Canterbury Aftershock

A moderately strong earthquake struck Canterbury this morning, the seventh largest aftershock of the Darfield earthquake.

Thursday 20th January 2011

A magnitude 5.1 earthquake roused Cantabrians from their beds at 6:03 this morning. GeoNet reports that the earthquake, an aftershock in the Darfield earthquake sequence, was located 10 km south-west of Christchurch at a depth of 10 km.

The earthquake is the seventh largest aftershock resulting from the Darfield earthquake of September 4th 2010, equal in size to aftershocks which struck on September 4th and 5th last year. Light damage has been reported.

Initial analysis by GeoNet shows that the strongest ground shaking was measured at Lincoln, where this morning’s earthquake was measured at 0.14g. This Peak Ground Acceleration (PGA) measurement shows how strongly the earth shook at a particular location according to readings taken by instruments. When expressed in terms of g (gravities) it effectively shows the lurch or jolt that was experienced on the ground at the time, much the same as if someone was given a violent shove.

By comparison, Lincoln experienced a PGA of 0.91g as a result of the main Darfield earthquake.

This morning’s earthquake was less strongly felt in Christchurch city, where the magnitude 4.9 aftershock of December 26th caused much more damage. The Boxing Day quake was closer to the city and, at 12 km, of a similar depth to today’s event.

Proximity to a shallow earthquake is an important factor in determining how much shaking will be felt. For example the Peak Ground Acceleration in the Christchurch Botanic Gardens resulting from the Boxing Day earthquake was 0.48g, much stronger than the 0.18g which resulted from the magnitude 7.1 main event of September 4th which was centred 40 km west of the city. This morning’s aftershock was measured at 0.02g at the gardens.

However, it should be noted that ground acceleration is only one component of the earthquake experience, and the sheer size of the main earthquake produced sustained shaking over a long period of time and hence caused serious damage over a wide area.

The Canterbury Quake Live website, operated by Chris Crowe, reports that this morning’s aftershock was located near Hay’s Road in Halswell, 11.7 km away from The Square in Christchurch. The Boxing Day aftershock was located near Huxley Street in Sydenham, only 2.6 km from The Square.

This morning’s earthquake was felt from Marlborough to Southland and on the West Coast, attracting 1770 reports to GeoNet from the public by 10 a.m. Residents from Rolleston to New Brighton reporting MM6 shaking this morning. Six on the Modified Mercalli scale is regarded as “slightly damaging” and defined as, “Felt by all. People and animals are alarmed, and many run outside. Walking steadily is difficult. Furniture and appliances may move on smooth surfaces, and objects fall from walls and shelves. Glassware and crockery break. Slight non-structural damage to buildings may occur.”

GeoNet Seismograph Network 20 January 2011

The aftershock showed strongly on New Zealand’s seismograph network. [Click on image for larger view]

Earthquake activity in Canterbury has increased as a result of this morning’s aftershock with a further four events reported by GeoNet. A magnitude 3.2 quake struck at 6:28 a.m., and two quakes of magnitude 3.4 and 4.0 struck four minutes apart at 8:02 and 8:06 a.m. A magnitude 3.2 earthquake struck at 8:14 a.m.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST, and Chris Crowe’s Canterbury Quake Live website.]

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