QuakeIndex

Earthquake · reviewed by a seismologist

M 1.7md

Earthquake near Three Forks, MT

No earthquake was likely felt in Three Forks in the 24 hours to 10:36 am MST (17:36 UTC). The last one was M 1.6, 7 km (5 mi) NE, at 4:03 pm MDT on 11 August 2026, estimated intensity II (weak) in town. Every earthquake near Three Forks

This page: a magnitude 1.7 earthquake was recorded 41 km (26 mi) south-southwest of Three Forks, MT, at 8:38 am MDT on 2 October 2021 (14:38 UTC), 8.1 km deep.

USGS: 10 km SSE of Pony, Montana

8:38 am MDT, Oct 2, 2021 · · 8.1 km deep · released about 9.6 lb of TNT

Every earthquake near Three Forks Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Madison County 29 kmGallatin County 53 kmJefferson County 63 kmSilver Bow County 73 kmBroadwater County 87 kmBeaverhead County 96 kmPark County 102 kmDeer Lodge County 116 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
8:38 am MDT on 2 October 2021 ()
Where
41 km (26 mi) south-southwest of Three Forks, MT · earthquakes near Three Forks
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 1.7 (md)
Depth
8.1 km below the surface
Felt
no reports yet
Nearest
Madison County, 29 km SSW
Nearest mapped fault
Madison fault, 22 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 1866 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 9 km (6 mi) from the epicentre. In Three Forks, 41 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 9.6 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 2.7 would have released about 32 times as much.

  1. Mineral, Virginia 2011 M 5.8 1,600,000× the energy of this one
  2. Magna, Utah 2020 M 5.7 1,100,000× the energy of this one
  3. Around the size people start to feel M 3.0 99× the energy of this one
  4. This earthquake M 1.7 about 9.6 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 4.1 on 2019-08-06 — about 4,400 times the energy of this one.

Before 2016, the USGS record since 1900 lists no earthquake of M5 or more within 50 km.

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 45.5787, -111.8330. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Madison County, MT 29 km (18 mi) SSW earthquakes near Madison County
Gallatin County, MT 53 km (33 mi) E earthquakes near Gallatin County
Jefferson County, MT 63 km (39 mi) NNW earthquakes near Jefferson County
Silver Bow County, MT 73 km (46 mi) WNW earthquakes near Silver Bow County
Broadwater County, MT 87 km (54 mi) NNE earthquakes near Broadwater County
Beaverhead County, MT 96 km (60 mi) WSW earthquakes near Beaverhead 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. 2.0 3 km WNW of Pony, Montana
  2. 1.8 18 km N of Ennis, Montana
  3. 2.5 3 km WNW of Pony, Montana
  4. 2.4 20 km N of Twin Bridges, Montana
  5. 1.8 19 km N of Twin Bridges, Montana
  6. 1.7 16 km N of Big Sky, Montana
  7. 1.6 14 km ESE of Harrison, Montana
  8. 2.1 17 km NNE of Ennis, Montana
  9. 1.5 17 km SSW of Whitehall, Montana
  10. 1.5 10 km WNW of Four Corners, Montana
  11. 1.6 5 km SE of Three Forks, Montana
  12. 1.6 5 km WSW of Logan, Montana
  13. 2.0 4 km NNW of Logan, Montana
  14. 1.6 18 km WNW of Pony, Montana
  15. 1.8 20 km WSW of Gallatin Gateway, Montana

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.