QuakeIndex

Earthquake · reviewed by a seismologist

M 3.8ml

Earthquake near Helena, MT

No earthquake was likely felt in Helena in the 24 hours to 7:45 am MST (14:45 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 3.8 earthquake was recorded 39 km (24 mi) west of Helena, MT, at 9:02 pm MDT on 12 March 2017 (03:02 UTC), 20 km deep. 231 people have reported feeling it to USGS.

USGS: 4 km ESE of Avon, Montana

9:02 pm MDT, Mar 12, 2017 · · 20 km deep · released about 7.8 tons of TNT

Every earthquake near Helena Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Powell County 42 kmLewis and Clark County 62 kmJefferson County 62 kmDeer Lodge County 70 kmGranite County 70 kmSilver Bow County 76 kmBroadwater County 85 kmMissoula County 114 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
9:02 pm MDT on 12 March 2017 ()
Where
39 km (24 mi) west of Helena, MT · earthquakes near Helena
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 3.8 (ml)
Depth
20 km below the surface
Felt
231 reports · strongest reported intensity IV
Measured shaking
intensity IV (ShakeMap)
Nearest
Powell County, 42 km NW
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 2008 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 81 km (50 mi) from the epicentre. In Helena, 39 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: Helena 39 km, intensity II · Lincoln 44 km, intensity II · Boulder 50 km, intensity II. Estimated for each town centre; each link is that town's own record.

How big was it?

This earthquake released about 7.8 tons of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 4.8 would have released about 32 times as much.

  1. Northridge, California 1994 M 6.7 22,000× the energy of this one
  2. Mineral, Virginia 2011 M 5.8 970× the energy of this one
  3. Magna, Utah 2020 M 5.7 680× the energy of this one
  4. This earthquake M 3.8 about 7.8 tons of TNT
  5. Around the size people start to feel M 3.0 this one released 16× more

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 4.3 on 2016-01-30 — about 6 times the energy of this one.

Before 2016, the USGS record lists 2 earthquakes of M5 or more within 50 km:

  1. 5.9 2 km ENE of Helena, Montana · 42 km E of this epicentre
  2. 5.0 2 km ENE of Helena, Montana · 42 km E 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 46.5730, -112.5485. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Powell County, MT 42 km (26 mi) NW earthquakes near Powell County
Lewis and Clark County, MT 62 km (39 mi) NNE earthquakes near Lewis and Clark County
Jefferson County, MT 62 km (39 mi) SE earthquakes near Jefferson County
Deer Lodge County, MT 70 km (44 mi) SW earthquakes near Deer Lodge County
Granite County, MT 70 km (44 mi) WSW earthquakes near Granite County
Silver Bow County, MT 76 km (47 mi) S earthquakes near Silver Bow 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. 3.0 13 km ESE of Lincoln, Montana
  2. 2.3 13 km ESE of Lincoln, Montana
  3. 2.3 15 km SE of Lincoln, Montana
  4. 2.4 15 km SE of Lincoln, Montana
  5. 2.6 14 km SE of Lincoln, Montana
  6. 2.5 14 km SE of Lincoln, Montana
  7. 2.5 13 km SSE of Lincoln, Montana
  8. 2.9 12 km SE of Lincoln, Montana
  9. 3.0 13 km SE of Lincoln, Montana
  10. 2.4 11 km S of Lincoln, Montana
  11. 2.9 16 km SE of Lincoln, Montana
  12. 1.9 7 km SE of Lincoln, Montana
  13. 2.7 Western Montana
  14. 3.5 7 km SE of Lincoln, Montana
  15. 2.6 14 km SE 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.