July 21, 1788 Tsunami: Alaska Peninsula, AK, United States
A tsunami caused by a magnitude 8 earthquake began at Alaska Peninsula, AK, United States on July 21, 1788. The water rose as high as 30 meters. Its effects were recorded at 4 places.
Where the waves reached
The source (white) and every runup with a place, colored by the water height recorded there. A ring is a runup NCEI marks doubtful.
- Under 1 m
- 1 to 3 m
- 3 to 10 m
- 10 to 30 m
- 30 m and more
- Height not recorded
The tsunami
- Date
- July 21, 1788
- Source
- Alaska Peninsula, AK, United States
- Cause
- Earthquake
- Earthquake
- Magnitude 8
- Maximum water height
- 30 m (98.4 ft)
- Runups recorded
- 4
- Tsunami intensity (Soloviev-Imamura)
- 4
- Tsunami magnitude (Iida)
- 3
- Waters
- Ocean
- How certain
- Definite tsunami
- Source location
- 57.000° N, 153.000° W
The toll
- Deaths
- Many (about 101 to 1,000)
- Damage
- Limited (under about $1 million)
- Houses destroyed
- Few (about 1 to 50)
Where the literature gives no number, NCEI records a description on a scale of 1 to 4, printed here in its words. Damage is in millions of US dollars of the year of the event, not adjusted for inflation.
What the record says
NCEI’s account, from the sources it cites.
1788, July 21. 57 N, 153 W, magnitude 8. This event is also dated July 11, which is the Julian date, and as July 22, the Gregorian date uncorrected for the now-existing
International Date Line. This great earthquake, which ruptured at least a 600 kilometer segment of the plate boundary (extending from Kodiak Island perhaps to Unga Island or beyond), produced strong ground motion and landslides at both Kodiak and Unga Islands. Extensive loss was reported on Kodiak Island due to multiple tsunami produced by this event. Subsidence was also reported at Kodiak near the harbor. Aftershocks on Kodiak Island lasted a month or longer. Clearly there was an earthquake and tsunami at Three Saints Harbor on July 11 (Julian date). It
is uncertain whether the runups described for Sanak, and other places in the Shumagin seismic gap area, relate to this date, or to a second event on July 27 (August 6, Gregorian date). (reference #239)
1788, July 22 (or 27). 54 N, 160 W. Validity definite. 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. "Before we had come to our "senses after the tremor, a flood came in from the sea." In Three Prelates Harbor at Kodiak Island, several buildings were washed away, and the soil was partially washed out and littered; two large waves were observed. On Unga Island, the water rose 115 m (50 fathoms) [?], and many Aleutians died. "The water passed over Sanak Island in strong, uncommon roller; "some swine brought to the island drowned. The tsunami was also observed at Pavlov village, but with considerably less force. There were no noticeable waves on the northern side of Unimak Island. (reference #414)
"In 1788 an earthquake caused a flood wave which moved from the Alaskan peninsula to Sanak Island and caused a big flood on Unga Island from which many natives died". (Dall, 1898; p. 310). (reference #1605)
Every runup
Nearest the source first. Heights are above sea level at the time; a tide gauge reads half the wave’s full height.
| Where | Country | Km from source | Water height, m | Inland, m | Measured by | First wave, UTC | Deaths |
|---|---|---|---|---|---|---|---|
| Three Saints Bay, Kodiak, AK, AK | United States | 33 | 10 | Eyewitness observation of water height | |||
| Unga Island, AK, AK | United States | 515 | 30 | Eyewitness observation of water height | |||
| Pavlof Village Alaska Peninsula, AK, AK | United States | 550 | 5 | Eyewitness observation of water height | |||
| Sanak Island, AK, AK | United States | 673 | 30 | Eyewitness observation of water height |
More
Every tsunami in 1788The 1700sTsunamis of United StatesThe earthquakeThis day in weather historyAlerts in force
Sources
Tsunami events and runups from NOAA’s National Centers for Environmental Information, Global Historical Hazard Database, cited as NCEI/WDS, doi:10.7289/V5PN93H7. Public domain. Read 2026-09-24. NCEI tsunami event 549.