Groundtruthing the August 6th 1788 Alaskan Earthquake: Missing Evidence, Mislocation or #Fakequake?

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

A recent SAFRR report highlights the Semidi segment of the Alaska-Aleutian subduction zone as having potential for producing a tsunami that would damage the Port of Los Angeles, California. First-hand historical accounts exist for an earthquake on July 21^st 1788 at Three Saints Bay, Kodiak Island, which caused coastal subsidence (~1m) and tsunami inundation (~6-8m). Greater uncertainty surrounds reports of a second earthquake on August 6^th, presumably the source of a 1788 tsunami that Russian church records from Unimak Island (800 km SW of Kodiak) suggest inundated the Shumagin and Sanak islands (570 and 730 km SW of Kodiak) up to 10–50 m. Here, we synthesize field evidence of coseismic land-level changes and tsunami inundation in 1788 from our investigations in the eastern Aleutians (2010-2018) with historical, archeological, and other published findings from Kodiak Island.

Stratigraphic records of coastal subsidence and tsunami deposits on Kodiak Island corroborate Russian accounts of a great earthquake in 1788. New field evidence from Sitkalidak Island (80 km SW of Kodiak) and nearby Old Harbor are consistent with analyses of archaeological findings and historical records of subsidence and tsunami inundation at Three Saints Bay (15 km SW of Old Harbor) and, when combined with results of earlier studies at Sitkinak Island and Chirikof Island, suggest a rupture length of >300 km. In contrast, there is no evidence for land-level changes or tsunami inundation in 1788 at Sanak, Simeonof, or Unga islands, where brief, second-and-third-hand Russian accounts report the highest tsunami waves. Tsunami deposits as high as 6 m that extend >800m inland at Sanak either date to the M8.6 1946 earthquake or pre-date the 1788 earthquake by >2000 years. Here we present alternative hypotheses that may explain the absence of stratigraphic evidence in the Shumagin and Sanak islands for a western rupture in August of 1788. These include an absence of suitable sedimentary recorders, mislocation in the historical reports, and foul play.

Presenting Author: Simon E. Engelhart

Authors