Monday, February 28, 2011


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.

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.

Christchurch earthquake not on 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.

Planned portable earthquake recorder deployment sites are shown as yellow pins (short period instruments) and red pins (strong motion instruments). Existing GeoNet sites are shown as red squares (Canterbury Network strong motion sites) and red crosses (GeoNet National Network sites). Portable strong motion instruments from the 4 September 2010 Darfield earthquake deployment are shown as white squares.

Four GeoNet staff are travelling to Christchurch to deploy 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 are planned to be placed around the outskirts of Christchurch. The short-period instruments will be 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 will be 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.

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