Feb 22 2011 - Christchurch badly damaged by magnitude 6.3 earthquake

Christchurch experienced a major earthquake centred south of the city and severe damage and casualties occurred. A National State of Emergency has been declared.

Map showing the mainshock and aftershocks above magnitude 3, and fault ruptures in Canterbury.

Map showing the mainshock and aftershocks above magnitude 3, and fault ruptures in Canterbury.

McQueens Valley seismograph recording of the earthquake.

The McQueen's Valley seismograph recording of the earthquake. It can be seen starting on the fifth line down just before 12:52. A few minutes later at 12:57, a magnitude 4.9 aftershock occurs. The other major feature is seen on the sixth line at 13:04, a magnitude 5.7 earthquake. Lots of smaller ones follow.

Last updated April 5, 7:30 pm

Why was this earthquake so damaging; wasn't it smaller?

The city had been comparatively lucky with both the location and timing of last September's magnitude 7.1 Darfield (Canterbury) earthquake; the location of this one within 10 km of the city and at a shallow depth of 5 km during the middle of a working day has resulted in destruction, injuries and deaths.

Why do you call this an aftershock; surely this is a major earthquake in its own right?

Seismologically, this is classed as an aftershock because of its relationship to the ongoing activity since September last year. Its occurrence was always statistically possible, but the long time interval and slow decrease in general activity had made it less likely. Unfortunately, it has happened after all and in a location that has brought the worst result.

Was the Christchurch earthquake on an extension of the Greendale Fault?

Preliminary analysis of seismograph records of the devastating magnitude 6.3 earthquake on Tuesday show that it was just 9 km from the centre of Christchurch on a buried fault oriented roughly east-west.

There is no obvious structure directly connecting the faults that ruptured in the September’s magnitude 7.1 earthquake with the fault that generated the magnitude 6.3 event. On the contrary, precise aftershock relocations suggest that at least two north-east/south-west trending faults lie between the two and that there is no evidence from the earthquake data of an extension of the Greendale Fault.

Portable seismometer deployment in the Christchurch region

Planned portable earthquake recorder deployment map.

Portable earthquake recorder deployment sites are shown as yellow circles (short-period instruments) and yellow squares (strong-motion instruments).

GeoNet staff have deployed further portable earthquake recorders. These instruments will collect information on the continuing aftershock sequence of the M6.3 Christchurch earthquake. Earthquake data from the portable instruments will be used in conjunction with data from permanent GeoNet monitoring stations and portable instruments that remain deployed following the M7.1 Darfield earthquake. This combined earthquake dataset will be used in scientific studies of fault structures and ground motion.

In total, six short-period seismometers and four strong-motion accelerometers have been placed around the outskirts of Christchurch. The short-period instruments are installed in the area of aftershocks to enable increased precision of earthquake locations and an enhanced understanding of local fault structure. The strong-motion instruments are deployed in the Port Hills to reveal ground motion differences between the hills and the plains as well as measure ground accelerations associated with any landslides that are caused. The instruments will record seismic activity for at least the next two months.

The first download of data for analysis took place late in the week of March 14; this is currently being analysed.

Portable GPS deployment in Clifton and Redcliffs

Land above the cliffs at Clifton and Redcliffs is being monitored for movement with portable GPS receivers. A weather station has also been deployed to correlate any movement with weather events. Data from this will also contribute to the longer term understanding of landslides and their failure mechanisms.

  • Read about the installation trips on the GeoNet blog.

Scientists talk about the Christchurch earthquake

Media links

Map showing measured Peak Ground Accelerations in the Christchurch area.

Map showing measured Peak Ground Accelerations in the Christchurch area. Shaking intensity in the city was much greater for this earthquake than the magnitude 7.1 earthquake for any of its other aftershocks. This is due to the proximity of the epicentre to the city and its shallow depth. The highest shaking was recorded at Heathcote Valley Primary School at 220 %g, with readings of 188 %g at Pages Road Pumping Station and 107 %g at Hulverstone Drive Pumping Station. Data from these instruments are available on our strong-motion FTP site.

Focal mechanisms for the bigger aftershocks since the Christchurch earthquake.

Focal mechanisms for the February 22 M 6.3 earthquake (red) and the larger aftershocks (blue). Yellow dots are the locations of all the aftershocks. Note that many of the focal mechanisms are offset from the true location in order to make them easier to see. The M 6.3 mainshock and a couple other events in the aftershock sequence are reverse (thrust) faulting mechanisms. Also the two events furthest to the west are mainly reverse faulting and may be considered as part of the aftershock sequence to the September M 7.1 earthquake rather than the February 22 earthquake. Most of the remaining mechanisms are strike-slip or mainly strike-slip solutions although the orientation of the fault plane is variable.