QuakeIndex

Earthquake · reviewed by a seismologist

M 2.1ml

Earthquake near Helena, MT

No earthquake was likely felt in Helena in the 24 hours to 10:36 am MST (17:36 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 2.1 earthquake was recorded 40 km (25 mi) north-northeast of Helena, MT, at 8:11 pm MDT on 28 July 2018 (02:11 UTC), 12 km deep.

USGS: 15 km SSE of Craig, Montana

8:11 pm MDT, Jul 28, 2018 · · 12 km deep · released about 38 lb of TNT

Every earthquake near Helena Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Lewis and Clark County 41 kmCascade County 60 kmBroadwater County 75 kmPowell County 78 kmMeagher County 85 kmJefferson County 91 kmTeton County 102 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
8:11 pm MDT on 28 July 2018 ()
Where
40 km (25 mi) north-northeast of Helena, MT · earthquakes near Helena
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 2.1 (ml)
Depth
12 km below the surface
Felt
no reports yet
Nearest
Lewis and Clark County, 41 km WNW
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 3026 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 Helena, 40 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 Northeast.

How big was it?

This earthquake released about 38 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.1 would have released about 32 times as much.

  1. Mineral, Virginia 2011 M 5.8 390,000× the energy of this one
  2. Magna, Utah 2020 M 5.7 280,000× the energy of this one
  3. Around the size people start to feel M 3.0 25× the energy of this one
  4. This earthquake M 2.1 about 38 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 4.3 on 2016-01-30 — about 2,500 times the energy of this one.

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

  1. 6.1 The 1935 Helena, Montana Earthquakes (Mainshock) · 43 km SSE of this epicentre
  2. 5.9 2 km ENE of Helena, Montana · 38 km S of this epicentre
  3. 5.9 The 1935 Helena, Montana Earthquakes (Largest Aftershock) · 44 km S of this epicentre
  4. 5.0 2 km ENE of Helena, Montana · 38 km S 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 46.9382, -111.9110. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Lewis and Clark County, MT 41 km (26 mi) WNW earthquakes near Lewis and Clark County
Cascade County, MT 60 km (37 mi) NE earthquakes near Cascade County
Broadwater County, MT 75 km (46 mi) SSE earthquakes near Broadwater County
Powell County, MT 78 km (49 mi) W earthquakes near Powell County
Meagher County, MT 85 km (53 mi) ESE earthquakes near Meagher County
Jefferson County, MT 91 km (57 mi) S earthquakes near Jefferson County

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

Aftershocks so far

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

  1. 1.8 8 km SSE of Marysville, Montana
  2. 1.6 3 km SW of Helena Valley Northwest, Montana
  3. 1.6 1 km SSE of South Hills, Montana
  4. 1.6 1 km SSE of South Hills, Montana
  5. 1.7 2 km NNW of Montana City, Montana
  6. 2.2 6 km W of Helena Valley West Central, Montana
  7. 2.4 8 km NNE of Helena Valley Northwest, Montana
  8. 1.7 15 km NW of Marysville, Montana
  9. 2.3 2 km WNW of East Helena, Montana
  10. 1.8 23 km WSW of Craig, Montana
  11. 1.6 2 km NW of Montana City, Montana
  12. 2.0 Western Montana
  13. 3.1 13 km NNW of Marysville, Montana
  14. 1.7 4 km WSW of South Hills, Montana
  15. 2.4 4 km NNW 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.