Intense Seismic Study in Canterbury

Scientists completed an intense study of aftershocks and seismic risk in Canterbury during May.

Monday 6th June 2011

Scientists have released preliminary findings from surveys carried out in the Canterbury area and in Pegasus Bay since the Darfield earthquake of September last year and the Christchurch earthquake in February.

Teams from GNS Science, NIWA, the University of Canterbury, University of Otago and the University of Calgary (Canada) have contributed to the Natural Hazards Research Platform project. The aim has been to understand more about the pattern of aftershocks and indicate where future earthquakes might occur.

Seismic, gravity and magnetic measurements were taken at several locations in Canterbury and Christchurch city. Seismic reflection surveys were conducted from a vehicle that generates vibrations similar to small earthquakes so that a network of microphones can detect how the underlying soil and rock transmits and impedes the waves as they travel through the ground. This allowed the scientists to produce images of the subsurface geological structure to a depth of 2 kilometres. The gravity and magnetic measurements provided information about the density of the material beneath the city and plains.

When combined, the data yielded information about the geological structure and revealed the location of previously unknown earthquake faults trending north-east to south-west. It is also thought that the bedrock structures on which the magnitude 5.3 Boxing Day earthquake and the magnitude 6.3 quake of 22nd February occurred have also been identified. These appear to be very old faults that have been weakly reactivated.

Most of the faults in the bedrock under Canterbury and beneath Pegasus Bay had their major period of movement about 50 million years ago. The off-shore study of the bay by NIWA and the University of Otago confirmed widespread offshore faulting in basement rock with a small number of faults showing evidence of reactivation in recent geological times. Like those onshore, it appears that younger fault structures are causing the reactivation of these old faults.

The offshore survey has revealed a previously unknown fault about 25 km long, to the east of Kaiapoi. The fault is similar in length to the Greendale fault which ruptured during the September earthquake and is capable of generating an earthquake of magnitude 6 to 7. However, there is no strong eveidence at present that the aftershocks are migrating offshore onto the fault structures of Pegasus Bay.

Nevertheless, it is now thought that the area’s earthquake risk is similar to Wellington’s for events occurring along faults in the Earth’s crust. Wellington has an additional risk profile for a subduction thrust earthquake that is currently being studied.

GeoNet seismic network 6th June 2011

A magnitude 5.5 earthquake struck 20 km south-west of Christchurch at 9:09 this morning. The quake, centred near Rolleston, was 15 km deep. Damage was reported from Templeton and Rolleston and neighbouring areas. Since then, the risk profile for Christchurch and parts of the Canterbury Plains experiencing MM6 shaking (slight damage) again during the next 24 hours has risen to about 1%.

Once again, this morning’s earthquake was not entirely unexpected, with an aftershock of magnitude 5.0 or above thought possible. The forecast on GeoNet”s website had noted that as many as two such aftershocks might be expected between May 19 and June 18th, but the expected average had fallen below one such event. This was also the situation when a magnitude 5.3 aftershock struck near Rolleston on May 10th.

Today’s aftershock was the second strongest to follow the magntude 6.3 earthquake of February 22nd, equal in strength to another magnitude 5.5 shock that occurred in conjunction with a magnitude 5.7 quake within 2 hours of the main event.

Another finding of the recent study is that the chance of a magnitude 6 to 7 earthquake occurring in the Canterbury aftershock area during the coming year is now thought to be about 23% and 6% for the Christchurch city area, an important finding for infrastructure planners and owners of damaged buildings. This is probably the first time that New Zealand scientists have been able to use an aftershock study to produce a medium-term forecast of activity. The media release from GNS Science can be found here.

Despite the increased activity in the area, most of the offshore faults in Pegasus Bay are slow-moving with rates of activity similar to the slowest moving faults anywhere in New Zealand. In the bigger picture, movements are greater in the Marlborough Fault Zone where land is deforming to take up the continued motion between the Pacific and Australian tectonic plates. The nearby transform zone, where subduction off the East Coast of the North Island changes to sliding past movement along faults in Marlborough and along the Alps is thought to be moving southward but over an incredibly long period of time. There is some debate amongst geophysicists that the burst of activity in Canterbury is just one (geologically brief) result of this very slow migration.

[sources: GNS Science media release of June 3rd, discussion, data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST. ]

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