The Sedimentary Record of the 2018 Anchorage Earthquake in Eklutna Lake, Alaska: Calibrating the Lacustrine Seismograph

The 30 November 2018 M-w 7.1 Anchorage earthquake caused modified Mercalli intensities of V1/4 to V1/2 at Eklutna Lake (south central Alaska). A few hours after the earth-quake, a "dirt streak" was observed on the lake surface, followed by a peak in sediment turbidity values (similar to 80 times normal) at a drinking water facility, which receives water from the lake through a pipe. These observations hint toward turbidity currents triggered by the earthquake in Eklutna Lake. Here, we study 32 short sediment cores retrieved from across Eklutna Lake and observe a millimeter-to-centimeter scale turbidite that can be confidently attributed to the 2018 earthquake in all coring locations. X-ray computed tomography, grain-size, and color-spectral analyses of the turbidite show that it shares physical characteristics with the turbidite generated by the 1964 M-w 9.2 Great Alaska earthquake, while it is considerably different from turbidites caused by historical floods. The 2018 turbidite reaches its largest thickness in the inflow-proximal basin, but when compared to the 1964 turbidite and thereby canceling out local site effects, it is relatively thick in the inflow-distal sub-basin. The latter was exposed to stronger shaking during the 2018 earthquake, and this relative thickness trend may therefore be attributed to shaking intensity and gives an indication of the location of the earthquake epicenter relative to the basin axis. Furthermore, in contrast to the 1964 turbidite, which was sourced from both deltas and hemipelagic slopes, the 2018 turbidite was sourced from deltas only, as evidenced by its distribution. These results confirm that while it is generally accepted that shaking intensities of >= VI are needed to trigger turbidity currents from hemipelagic slopes, intensities as low as V1/4 can be sufficient to trigger turbidity currents from deltaic slopes. Our results show that proglacial lakes can sensitively record differences in shaking intensity and that investigating deposits from recent earthquakes is crucial to calibrate the lacustrine seismograph.