• Project The tephra analyses were conducted within the framework of the FWO-funded project (2012-2017; G042812N), titled 'Decoding and calibrating the "lacustrine seismometer": towards a better understanding of the recurrence pattern of megathrust earthquakes along subduction zones.' The tephra were used to help develop age models for the lake sediments discussed in Praet et al. (2022) but are also an important new contribution towards developing a tephrostratigraphic framework for southwestern Alaska. Reported in this dataset are 15 new primary tephra deposits with highly constrained ages (Thomas et al., 2021) and well-defined geochemistry that will be helpful for future studies in the region. In Praet et al. (2022), these tephras were assigned sources using the machine-learning classifiers and data presented in Bolton et al. (2020) and confirmed with manual plotting. It should be noted that detrital glass is present as a minor background component of the ambient sediment in Skilak Lake and can complicate tephra interpretations. Therefore, samples comprising a substantial amount of detrital glass were omitted and only primary tephra material is presented, i.e., well-defined units that show consistent geochemistry, with little, if any, detrital glass contamination. • Sediment cores In the summer (May/June) of 2016, two long composite piston cores and three short gravity cores were collected using a UWITEC coring platform, from a proximal and distal coring site, i.e. location 1 (194 m water depth; 60°24'35,89" N; 150°20'10,54" W) and location 2 (146 m water depth; 60°25'59,77" N, 150°21'12,78" W), respectively, in Skilak Lake, Alaska. The composite piston cores in location 1 (Core 1) and location 2 (Core 2) are 16 m and 13 m long, respectively. • Tephra geochemistry analysis Glass major-element geochemistry was determined with wavelength-dispersive spectrometry (WDS) on a JEOL 8900R electron microprobe at the University of Alberta. Samples were analysed using a current of 6 nA, 15 keV accelerating voltage and defocused beam of either 5 or 10 µm. The software "Probe for EPMA" was employed to apply time-dependent intensity corrections to compensate for potential Na-loss caused by the 5 µm beam (Donovan et al., 2015). Two natural glasses of known composition, Lipari obsidian (ID3506) and Old Crow tephra (UT1434), were analysed with the samples to ensure proper calibration and track potential drift. All these data are available in this dataset.

This dataset comprises individual glass shard and secondary standard analyses for tephra collected in sediment cores from Skilak Lake, Alaska. These geochemical analyses were used in the research article in Sedimentology titled “Unravelling a 2300 year long sedimentary record of megathrust and intraslab earthquakes in proglacial Skilak Lake, south-central Alaska” by Praet et al. (2022). DOI: 10.1111/sed.12986