M4.8 earthquake 137 km ESE of Chignik, Alaska

Friday, October 1, 2021, 14:12:52 UTC · 55.900°N, 156.312°W

A magnitude 4.8 earthquake struck 137 km ESE of Chignik, Alaska on Friday, October 1, 2021, 14:12:52 UTC, at a depth of 44.9 km (shallow). USGS received 1 “Did You Feel It?” report, with a maximum reported intensity of II (weak). The USGS PAGER alert is green: low likelihood of casualties and damage. It is the strongest earthquake within 300 km since an M5.1 on September 16, 2021.

Tsunami flagUSGS sets this flag for large events in oceanic regions so tsunami warning centers review them. It does not by itself mean a tsunami occurred. Check tsunami.gov and local authorities for official messages.

Magnitude: 4.8; Light · ml
Depth: 45 km; Shallow
Felt reports: 1; Max intensity II
Est. shaking: IV; Light (ShakeMap)
PAGER alert: Green; Impact estimate
Tsunami: Flag; Check tsunami.gov

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

239 tons of TNT

That is about 1/63 of the Hiroshima bomb. A magnitude 5.0 releases about 2 times more energy.

Tectonic setting

139 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

45 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
99709<br>0.330	II · Weak	1	1101 km

I felt it — report to USGS ↗

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

KodiakPopulation 6,100I · Not felt
Kodiak StationPopulation 1,300I · Not felt

After the main event

Aftershocks within 50 km

USGS lists 51 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M6.9 on October 11, 2021. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.

Time after the mainshock on a logarithmic scale (first hours stretched), magnitude on the vertical axis. M2.5+ events from USGS.

Regional history

Past M5+ earthquakes within 300 km

USGS lists 409 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:

History · NOAA

Deadliest earthquakes in this area

NOAA’s significant-earthquake database lists 13 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700. The most severe:

1900M8.3
Kodiak Island
9 Oct 1900 · 218 km away · intensity VIII
Limited damage (under $1M)
1788M8.0
Unga Island
21 Jul 1788 · 237 km away · intensity VII
Limited damage (under $1M)tsunami
2020M7.8
Alaska
22 Jul 2020 · 171 km away · intensity VII
Limited damage (under $1M)tsunami
1970M6.0
Andreanof Islands
11 Mar 1970 · 231 km away · intensity V
Limited damage (under $1M)tsunami
1792
Kodiak Island
1792 · 292 km away · intensity VII
Limited damage (under $1M)tsunami
1938M8.2
Alaska
10 Nov 1938 · 137 km away · intensity VI
tsunami

NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.

Nearby trackers