July 21, 1788 Earthquake: Unga Island, Alaska Peninsula, United States

1788, July 22. 56.0 N, 157.0 W, intensity VII. Shumagin Islands-Kodiak Island region, Alaska. Strong ground motion and landslides occurred on both Kodiak and Unga Islands. Land subsidence also was reported on Kodiak near the harbor. The earthquake is described as "a great shock that ruptured at least a 600-km segment of the plate boundary" (extending from Kodiak Island perhaps to Unga Island or beyond). A tsunami perhaps as high as 10 m inundated Three

Saints Bay and caused extensive loss on Kodiak Island. Aftershocks on Kodiak Island lasted a month or longer. (reference #542)

1788, July 21. Veniaminov, writing in the 1830s, states: "Here in the islands (except for one in the Pribylov Islands), there have been no extraordinarily powerful earthquakes for a long time, but in one record I saw, it says that, "On the 11th of July 1788" (that is 16 days earlier than the inundation on Unga), "also on Unga there was so strong an earthquake that one could not keep on one's feet. Many mountains crumbled. Some time after this event there was a terrible flood." It is interesting that such frightful earthquakes (as, for instance, the once on the Pribylov Islands) occurred on islands situated far from volcanoes." (Ref #1604)

1788, July 22 (or 27). A catastrophic earthquake occurred with a source apparently extending from the Sanak islands to Kodiak Island. It was impossible to remain standing on Unga and Kodiak Islands; on Unga Island, "many mountains crumbled." The subsequent shocks, at least two or three or more per day, were felt for at least a month on Kodiak Island. The coast of the island may have subsided somewhat (Reference #414).