QuakeIndex

experimental explosion · reviewed by a seismologist

M 3.9ml

163 km E of Saint Augustine Beach, Florida

No: USGS classifies this as a experimental explosion, not an earthquake. A magnitude 3.9 experimental explosion was recorded 163 km E of Saint Augustine Beach, Florida at 19:55 UTC on 2021-08-08, less than 1 km deep. 2 people have reported feeling it to USGS.

· 0.0 km deep · released about 11 tons of TNT

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10 km30 km60 km120 km N
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
Florida today
every earthquake in Florida in the last 24 hours
Magnitude
M 3.9 (ml)
Depth
0.0 km below the surface
Felt
2 reports · strongest reported intensity V
Status
Reviewed by a seismologist

USGS · last changed by USGS · we first saw it 1823 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 431 km (268 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.

USGS catalogues this as experimental explosion, not an earthquake.

How big was it?

This earthquake released about 11 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 16,000× the energy of this one
  2. Mineral, Virginia 2011 M 5.8 710× the energy of this one
  3. Magna, Utah 2020 M 5.7 500× the energy of this one
  4. This earthquake M 3.9 about 11 tons of TNT
  5. Around the size people start to feel M 3.0 this one released 22× 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

No earthquake of M4 or more has been recorded within 50 km since our catalogue begins in 2016.

Before 2016, the USGS record since 1900 lists no earthquake of M5 or more within 50 km.

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 30.1465, -79.6066. Map © OpenStreetMap contributors.

Aftershocks so far

1 earthquake has been recorded within 50 km since this one.

  1. 3.9 168 km ENE of Flagler Beach, Florida

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.