QuakeIndex

Earthquake · reviewed by a seismologist

M 3.9mw

76km W of Petrolia, CA

Yes. A magnitude 3.9 earthquake was recorded 76km W of Petrolia, CA, at 8:21 am PDT on 5 August 2021 (15:21 UTC), 7.9 km deep. 2 people have reported feeling it to USGS.

· 7.9 km deep · released about 10 tons of TNT

Felt it? Tell USGS Check my area
10 km30 km60 km120 km N Humboldt County 110 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
Magnitude
M 3.9 (mw)
Depth
7.9 km below the surface
Felt
2 reports · strongest reported intensity III
Measured shaking
intensity II (ShakeMap)
Nearest
Humboldt County, 110 km ENE
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 1856 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 88 km (55 mi) from the epicentre. 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.

How big was it?

This earthquake released about 10 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.

  1. Northridge, California 1994 M 6.7 17,000× the energy of this one
  2. Mineral, Virginia 2011 M 5.8 760× the energy of this one
  3. Magna, Utah 2020 M 5.7 540× the energy of this one
  4. This earthquake M 3.9 about 10 tons of TNT
  5. Around the size people start to feel M 3.0 this one released 21× 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 5.8 on 2020-03-09 — about 680 times the energy of this one.

Before 2016, the USGS record lists 19 earthquakes of M5 or more within 50 km; the five largest:

  1. 6.5 The 2010 Eureka, California Earthquake · 49 km ENE of this epicentre
  2. 6.0 Offshore Humboldt County, California · 48 km SSE of this epicentre
  3. 6.0 53 km SW of Ferndale, California · 43 km ESE of this epicentre
  4. 6.0 near the coast of Northern California · 48 km NE of this epicentre
  5. 6.0 33 km WSW of Ferndale, California · 45 km ENE 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 40.4107, -125.1705. Map © OpenStreetMap contributors.

Nearby places

PlaceDistanceDirection from the epicentre
Humboldt County, CA 110 km (68 mi) ENE earthquakes near Humboldt 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. 2.7 39km WNW of Petrolia, CA
  2. 2.7 40km W of Petrolia, CA
  3. 2.7 40km W of Petrolia, CA
  4. 2.8 42km W of Petrolia, CA
  5. 2.8 40km W of Petrolia, CA
  6. 4.6 50km W of Petrolia, CA
  7. 2.7 35km W of Petrolia, CA
  8. 2.7 31km W of Petrolia, CA
  9. 3.6 93km W of Petrolia, CA
  10. 2.7 31km W of Petrolia, CA
  11. 2.5 31km W of Petrolia, CA
  12. 2.4 29km W of Petrolia, CA
  13. 2.8 32km W of Petrolia, CA
  14. 2.9 36km W of Petrolia, CA
  15. 2.9 40km W 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

QuakeIndex does not issue warnings. For earthquake early warning see ShakeAlert.