QuakeIndex

Earthquake · reviewed by a seismologist

M 1.6ml

Earthquake near Big Sky, MT

No earthquake was likely felt in Big Sky in the 24 hours to 2:52 pm MST (21:52 UTC). The last one was M 3.7, 68 km (42 mi) SE, at 7:22 am MDT on 28 October 2025, estimated intensity II (weak) in town; 11 felt reports to USGS. Every earthquake near Big Sky

This page: a magnitude 1.6 earthquake was recorded 34 km (21 mi) southwest of Big Sky, MT, at 9:55 pm MDT on 21 July 2021 (03:55 UTC), 11 km deep.

USGS: 28 km S of Ennis, Montana

9:55 pm MDT, Jul 21, 2021 · · 11 km deep · released about 7.3 lb of TNT

Every earthquake near Big Sky Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Madison County 30 kmGallatin County 63 kmBeaverhead County 93 kmFremont County 99 kmPark County 100 kmClark County 103 kmSilver Bow County 116 kmJefferson County 118 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
9:55 pm MDT on 21 July 2021 ()
Where
34 km (21 mi) southwest of Big Sky, MT · earthquakes near Big Sky
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 1.6 (ml)
Depth
11 km below the surface
Felt
no reports yet
Nearest
Madison County, 30 km NNW
Nearest mapped fault
Madison fault, 10 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 1938 days ago

This was a minor earthquake. Quakes this small are rarely felt and do not cause damage.

At this magnitude and depth, people at rest would likely notice it out to about 1 km (1 mi) from the epicentre. In Big Sky, 34 km away, the estimate is intensity I (not felt). 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.

How big was it?

This earthquake released about 7.3 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 2.6 would have released about 32 times as much.

  1. Mineral, Virginia 2011 M 5.8 2,100,000× the energy of this one
  2. Magna, Utah 2020 M 5.7 1,500,000× the energy of this one
  3. Around the size people start to feel M 3.0 130× the energy of this one
  4. This earthquake M 1.6 about 7.3 lb of TNT

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 4.3 on 2016-06-13 — about 13,000 times the energy of this one.

Before 2016, the USGS record lists 5 earthquakes of M5 or more within 50 km:

  1. 6.1 30 km SSW of Big Sky, Montana · 13 km ESE of this epicentre
  2. 5.2 32 km WNW of Hebgen Lake Estates, Montana · 27 km SSE of this epicentre
  3. 5.2 26 km WNW of Hebgen Lake Estates, Montana · 32 km SSE of this epicentre
  4. 5.0 31 km W of Hebgen Lake Estates, Montana · 32 km SSE of this epicentre
  5. 5.0 39 km W of Hebgen Lake Estates, Montana · 44 km S 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 45.0953, -111.7053. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Madison County, MT 30 km (19 mi) NNW earthquakes near Madison County
Gallatin County, MT 63 km (39 mi) NE earthquakes near Gallatin County
Beaverhead County, MT 93 km (58 mi) W earthquakes near Beaverhead County
Fremont County, ID 99 km (62 mi) S earthquakes near Fremont County
Park County, MT 100 km (62 mi) ENE earthquakes near Park County
Clark County, ID 103 km (64 mi) SSW earthquakes near Clark County

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

Aftershocks so far

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

  1. 1.5 29 km S of Ennis, Montana
  2. 1.6 23 km WNW of Hebgen Lake Estates, Montana
  3. 1.5 Idaho-Montana border region
  4. 1.9 4 km WNW of Hebgen Lake Estates, Montana
  5. 1.8 18 km N of Ennis, Montana
  6. 1.8 53 km NNE of Spencer, Idaho
  7. 1.9 37 km SW of Big Sky, Montana
  8. 1.7 16 km N of Big Sky, Montana
  9. 2.5 46 km NW of Island Park, Idaho
  10. 1.6 28 km SE of Virginia City, Montana
  11. 1.8 41 km W of Hebgen Lake Estates, Montana
  12. 1.7 28 km S of Ennis, Montana
  13. 1.9 42 km NE of Spencer, Idaho
  14. 2.1 17 km NNE of Ennis, Montana
  15. 3.1 45 km NW of Island Park, Idaho

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.