Earthquake · reviewed by a seismologist
M 3.9mw
Earthquake near Fortuna, CA
No earthquake was likely felt in Fortuna in the 24 hours to 5:51 am PST (13:51 UTC). The last one was M 5.0, 123 km (76 mi) W, at 8:56 pm PST on 4 November 2026, estimated intensity II (weak) in town; 145 felt reports to USGS. Every earthquake near Fortuna
This page: a magnitude 3.9 earthquake was recorded 27 km (17 mi) south-southwest of Fortuna, CA, at 1:53 pm PST on 20 December 2021 (21:53 UTC), 27 km deep. 1 person has reported feeling it to USGS.
USGS: 6km NE of Petrolia, CA
- When
- 1:53 pm PST on 20 December 2021 ()
- Where
- 27 km (17 mi) south-southwest of Fortuna, CA · earthquakes near Fortuna
- Magnitude
- M 3.9 (mw)
- Depth
- 27 km below the surface
- Felt
- 1 report · strongest reported intensity I
- Measured shaking
- intensity III (ShakeMap)
- Nearest
- Humboldt County, 46 km NNE
- Nearest mapped fault
- Mattole Canyon fault, 18 km from its surface trace — near is not the same as on
- Status
- Reviewed by a seismologist
USGS · last changed by USGS · we first saw it 1717 days ago
Earthquake alerts for Fortuna, CA
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Alerts are on for .
Not a warning. An alert comes after USGS publishes a quake, usually minutes after the shaking. For early warning use ShakeAlert, through your phone's Wireless Emergency Alerts or the MyShake app.
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 87 km (54 mi) from the epicentre. In Fortuna, 27 km away, the estimate is intensity III (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: Rio Dell 19 km, intensity III · Ferndale 24 km, intensity III · Fortuna 27 km, intensity III · Humboldt Hill 41 km, intensity II · Redway 43 km, intensity II · Eureka 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 12 tons of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 4.9 would have released about 32 times as much.
- Northridge, California 1994 M 6.7 14,000× the energy of this one
- Mineral, Virginia 2011 M 5.8 640× the energy of this one
- Magna, Utah 2020 M 5.7 450× the energy of this one
- This earthquake M 3.9 about 12 tons of TNT
- Around the size people start to feel M 3.0 this one released 25× 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 6.2 on 2021-12-20 — about 2,500 times the energy of this one.
Before 2016, the USGS record lists 33 earthquakes of M5 or more within 50 km; the five largest:
- 7.2 The 1992 Petrolia, California Earthquake · 3 km S of this epicentre
- 6.6 32 km SW of Ferndale, California · 28 km W of this epicentre
- 6.5 The 1932 Eureka, California Earthquake · 42 km N of this epicentre
- 6.5 30 km WSW of Ferndale, California · 30 km WNW of this epicentre
- 6.0 Near Rio Dell, California · 16 km N 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 40.3585, -124.2258. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| Humboldt County, CA | 46 km (29 mi) | NNE | earthquakes near Humboldt County |
| Trinity County, CA | 99 km (62 mi) | ENE | earthquakes near Trinity County |
| Mendocino County, CA | 123 km (76 mi) | SSE | earthquakes near Mendocino 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.
- 2.8 11km N of Petrolia, CA
- 1.8 4km ENE of Petrolia, CA
- 2.4 14km S of Ferndale, CA
- 3.2 12km SSW of Ferndale, CA
- 2.3 17km WSW of Petrolia, CA
- 2.9 7km N of Petrolia, CA
- 2.7 32km W of Petrolia, CA
- 1.6 8km N of Petrolia, CA
- 1.6 10km SSW of Ferndale, CA
- 1.8 12km SSW of Ferndale, CA
- 1.6 7km S of Ferndale, CA
- 2.8 28km W of Petrolia, CA
- 2.2 7km WSW of Petrolia, CA
- 1.6 5km E of Petrolia, CA
- 2.0 7km N of Petrolia, CA
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.