Reviewed by USGSAlaska5y ago
M4.5 earthquake 64 km SE of Perryville, Alaska
Wednesday, August 25, 2021, 01:31:26 UTC · 55.470°N, 158.482°W
A magnitude 4.5 earthquake struck 64 km SE of Perryville, Alaska on Wednesday, August 25, 2021, 01:31:26 UTC, at a depth of 6.8 km (shallow). USGS received 2 “Did You Feel It?” reports, with a maximum reported intensity of IV (light). The USGS PAGER alert is green: low likelihood of casualties and damage. It is the strongest earthquake within 300 km since an M6.9 on August 14, 2021.
- Magnitude
- 4.5
- Light · ml
- Depth
- 7 km
- Shallow
- Felt reports
- 2
- Max intensity IV
- Est. shaking
- III
- Weak (ShakeMap)
- PAGER alert
- Green
- Impact estimate
- Tsunami
- No flag
- USGS feed
Outlined circle: this earthquakeDashed circle: 50 km aftershock search areaColoured lines: estimated shaking intensity (USGS ShakeMap)
In context
How big was it, and why here?
Energy released
85 tons of TNT
That is about 1/178 of the Hiroshima bomb. A magnitude 5.0 releases about 6 times more energy.
Tectonic setting
152 km
from the nearest mapped plate boundary, between the North American and Pacific plates. Earthquakes this far from a boundary happen on faults inside a plate, often less frequently but sometimes strongly.
Depth: shallow
7 km
Shallow earthquakes (under 70 km) produce the strongest surface shaking for their size, so they are the ones most likely to cause damage near the epicenter.
Did You Feel It?
Where it was felt
Reports sent by people to USGS, grouped by place (most reports first). Intensity uses the Modified Mercalli scale (I–X): what people felt, not the magnitude.
| Place | Intensity | Reports | Distance |
|---|---|---|---|
| 99565<br>0.330 | IV · Light | 1 | 93 km |
| 99926<br>0.330 | II · Weak | 1 | 1713 km |
USGS PAGER estimate
Who was exposed to shaking
Modelled from the ShakeMap and population data. Alert level green: low likelihood of casualties and damage.
Estimated shaking in nearby towns
- Sand PointPopulation 976I · Not felt
After the main event
Aftershocks within 50 km
USGS lists 2 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M2.7 on September 10, 2021. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.
Regional history
Past M5+ earthquakes within 300 km
USGS lists 325 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:
- 8.21938 Semidi Islands, Alaska EarthquakeNovember 10, 1938 · 38 km SE of this epicenter
- 8.22021 Chignik, Alaska EarthquakeJuly 29, 2021 · 39 km E of this epicenter
- 7.82020 Perryville, Alaska EarthquakeJuly 22, 2020 · 45 km S of this epicenter
- 7.62020 Sand Point, Alaska EarthquakeOctober 19, 2020 · 121 km SW of this epicenter
- 7.454 km SE of Sand Point, AlaskaMay 31, 1917 · 103 km SW of this epicenter
- 7.190 km SE of King Cove, AlaskaMay 14, 1948 · 197 km SW of this epicenter
- 7.0113 km ESE of Chignik, AlaskaMay 4, 1923 · 115 km E of this epicenter
- 7.071 km SE of King Cove, AlaskaMay 30, 1991 · 223 km SW of this epicenter
History · NOAA
Deadliest earthquakes in this area
NOAA’s significant-earthquake database lists 9 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700. The most severe:
- 2020M7.8
Alaska
22 Jul 2020 · 45 km away · intensity VII
Limited damage (under $1M)tsunami
- 1938M8.2
Alaska
10 Nov 1938 · 7 km away · intensity VI
tsunami
- 2021M8.2
Alaska Peninsula
29 Jul 2021 · 44 km away · intensity VIII
tsunami
- 1788M8.0
Alaska Peninsula
6 Aug 1788 · 168 km away
tsunami
- 2020M7.6
Alaska
19 Oct 2020 · 121 km away · intensity IX
tsunami
- 1948M7.5
Alaska Peninsula
14 May 1948 · 193 km away
NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.