‘megaturbidite’ refers to thick, extensive, relatively homogenous sediments deposited by a turbidity current (Bouma, 1987).…”