QuakeIndex

Earthquake · reviewed by a seismologist

M 1.8ml

Earthquake near Helena, MT

No earthquake was likely felt in Helena in the 24 hours to 10:34 am MST (17:34 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 1.8 earthquake was recorded 31 km (19 mi) west of Helena, MT, at 4:54 am MDT on 21 July 2017 (10:54 UTC), 17 km deep.

USGS: 8 km N of Elliston, Montana

4:54 am MDT, Jul 21, 2017 · · 17 km deep · released about 15 lb of TNT

Every earthquake near Helena Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Powell County 45 kmLewis and Clark County 54 kmJefferson County 64 kmBroadwater County 79 kmGranite County 81 kmDeer Lodge County 81 kmSilver Bow County 84 kmCascade County 112 kmMeagher County 116 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
4:54 am MDT on 21 July 2017 ()
Where
31 km (19 mi) west of Helena, MT · earthquakes near Helena
Montana today
every earthquake in Montana in the last 24 hours
Magnitude
M 1.8 (ml)
Depth
17 km below the surface
Felt
no reports yet
Nearest
Powell County, 45 km WNW
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 3399 days ago

This was a minor earthquake. Quakes this small are rarely felt and do not cause damage.

At this magnitude and depth it was probably too small to be felt, even directly above it. 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.

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

How big was it?

This earthquake released about 15 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 2.8 would have released about 32 times as much.

  1. Mineral, Virginia 2011 M 5.8 1,000,000× the energy of this one
  2. Magna, Utah 2020 M 5.7 730,000× the energy of this one
  3. Around the size people start to feel M 3.0 65× the energy of this one
  4. This earthquake M 1.8 about 15 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 1,000,000 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 E of this epicentre
  2. 5.9 The 1935 Helena, Montana Earthquakes (Largest Aftershock) · 46 km ESE of this epicentre
  3. 5.0 2 km ENE of Helena, Montana · 33 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 46.6353, -112.4322. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Powell County, MT 45 km (28 mi) WNW earthquakes near Powell County
Lewis and Clark County, MT 54 km (34 mi) N earthquakes near Lewis and Clark County
Jefferson County, MT 64 km (40 mi) SSE earthquakes near Jefferson County
Broadwater County, MT 79 km (49 mi) ESE earthquakes near Broadwater County
Granite County, MT 81 km (50 mi) WSW earthquakes near Granite County
Deer Lodge County, MT 81 km (50 mi) SW earthquakes near Deer Lodge 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. 2.5 12 km SE of Lincoln, Montana
  2. 1.6 12 km ESE of Lincoln, Montana
  3. 2.1 15 km SE of Lincoln, Montana
  4. 2.1 13 km ESE of Lincoln, Montana
  5. 2.7 17 km WNW of Marysville, Montana
  6. 1.8 15 km ESE of Lincoln, Montana
  7. 2.9 15 km SE of Lincoln, Montana
  8. 1.6 14 km SE of Lincoln, Montana
  9. 1.6 14 km SE of Lincoln, Montana
  10. 1.5 14 km SE of Lincoln, Montana
  11. 1.6 17 km ESE of Lincoln, Montana
  12. 2.3 15 km SE of Lincoln, Montana
  13. 2.4 14 km SE of Lincoln, Montana
  14. 1.9 12 km ESE of Lincoln, Montana
  15. 1.8 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.