Earthquakes occur frequently in Alaska and fall into three general categories:

• giant megathrust earthquakes, which occur on the subduction plate boundary (the 1964 Good Friday earthquake is a notable example);
• shallow crustal earthquakes, analogous to those that occur along faults in California; and
• deep intraslab earthquakes, which originate inside down-going subducting oceanic slabs.

While techniques exist to reconstruct seismic histories of megathrust and crustal earthquakes, determining the history—and potential recurrence—of intraslab earthquakes is trickier, as they often occur deep beneath the surface and leave little evidence in the geologic record.

To better understand these intraslab earthquakes, USGS researchers in Alaska, including geologist and Mendenhall Fellow Drake Singleton of the Pacific Coastal and Marine Science Center, are using techniques from a discipline known as lake paleoseismology, studying cores of lake sediment to reconstruct past seismic activity. In much the same way that annual growth rings of a tree record past events such as fire and drought, annually deposited lake sediment can be rearranged by earthquakes, leaving a distinctive signature in the stratigraphy. The magnitude-7.1 earthquake that occurred near Anchorage in November 2018 is clearly represented in lake-sediment cores collected near the epicenter; the researchers plan to continue sampling in coming years to better understand how sediments respond to earthquakes and what they can reveal about an area’s past and future seismology.