Earthquake · reviewed by a seismologist
M 2.6ml
Earthquake near Helena, MT
USGS: 4 km SE of Avon, Montana
- When
- 8:31 pm MDT on 21 March 2017 ()
- Where
- 40 km (25 mi) W of Helena, MT · earthquakes near Helena
- Magnitude
- M 2.6 (ml)
- Depth
- 14 km below the surface
- Felt
- 2 reports · strongest reported intensity II
- Nearest
- Powell County, 42 km NW
- Status
- Reviewed by a seismologist
USGS · last changed by USGS · we first saw it 1968 days ago
A magnitude 2.6 earthquake was recorded 40 km (25 mi) W of Helena, MT, at 8:31 pm MDT on 21 March 2017 (02:31 UTC). 2 people have reported feeling it to USGS. This was a small earthquake. Shaking like this is often felt indoors nearby and very rarely causes damage.
Earthquakes near other towns within 30 km: Deer Lodge.
How big was it?
This earthquake released about 230 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.
- Mineral, Virginia 2011 M 5.8 65,000× the energy of this one
- Magna, Utah 2020 M 5.7 46,000× the energy of this one
- Around the size people start to feel M 3.0 4.1× the energy of this one
- This earthquake M 2.6 about 230 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 410 times the energy of this one.
Before 2016, the USGS record lists 2 earthquakes of M5 or more within 50 km:
- 5.9 2 km ENE of Helena, Montana · 43 km E of this epicentre
- 5.0 2 km ENE of Helena, Montana · 43 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 46.5672, -112.5615. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| Powell County, MT | 42 km (26 mi) | NW | earthquakes near Powell County |
| Jefferson County, MT | 63 km (39 mi) | SE | earthquakes near Jefferson County |
| Lewis and Clark County, MT | 63 km (39 mi) | NNE | earthquakes near Lewis and Clark County |
| Deer Lodge County, MT | 69 km (43 mi) | SW | earthquakes near Deer Lodge County |
| Granite County, MT | 69 km (43 mi) | WSW | earthquakes near Granite County |
| Silver Bow County, MT | 75 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
59 earthquakes have been recorded within 50 km since this one.
- 2.0 12 km ESE of Lincoln, Montana
- 3.1 14 km SE of Lincoln, Montana
- 2.6 16 km SSE of Lincoln, Montana
- 2.0 14 km SE of Lincoln, Montana
- 3.0 13 km ESE of Lincoln, Montana
- 2.3 13 km ESE of Lincoln, Montana
- 2.3 15 km SE of Lincoln, Montana
- 2.4 15 km SE of Lincoln, Montana
- 2.6 14 km SE of Lincoln, Montana
- 2.5 14 km SE of Lincoln, Montana
- 2.5 13 km SSE of Lincoln, Montana
- 2.9 12 km SE of Lincoln, Montana
- 3.0 13 km SE of Lincoln, Montana
- 2.4 11 km S of Lincoln, Montana
- 2.9 16 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
- 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.