Alluvial layers might be earthquake faults

BY MARCIA AHERN

Southland had few mapped fault lines, but that did not mean they weren't there, a GNS Science geologist said.

The 7.1 magnitude earthquake that damaged much of Christchurch on Saturday occurred along a previously unknown fault line, beneath Darfield.

GNS Science geologist Jane Forsyth said fault lines, such as the Darfield one can exist under alluvial layers – sediment deposited by flowing water, such as riverbeds and floodplains.

These layers can hide the surface evidence created by earthquakes.

Southland's Oreti Plains, like the Canterbury Plains, consist of alluvial layers, she said.

"For Southland (there is) nothing showing up south of Hokonui. However, from there down to Invercargill there are big alluvial plains.

"They could be hiding the fault lines, because that is pretty much what we think has happened up at Canterbury," she said.

Observations of physical damage to roads, shelter belts and paddocks were used to map fault lines.

Ms Forsyth said geologists have mapped active fault lines based on surface evidence such as ruptures on the surface of the earth.

An active fault line is one that has moved in the past 125,000 years, she said.

"That information will stay there at the surface for thousands of years."

Ms Forsyth said people needed to prepare for a major earthquake, no matter where they were in New Zealand.

"I am not just panic-mongering. Everywhere in New Zealand has a seismic pattern. You can be very badly shaken by an earthquake not necessarily close to you," she said.

Marcia Ahern is an SIT journalism student

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