The first M5.8 quake was situated 220 kilometres west of Southland’s most southern city of Invercargill, at a depth of 18.1 kilometres along a major Pacific plate boundary fault line. It was timed at 7.48pm NZT on January 19th, 2012.
No reports of damage were received, although the earthquake was felt widely throughout the southern half of the South Island and as far north as Christchurch. In Invercargill, residents reported the quake as swaying for about 20-30 seconds with hundreds of reports to GeonetNZ.
Large Aftershocks Rock South
It was followed two hours later by a M5.5 tremblor in the same region, at 10.01pm NZT that was at a depth of 12km and reported from as far north as Kaikoura, on the South Island’s east coast.
Nearly fours hours later, in the early hours of January 20th, a M5 quake rocked the area at a depth of 21 km, and was described by Invercargill residents as another swaying, side to side tremor.
The nearest landfall to the earthquakes’ epicentre are the uninhabited sub-Antarctic Snares Islands that are a wildlife sanctuary, 150km northwest of the epicentre, and part of a UNESCO World Heritage Site.
Active Plate Boundary
The first earthquake was assessed at M5.8 by New Zealand seismologists and M5.9 by the United States Geological Service that uses a different system known as the Moment Tensor method.
This series of quakes follow several other major earthquakes along this Indo-Australian/Pacific plate boundary in the past week, including a M4.9 on January 17th, (87km south of Te Anau), on the southwest coast of the South Island.
North of New Zealand, a M5.4 near Tonga occurred on January 14th, and two M5.1 quakes just south of the Kermadec Islands on January 13th.
Christchurch Activity Continues
Meanwhile, Christchurch continues to be struck by after-shocks with hundreds recorded since the renewal of major seismic activity in the city on December 23rd, 2011.
The majority are M2 - M4, but exceptions in the past week include; a M4.7 on January 12th (depth 13km), that was felt widely in Canterbury and on the north-west coast of the South Island; and a M5.1 at 2.47am on January 15th (12km from Christchurch and just 6km deep) that was felt widely in Canterbury, Otago and Southland, as well as the northern West Coast.
Late last year, GNS scientists adjusted their earthquake forecasting modelling for the Canterbury region to a long term outlook. Updated figures were based on a combination of two different statistical forecasting models.
Modeling Helps Predictions
Earlier, the earthquake probabilities were dominated by a short-term clustering model, but in October 2011, the medium-term clustering model had more influence on the earthquake probabilities.
The medium-term model estimated slightly higher probabilities than the short-term model which had caused a slight increase in the combined figure, said Dr Matt Gerstenberger, a senior natural hazard scientist at GNS Science.
Updated aftershock probabilities were the result of the change in mathematical modelling rather than an increased risk of earthquakes. Nothing had changed inside the earth to increase the risk of an earthquake.
Canterbury Zone Probabilities
The figures were for the entire aftershock zone and not just for Christchurch city, said Dr Kelvin Berryman, Manager of the Natural Hazards Research Platform. The zone extends from Hororata in the west of Canterbury to large parts of Banks Peninsula and from Kaiapoi in the north to Lincoln in the south.
The combined earthquake forecast model for the region would continue to be used, he said. The updated figures based on the combined model show for the 12 months to October 15 next year there were:
- an 88% probability of a magnitude 5.0 to 5.4 aftershock
- a 46% probability of a magnitude 5.5 to 5.9 quake
- a 15% probability of a magnitude 6.0 to 6.4 quake
- a 4% probability of a magnitude 6.5 to 6.9 quake
- a 1%, or one in 100 chance, of a magnitude 7.0 or higher
Sources
United States Geological Survey
GNS Science NZ
Geonet NZ
