QuakeIndex

Earthquake · reviewed by a seismologist

M 2.3ml

Earthquake near East Helena, MT

No earthquake was likely felt in East Helena in the 24 hours to 9:53 am MST (16:53 UTC). The last one was M 1.6, 7 km (4 mi) SW, at 11:34 am MDT on 14 October 2026, estimated intensity II (weak) in town. Every earthquake near East Helena

This page: a magnitude 2.3 earthquake was recorded 3 km (2 mi) west-northwest of East Helena, MT, at 5:03 am MDT on 29 May 2021 (11:03 UTC), 6.2 km deep. 2 people have reported feeling it to USGS.

USGS: 2 km WNW of East Helena, Montana

5:03 am MDT, May 29, 2021 · · 6.2 km deep · released about 85 lb of TNT

Every earthquake near East Helena Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Broadwater County 46 kmJefferson County 53 kmLewis and Clark County 67 kmMeagher County 78 kmPowell County 80 kmCascade County 92 kmSilver Bow County 95 kmDeer Lodge County 107 kmGranite County 115 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
5:03 am MDT on 29 May 2021 ()
Where
3 km (2 mi) west-northwest of East Helena, MT · earthquakes near East Helena
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 2.3 (ml)
Depth
6.2 km below the surface
Felt
2 reports · strongest reported intensity III
Nearest
Broadwater County, 46 km SE
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 1922 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 19 km (12 mi) from the epicentre. In East Helena, 3 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: Helena 7 km, intensity II. Estimated for each town centre; each link is that town's own record.

How big was it?

This earthquake released about 85 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.3 would have released about 32 times as much.

  1. Mineral, Virginia 2011 M 5.8 180,000× the energy of this one
  2. Magna, Utah 2020 M 5.7 130,000× the energy of this one
  3. Around the size people start to feel M 3.0 11× the energy of this one
  4. This earthquake M 2.3 about 85 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

No earthquake of M4 or more has been recorded within 50 km since our catalogue begins in 2016.

Before 2016, the USGS record lists 4 earthquakes of M5 or more within 50 km:

  1. 6.1 The 1935 Helena, Montana Earthquakes (Mainshock) · 23 km E of this epicentre
  2. 5.9 2 km ENE of Helena, Montana · 4 km W of this epicentre
  3. 5.9 The 1935 Helena, Montana Earthquakes (Largest Aftershock) · 10 km SE of this epicentre
  4. 5.0 2 km ENE of Helena, Montana · 4 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 46.5973, -111.9487. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Broadwater County, MT 46 km (28 mi) SE earthquakes near Broadwater County
Jefferson County, MT 53 km (33 mi) S earthquakes near Jefferson County
Lewis and Clark County, MT 67 km (42 mi) NNW earthquakes near Lewis and Clark County
Meagher County, MT 78 km (49 mi) E earthquakes near Meagher County
Powell County, MT 80 km (50 mi) WNW earthquakes near Powell County
Cascade County, MT 92 km (57 mi) NNE earthquakes near Cascade County

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

Aftershocks so far

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

  1. 1.9 8 km W of Marysville, Montana
  2. 1.6 7 km WSW of Townsend, Montana
  3. 1.6 8 km WSW of Townsend, Montana
  4. 1.6 Western Montana
  5. 1.8 5 km NW of Helena Valley Southeast, Montana
  6. 1.7 Western Montana
  7. 2.1 3 km NW of Elkhorn, Montana
  8. 1.6 4 km WNW of Elkhorn, Montana
  9. 2.2 11 km NE of Townsend, Montana
  10. 1.7 10 km N of Elliston, Montana
  11. 1.6 2 km E of South Hills, Montana
  12. 1.7 9 km WSW of Townsend, Montana
  13. 1.6 2 km E of South Hills, Montana
  14. 1.8 31 km ENE of Helena Valley Northeast, Montana
  15. 1.9 30 km ENE of Helena Valley Northeast, 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.