Earthquake · reviewed by a seismologist
M 2.9ml
Earthquake near Helena, MT
No earthquake was likely felt in Helena in the 24 hours to 9:12 am MST (16:12 UTC). The last one was M 1.6, 7 km (4 mi) SE, at 11:34 am MDT on 14 October 2026, estimated intensity II (weak) in town. Every earthquake near Helena
This page: a magnitude 2.9 earthquake was recorded 31 km (19 mi) southeast of Helena, MT, at 5:10 pm MDT on 28 August 2021 (23:10 UTC), 4.6 km deep. 4 people have reported feeling it to USGS.
USGS: 11 km SW of Winston, Montana
- When
- 5:10 pm MDT on 28 August 2021 ()
- Where
- 31 km (19 mi) southeast of Helena, MT · earthquakes near Helena
- Montana today
- every earthquake in Montana in the last 24 hours
- Magnitude
- M 2.9 (ml)
- Depth
- 4.6 km below the surface
- Felt
- 4 reports · strongest reported intensity II
- Nearest
- Broadwater County, 21 km ESE
- Status
- Reviewed by a seismologist
USGS · last changed by USGS · we first saw it 1556 days ago
Earthquake alerts for Helena, MT
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Alerts are on for .
Not a warning. An alert comes after USGS publishes a quake, usually minutes after the shaking. For early warning use ShakeAlert, through your phone's Wireless Emergency Alerts or the MyShake app.
This was a small earthquake. Shaking like this is often felt indoors nearby and very rarely causes damage.
At this magnitude and depth, people at rest would likely notice it out to about 32 km (20 mi) from the epicentre. In Helena, 31 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: Townsend 19 km, intensity II · Helena 31 km, intensity II. Estimated for each town centre; each link is that town's own record.
How big was it?
This earthquake released about 680 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.9 would have released about 32 times as much.
- Mineral, Virginia 2011 M 5.8 22,000× the energy of this one
- Magna, Utah 2020 M 5.7 16,000× the energy of this one
- Around the size people start to feel M 3.0 1.4× the energy of this one
- This earthquake M 2.9 about 680 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 9 earthquakes of M5 or more within 50 km; the five largest:
- 6.6 The 1925 Clarkston Valley, Montana Earthquake · 40 km SSE of this epicentre
- 6.4 Clarkston Valley, Montana (Aftershock) · 40 km SSE of this epicentre
- 6.1 The 1935 Helena, Montana Earthquakes (Mainshock) · 23 km NNE of this epicentre
- 5.9 2 km ENE of Helena, Montana · 30 km NW of this epicentre
- 5.9 The 1935 Helena, Montana Earthquakes (Largest Aftershock) · 18 km NNW 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 46.3945, -111.7493. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| Broadwater County, MT | 21 km (13 mi) | ESE | earthquakes near Broadwater County |
| Jefferson County, MT | 38 km (24 mi) | SW | earthquakes near Jefferson County |
| Meagher County, MT | 67 km (42 mi) | ENE | earthquakes near Meagher County |
| Silver Bow County, MT | 89 km (56 mi) | SW | earthquakes near Silver Bow County |
| Lewis and Clark County, MT | 94 km (59 mi) | NNW | earthquakes near Lewis and Clark County |
| Powell County, MT | 103 km (64 mi) | WNW | earthquakes near Powell County |
Places and coordinates from the US Census Bureau Gazetteer (2024). Distances are from the epicentre.
Aftershocks so far
54 earthquakes have been recorded within 50 km since this one.
- 2.4 3 km NE of Helena West Side, Montana
- 1.6 Western Montana
- 1.6 3 km WSW of Ponderosa Pines, Montana
- 1.6 8 km W of Townsend, Montana
- 1.7 6 km WSW of Townsend, Montana
- 1.5 2 km NW of Montana City, Montana
- 2.4 12 km E of Elkhorn, Montana
- 1.5 5 km W of Townsend, Montana
- 1.7 2 km NW of Montana City, Montana
- 1.5 18 km SSW of Radersburg, Montana
- 1.5 Western Montana
- 1.6 7 km WSW of Townsend, Montana
- 1.6 8 km WSW of Townsend, Montana
- 1.6 Western Montana
- 1.7 5 km W of Ponderosa Pines, 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
- The official USGS event page — the source of record for every value above
- Did you feel it? Report it to USGS — reports go to USGS, not to us
- ShakeMap for this event, when USGS has produced one
QuakeIndex does not issue warnings. For earthquake early warning see ShakeAlert.