QuakeScout
Magnitude5.6

Reviewed by USGSTonga45y ago

M5.6 earthquake 55 km N of Hihifo, Tonga

Wednesday, December 23, 1981, 18:47:18 UTC · 15.452°S, 173.754°W

A magnitude 5.6 earthquake struck 55 km N of Hihifo, Tonga on Wednesday, December 23, 1981, 18:47:18 UTC, at a depth of 73.3 km (intermediate). It is the strongest earthquake within 300 km since an M5.8 on November 25, 1981.

Magnitude
5.6
Moderate · mb
Depth
73 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 earthquake

In context

How big was it, and why here?

Energy released

4 kilotons of TNT

That is about 1/4 of the Hiroshima bomb. It released about 8 times the energy of a magnitude 5.0.

Tectonic setting

56 km

from the nearest mapped plate boundary, between the Tonga and Niuafo'ou plates. Earthquakes cluster along boundaries like this one, where plates collide, pull apart or slide past each other.

Depth: intermediate

73 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 75 km

USGS has not published any M2.5+ earthquake within 75 km in the 30 days after this event.

Regional history

Past M5+ earthquakes within 300 km

USGS lists 488 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:

  1. 8.0The 1917 Samoa-Tonga EarthquakeJune 26, 1917 · 73 km NE of this epicenter
  2. 7.8146 km ESE of Hihifo, TongaDecember 26, 1975 · 165 km SE of this epicenter
  3. 7.7133 km NE of Hihifo, TongaSeptember 1, 1981 · 90 km NE of this epicenter
  4. 7.6199 km ESE of Hihifo, TongaApril 2, 1977 · 225 km SE of this epicenter
  5. 7.1112 km NNE of Hihifo, TongaApril 14, 1957 · 63 km NE of this epicenter
  6. 7.0146 km NW of Neiafu, TongaMay 22, 1972 · 292 km SW of this epicenter
  7. 7.0206 km SE of Hihifo, TongaJune 17, 1978 · 242 km SE of this epicenter
  8. 6.9127 km S of Hihifo, TongaMay 8, 1913 · 184 km S of this epicenter

History · NOAA

Deadliest earthquakes in this area

NOAA’s significant-earthquake database lists 5 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 2 recorded deaths. The most severe:

  1. 1917M8.3

    Samoa Islands

    26 Jun 1917 · 81 km away

    2 deathsModerate damage ($1–5M)tsunami

  2. 1981M7.7

    Apia; American Samoa: Pago Pago

    1 Sep 1981 · 90 km away · intensity VI

    Limited damage (under $1M)tsunami

  3. 1977M7.6

    Apia

    2 Apr 1977 · 225 km away · intensity VI

    Limited damage (under $1M)tsunami

  4. 1975M7.8

    Samoa Islands

    26 Dec 1975 · 165 km away · intensity V

    tsunami

  5. 1957M7.5

    Samoa Islands

    14 Apr 1957 · 81 km away

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

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