Dig shows another quake was on fault

Evidence of a second major Greendale Fault earthquake some time in the last 10,000 to 15,000 years has been uncovered in a deep trench on the Canterbury Plains.

Canterbury University and GNS Science geologists are studying the fault that ruptured in the September 4, 2010, magnitude-7.1 quake.

They have found that features at the surface and in an old stream channel about a metre down were displaced about 65 centimetres horizontally and 10cm vertically by the fault.

However, a much older stream channel towards the bottom of their 4m-deep trench has been offset by the fault horizontally by about 120cm and vertically by 20cm, about double the amount closer to the surface.

That means large quakes have shifted the land twice in the roughly 16,000 years the deeper gravel layers have been there.

When compared to the 2010 movement at the surface, it also means the size of each shunt was about the same, implying the magnitude of the Greendale Fault quakes were similar.

PhD student Sharon Hornblow said those results would not have been discovered without excavating the 30m-long trench.

"It shows how easily the evidence of former quakes can be washed away or removed at or near the surface and altered by farming and forestry. But the movements deeper down have been preserved."

Organic material found in the trench next to Highfield Rd near Grange Rd was being radiocarbon dated to allow researchers to date the movements more accurately. That would take another month or two, she said.

Hornblow's supervisor, Mark Quigley, said the movement of surface objects and 1m-deep channel showed the September 2010 quake was the first to rupture the ground surface for more than 5000 years.

"The easiest explanation is that a major, surface-rupturing earthquake occurred at this location sometime after the deposition of the older sand channels and prior to deposition of the younger sand channels.

"It suggests that the last Greendale Fault rupture was a similar earthquake magnitude to our 2010 quake because the ancient displacement we measured is consistent with the recent one."

Data collected by ground-penetrating radar showed sediments deeper in the trench were tilted more than shallower sediments.

"This could just be an artefact of the river environment in which these sediments were deposited. But another equally plausible explanation is that they have been folded by previous earthquakes in a similar fashion to the folding of the sediments in the Darfield earthquake."

More excavations of the fault at different locations would be required to pinpoint the magnitude and timing of the last earthquake more conclusively, he said.

The research is being funded by the Earthquake Commission. Hornblow will present this research in Hamilton this month.

The Press