Three big earthquakes over 1000 years found on ruptured Kekerengu faultline

Geologists discovering evidence of three large historic quakes on an active southern fault suspected another big one was due when the devastating Kaikoura tremor struck.

In a blog on Government Science Geonet details have emerged of the work undertaken by geologists in recent months on the Kekerengu Fault after it had been identified as the fastest slipping fault within 100 km of Wellington apart from the Hikurangi subduction zone.

The North Canterbury fault was seen to pose a significant seismic hazard to north-eastern regions of the South Island and Wellington if linking faults in the South Island ruptured at the same time.

Geologists examining the fault dug trenches deep into the earth across the Kekerengu Fault and found evidence that at least three big earthquakes had struck in the last 1250 years.

These initial results confirmed that the Kekerengu Fault was capable of producing large earthquakes frequently every 300 to 400 years and was likely to do so again in future.

Just over a fortnight ago the fault ruptured right through those same trenches.

"I had often wondered what it would look like if a fault moved while we were working on a trench cut across it but I had never expected this to happen to me," said Victoria University's Tim Little.

The scientists described the impact on the landscape as dramatic with one side of the fault moving as much as 11m.

The geologists said they knew the fault posed a seismic hazard and that it was possible it might rupture jointly with other faults.

A national seismic hazard model included scenarios that involved a joint rupture of the Jordon, Kekerengu, and Needles Faults.

NIWA's offshore survey of the Needles Fault confirmed this scenario played during the recent 7.8 earthquake.

However geologists were surprised that even more faults were involved in the single earthquake sequence.