QuakeIndex

Earthquake · reviewed by a seismologist

M 2.6ml

Earthquake near Helena, MT

No earthquake was likely felt in Helena in the 24 hours to 3:02 pm MST (22:02 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 2.6 earthquake was recorded 32 km (20 mi) northwest of Helena, MT, at 11:40 pm MDT on 29 September 2017 (05:40 UTC), 8.2 km deep. 4 people have reported feeling it to USGS.

USGS: 7 km N of Marysville, Montana

11:40 pm MDT, Sep 29, 2017 · · 8.2 km deep · released about 240 lb of TNT

Every earthquake near Helena Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Lewis and Clark County 35 kmPowell County 49 kmJefferson County 79 kmBroadwater County 81 kmCascade County 90 kmGranite County 99 kmDeer Lodge County 103 kmSilver Bow County 106 kmMeagher County 108 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
11:40 pm MDT on 29 September 2017 ()
Where
32 km (20 mi) northwest of Helena, MT · earthquakes near Helena
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 2.6 (ml)
Depth
8.2 km below the surface
Felt
4 reports · strongest reported intensity III
Nearest
Lewis and Clark County, 35 km NNW
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 3246 days ago

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 22 km (14 mi) from the epicentre. In Helena, 32 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.

Earthquakes near other towns within 30 km: Helena Valley Northwest, Helena Valley West Central, Helena West Side, Helena Valley Northeast.

How big was it?

This earthquake released about 240 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.6 would have released about 32 times as much.

  1. Mineral, Virginia 2011 M 5.8 63,000× the energy of this one
  2. Magna, Utah 2020 M 5.7 45,000× the energy of this one
  3. Around the size people start to feel M 3.0 4× the energy of this one
  4. This earthquake M 2.6 about 240 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 5.8 on 2017-07-06 — about 63,000 times the energy of this one.

Before 2016, the USGS record lists 3 earthquakes of M5 or more within 50 km:

  1. 5.9 2 km ENE of Helena, Montana · 33 km SE of this epicentre
  2. 5.9 The 1935 Helena, Montana Earthquakes (Largest Aftershock) · 46 km SE of this epicentre
  3. 5.0 2 km ENE of Helena, Montana · 33 km SE 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 46.8173, -112.2926. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Lewis and Clark County, MT 35 km (21 mi) NNW earthquakes near Lewis and Clark County
Powell County, MT 49 km (30 mi) W earthquakes near Powell County
Jefferson County, MT 79 km (49 mi) SSE earthquakes near Jefferson County
Broadwater County, MT 81 km (51 mi) SE earthquakes near Broadwater County
Cascade County, MT 90 km (56 mi) NE earthquakes near Cascade County
Granite County, MT 99 km (61 mi) WSW earthquakes near Granite 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. 1.6 12 km ESE of Lincoln, Montana
  2. 2.4 12 km ESE of Lincoln, Montana
  3. 2.4 14 km ESE of Lincoln, Montana
  4. 1.9 13 km SE of Lincoln, Montana
  5. 1.6 14 km SE of Lincoln, Montana
  6. 1.5 13 km ESE of Lincoln, Montana
  7. 2.4 11 km ESE of Lincoln, Montana
  8. 2.2 13 km ESE of Lincoln, Montana
  9. 1.8 15 km ESE of Lincoln, Montana
  10. 3.1 11 km ESE of Lincoln, Montana
  11. 2.4 14 km SE of Lincoln, Montana
  12. 1.7 14 km SE of Lincoln, Montana
  13. 1.8 14 km ESE of Lincoln, Montana
  14. 2.6 16 km ESE of Lincoln, Montana
  15. 1.7 14 km ESE of Lincoln, 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.