QuakeScout
Magnitude4.6

Reviewed by USGS5y ago

M4.6 earthquake in South Sandwich Islands region

Thursday, September 23, 2021, 02:06:41 UTC · 56.279°S, 27.854°W

A magnitude 4.6 earthquake struck in South Sandwich Islands region on Thursday, September 23, 2021, 02:06:41 UTC, at a depth of 108.4 km (intermediate). It is the strongest earthquake within 300 km since an M5.0 on September 22, 2021.

Magnitude
4.6
Light · mb
Depth
108 km
Intermediate
Felt reports
0
Did You Feel It?
Est. shaking
—
No ShakeMap
PAGER alert
None
Not issued
Tsunami
No flag
USGS feed

Outlined circle: this earthquakeDashed circle: 50 km aftershock search area

In context

How big was it, and why here?

Energy released

120 tons of TNT

That is about 1/126 of the Hiroshima bomb. A magnitude 5.0 releases about 4 times more energy.

Tectonic setting

127 km

from the nearest mapped plate boundary, between the Sandwich and South American plates. Earthquakes this far from a boundary happen on faults inside a plate, often less frequently but sometimes strongly.

Depth: intermediate

108 km

Intermediate-depth earthquakes usually break inside a plate that is sinking into the mantle. Shaking spreads over a wide area but is gentler at the epicenter than a shallow quake of the same size.

Did You Feel It?

Where it was felt

No “Did You Feel It?” reports have been published for this event yet. If you felt it, you can add your report on the USGS website.

I felt it — report to USGS ↗

USGS PAGER estimate

Who was exposed to shaking

USGS publishes PAGER impact estimates for significant earthquakes, usually within an hour. No PAGER estimate is available for this event.

After the main event

Aftershocks within 50 km

USGS lists 3 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M5.2 on October 23, 2021. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.

M3M4M51h6h1d3d7d14d30dMainshock M4.6
Time after the mainshock on a logarithmic scale (first hours stretched), magnitude on the vertical axis. M2.5+ events from USGS.
  1. 5.2South Sandwich Islands region5y ago · 4 km NW of this epicenter
  2. 4.4South Sandwich Islands region5y ago · 48 km NE of this epicenter
  3. 4.5South Sandwich Islands region5y ago · 32 km NE of this epicenter

Regional history

Past M5+ earthquakes within 300 km

USGS lists 1,382 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:

  1. 8.12021 South Sandwich Islands EarthquakeAugust 12, 2021 · 280 km SE of this epicenter
  2. 8.11929 South Sandwich Islands EarthquakeJune 27, 1929 · 137 km NW of this epicenter
  3. 7.8South Sandwich Islands regionMay 26, 1964 · 13 km E of this epicenter
  4. 7.5South Sandwich Islands regionAugust 12, 2021 · 223 km SE of this epicenter
  5. 7.4South Georgia Island regionAugust 19, 2016 · 275 km W of this epicenter
  6. 7.2South Sandwich Islands regionDecember 27, 1991 · 163 km E of this epicenter
  7. 7.2South Sandwich Islands regionMay 28, 2016 · 57 km E of this epicenter
  8. 7.1South Sandwich Islands regionSeptember 8, 1961 · 32 km E of this epicenter

History · NOAA

Deadliest earthquakes in this area

NOAA’s significant-earthquake database lists 6 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700. The most severe:

  1. 1929M8.1

    South Sandwich Islands

    27 Jun 1929 · 137 km away

  2. 2021M8.1

    South Sandwich Islands

    12 Aug 2021 · 282 km away

    tsunami

  3. 1961M7.7

    South Sandwich Islands

    5 Sep 1961 · 198 km away

  4. 1961M7.7

    South Sandwich Islands

    8 Sep 1961 · 35 km away

  5. 2021M7.5

    South Sandwich Islands

    12 Aug 2021 · 218 km away

  6. 2016M7.4

    South Sandwich Islands

    19 Aug 2016 · 275 km away · intensity III

    tsunami

NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.

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