QuakeIndex

Earthquake · reviewed by a seismologist

M 1.7ml

Earthquake near Three Forks, MT

No earthquake was likely felt in Three Forks in the 24 hours to 7:47 am MST (14:47 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 5 km (3 mi) west-southwest of Three Forks, MT, at 9:17 am MDT on 21 September 2018 (15:17 UTC), 2.6 km deep.

USGS: 4 km WSW of Three Forks, Montana

49 km (30 mi) WNW of Bozeman, MT

9:17 am MDT, Sep 21, 2018 · · 2.6 km deep · released about 11 lb of TNT

Every earthquake near Three Forks Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Jefferson County 44 kmBroadwater County 50 kmGallatin County 54 kmMadison County 66 kmSilver Bow County 81 kmMeagher County 95 kmPark County 96 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
9:17 am MDT on 21 September 2018 ()
Where
5 km (3 mi) west-southwest of Three Forks, MT · earthquakes near Three Forks
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 1.7 (ml)
Depth
2.6 km below the surface
Felt
no reports yet
Nearest
Jefferson County, 44 km NW
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 2973 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 12 km (7 mi) from the epicentre. In Three Forks, 5 km away, the estimate is intensity II (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: Three Forks 5 km, intensity II. Estimated for each town centre; each link is that town's own record.

How big was it?

This earthquake released about 11 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 1,300,000× the energy of this one
  2. Magna, Utah 2020 M 5.7 930,000× the energy of this one
  3. Around the size people start to feel M 3.0 83× the energy of this one
  4. This earthquake M 1.7 about 11 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 4.0 on 2016-09-03 — about 2,900 times the energy of this one.

Before 2016, the USGS record lists 6 earthquakes of M5 or more within 50 km; the five largest:

  1. 6.6 The 1925 Clarkston Valley, Montana Earthquake · 23 km NNE of this epicentre
  2. 6.4 Clarkston Valley, Montana (Aftershock) · 23 km NNE of this epicentre
  3. 5.5 Clarkston Valley, Montana (Aftershock) · 23 km NNE of this epicentre
  4. 5.5 Clarkston Valley, Montana (Aftershock) · 23 km NNE of this epicentre
  5. 5.5 Clarkston Valley, Montana (Aftershock) · 23 km NNE 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 45.8837, -111.6148. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Jefferson County, MT 44 km (27 mi) NW earthquakes near Jefferson County
Broadwater County, MT 50 km (31 mi) N earthquakes near Broadwater County
Gallatin County, MT 54 km (33 mi) SE earthquakes near Gallatin County
Madison County, MT 66 km (41 mi) SSW earthquakes near Madison County
Silver Bow County, MT 81 km (50 mi) W earthquakes near Silver Bow County
Meagher County, MT 95 km (59 mi) NE earthquakes near Meagher County

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

Aftershocks so far

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

  1. 2.3 5 km SSE of Logan, Montana
  2. 1.5 1 km W of Manhattan, Montana
  3. 1.5 7 km SSW of The Silos, Montana
  4. 1.6 1 km W of Manhattan, Montana
  5. 2.1 2 km W of Manhattan, Montana
  6. 1.7 1 km WSW of Manhattan, Montana
  7. 2.1 2 km W of Manhattan, Montana
  8. 2.2 1 km W of Manhattan, Montana
  9. 1.6 2 km WNW of Manhattan, Montana
  10. 1.7 3 km NW of Manhattan, Montana
  11. 1.6 1 km WNW of Manhattan, Montana
  12. 1.8 2 km WNW of Manhattan, Montana
  13. 1.6 3 km WNW of Manhattan, Montana
  14. 2.3 2 km WSW of Manhattan, Montana
  15. 1.6 1 km WSW of Manhattan, 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.