Significant Aftershock, Christchurch

A moderately strong earthquake struck Christchurch this evening.

Saturday, 16th April 2011

A magnitude 5.3 earthquake struck 10 km north-east of Diamond Harbour, 10 km east of Lyttelton, 20 km south-east of Christchurch at 5:49 this evening. GeoNet reports that the quake was 11 km deep, and was centred beneath the harbour entrance, just off the southern shore.

The earthquake was felt from Masterton, in the North Island, to Invercargill.

Damage has been reported from Avonside, Christchurch, Dallington, Ferrymead, Halswell, Linwood, South New Brighton, Sumner, Westmorland and Woolston, with slight damage reported from numerous other Christchurch suburbs. More than 1100 reports on the quake had been filed by the public by 7:30 p.m.

Radio New Zealand National reports no injuries, and that the Fire Service is attending incidents in the city. Water mains have burst, and a significant power outage occurred. Damage to the electricity infrastructure is unclear but, if the outage is due to protection mechanisms tripping, power should be restored by 8 p.m.

Residents report pictures falling from walls, items thrown from cupboards and shelves, and contents damage.

The U.S. Geological Survey reports the quake as magnitude 5.2 at a depth of 9 km, centred 16 km east-south-east of Christchurch.

Using data provided by GeoNet, the Canterbury Quake Live website reports the epicentre to be beneath the harbour near 257 Camp Bay Road, Port Levy.

GeoNet’s seismograph network Saturday 16 April 2011

[click to enlarge image] New Zealand’s seismograph network shows the trace of this evening’s earthquake near Christchurch, and the traces of the aftershocks which followed.

McQueen’s Valley Seimograph

[click to enlarge image] The McQueen’s Valley seismograph shows the main shock and aftershocks which occurred near Christchurch this evening.

Seven aftershocks of about magnitude 3 followed this evening’s earthquake over a period of five minutes. Most of these were reported felt by residents. GeoNet reports that a magnitude 3.4 earthquake then struck within 5 km of Lyttelton at 5:59 p.m. The quake was 8 km deep.

Tonight’s earthquake was not entirely unexpected. GeoNet’s projection for the week ending April 18, indicated that a magnitude five aftershock might be possible. The projection is based on observations of similar aftershock sequences in the past, and stated that the expected average for magnitude 5 events was less than 1. This shows that the chance of such a quake was fading, but had not been entirely eliminated in the calculation.

The aftershock is the third largest to follow the damaging magnitude 6.3 earthquake of February 22nd.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST; the US Geological Survey and its contributing agencies, and the Canterbury Quake Live website.]

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