QuakeIndex

Earthquake · reviewed by a seismologist

M 4.0ml

Earthquake near Big Lake, AK

No earthquake was likely felt in Big Lake in the 24 hours to 1:38 am AKST (10:38 UTC). The last one was M 2.8, 6 km (4 mi) NW, at 6:17 am AKST on 6 November 2026, estimated intensity II (weak) in town; 3 felt reports to USGS. Every earthquake near Big Lake

This page: a magnitude 4.0 earthquake was recorded 7 km (4 mi) south of Big Lake, AK, at 6:18 pm AKDT on 9 September 2020 (02:18 UTC), 32 km deep. 183 people have reported feeling it to USGS.

USGS: 7 km S of Big Lake, Alaska

27 km (17 mi) N of Anchorage, AK

6:18 pm AKDT, Sep 9, 2020 · · 32 km deep · released about 15 tons of TNT

Every earthquake near Big Lake Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Anchorage Municipality 47 kmMatanuska-Susitna Borough 93 km
Distance rings around the epicentre, with the nearest towns at their true direction and later quakes as coloured dots. Not a shaking map — see the USGS ShakeMap for that.
When
6:18 pm AKDT on 9 September 2020 ()
Where
7 km (4 mi) south of Big Lake, AK · earthquakes near Big Lake
Magnitude
M 4.0 (ml)
Depth
32 km below the surface
Felt
183 reports · strongest reported intensity III
Measured shaking
intensity III (ShakeMap)
USGS impact alert
green
Tsunami
USGS flagged this event for tsunami evaluation. Follow tsunami.gov for official messages.
Nearest
Anchorage Municipality, 47 km SE
Nearest mapped fault
Castle Mountain fault, 14 km from its surface trace — near is not the same as on
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 2183 days ago

This was a light earthquake. It is usually felt clearly nearby and can rattle windows and dishes, but damage is uncommon.

At this magnitude and depth, people at rest would likely notice it out to about 91 km (57 mi) from the epicentre. In Big Lake, 7 km away, the estimate is intensity III (weak). Estimated from magnitude, depth and distance with the intensity equation USGS Did You Feel It? uses; felt reports, where there are any, are the measurement.

Likely felt in: Big Lake 7 km, intensity III · Knik-Fairview 19 km, intensity III · Eagle River 25 km, intensity III · Anchorage 27 km, intensity III · Willow 33 km, intensity III · Tanaina 33 km, intensity III · Palmer 46 km, intensity II · Fishhook 49 km, intensity II. Estimated for each town centre; each link is that town's own record.

How big was it?

This earthquake released about 15 tons of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 5.0 would have released about 32 times as much.

  1. Northridge, California 1994 M 6.7 11,000× the energy of this one
  2. Mineral, Virginia 2011 M 5.8 500× the energy of this one
  3. Magna, Utah 2020 M 5.7 350× the energy of this one
  4. This earthquake M 4.0 about 15 tons of TNT
  5. Around the size people start to feel M 3.0 this one released 32× more

Energy from the Gutenberg–Richter relation (log10E = 1.5M + 4.8), rounded to two figures; bar length is magnitude, which is already a logarithm of energy. Magnitudes of past earthquakes from the USGS catalogue.

For this area

The largest earthquake within 50 km since 2016 was M 7.1 on 2018-11-30 — about 45,000 times the energy of this one.

Before 2016, the USGS record lists 18 earthquakes of M5 or more within 50 km; the five largest:

  1. 5.6 11 km NW of Big Lake, Alaska · 17 km NW of this epicentre
  2. 5.6 5 km N of Susitna, Alaska · 34 km WNW of this epicentre
  3. 5.5 2 km ENE of Eagle River, Alaska · 26 km ESE of this epicentre
  4. 5.5 9 km E of Susitna, Alaska · 23 km WNW of this epicentre
  5. 5.3 12 km S of Susitna, Alaska · 31 km W of this epicentre

Since 2016: the QuakeIndex catalogue (US, M1.5+), built from the USGS FDSN event service. Before 2016: USGS ComCat, M5+ from 1900. Distances are between epicentres. This is the record, not a forecast.

Where

Epicentre 61.4588, -149.9364. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Anchorage Municipality, AK 47 km (29 mi) SE earthquakes near Anchorage Municipality
Matanuska-Susitna Borough, AK 93 km (57 mi) NNE earthquakes near Matanuska-Susitna Borough

Places and coordinates from the US Census Bureau Gazetteer (2024). Distances are from the epicentre.

Aftershocks so far

58 earthquakes have been recorded within 50 km since this one.

  1. 1.7 22 km S of Susitna, Alaska
  2. 1.5 4 km E of Big Lake, Alaska
  3. 1.5 6 km SW of Big Lake, Alaska
  4. 1.5 14 km SW of Anchorage, Alaska
  5. 2.3 5 km NW of Point MacKenzie, Alaska
  6. 2.6 6 km S of Houston, Alaska
  7. 1.7 9 km ESE of Elmendorf Air Force Base, Alaska
  8. 2.3 5 km SE of Willow, Alaska
  9. 1.7 Southern Alaska
  10. 1.6 9 km SW of Point MacKenzie, Alaska
  11. 2.2 22 km WSW of Anchorage, Alaska
  12. 3.0 7 km NNE of Point MacKenzie, Alaska
  13. 2.1 8 km WSW of Point MacKenzie, Alaska
  14. 1.6 4 km SSW of Big Lake, Alaska
  15. 1.5 6 km N of Meadow Lakes, Alaska

Counted from the QuakeIndex database, which ingests the USGS feed every 30 seconds · as of . QuakeIndex does not forecast aftershocks. When USGS issues an operational aftershock forecast it appears on the USGS event page.

On USGS

QuakeIndex does not issue warnings. For earthquake early warning see ShakeAlert.