M4.0 earthquake 2 km SSW of Christchurch, New Zealand

Saturday, August 20, 2011, 07:31:12 UTC · 43.558°S, 172.625°E

A magnitude 4.0 earthquake struck 2 km SSW of Christchurch, New Zealand on Saturday, August 20, 2011, 07:31:12 UTC, at a depth of 9.9 km (shallow).

Magnitude: 4.0; Light · ml
Depth: 10 km; Shallow
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

15 tons of TNT

That is about 1/998 of the Hiroshima bomb. A magnitude 5.0 releases about 32 times more energy.

Tectonic setting

116 km

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

Depth: shallow

10 km

Shallow earthquakes (under 70 km) produce the strongest surface shaking for their size, so they are the ones most likely to cause damage near the epicenter.

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 11 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M4.8 on August 31, 2011. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.

Time after the mainshock on a logarithmic scale (first hours stretched), magnitude on the vertical axis. M2.5+ events from USGS.

Regional history

Past M5+ earthquakes within 300 km

USGS lists 72 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:

History · NOAA

Deadliest earthquakes in this area

NOAA’s significant-earthquake database lists 17 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 209 recorded deaths. The most severe:

2011M6.1
Christchurch, Lyttelton
21 Feb 2011 · 5 km away
185 deaths$15 billion damagetsunami
1929M7.5
South Island
16 Jun 1929 · 203 km away
17 deathsModerate damage ($1–5M)tsunami
1968M7.1
South Island: Nw Coast
23 May 1968 · 213 km away
3 deaths$3 million damage
1848M6.0
Awatere River, Marlborough
18 Oct 1848 · 238 km away
3 deathstsunami
2011M6.0
South Island: Canterbury
13 Jun 2011 · 10 km away
1 death$3.0 billion damage
2010M7.0
Christchurch
3 Sep 2010 · 64 km away · intensity IX
$6.5 billion damage

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

Nearby trackers