NIWA finds new ways of seeing past earthquakes – under the sea

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NIWA scientists are working at the cutting edge of earthquake research, developing new ways to interpret the history of undersea earthquakes occurring on major faultlines around New Zealand. This work will help scientists determine the likelihood of damaging earthquakes from underwater faults close to the coast.

This interdisciplinary work is done on undersea faults, using acoustic soundings to analyse the history of the fault’s activity.

“We are developing new ways of finding past earthquakes in the submarine realm. It’s the first time we have been able to do this. And in time these approaches will likely be used widely throughout the world,” says NIWA Principal Scientist Dr Philip Barnes.

The work of Dr Barnes, and his colleague Nicolas Pondard, has just been presented at the prestigious European Science Foundation conference.

Investigation of faults on land, and what they reveal about their earthquake history, is an established discipline. Approaches to submarine faults and their data are very much only now just being developed.

The Wairau Fault, which is the northern part of the Alpine Fault, together with the Cloudy and Vernon undersea faults in Cook Strait, are all part of the tectonic faulting structure that divides New Zealand on land and sea. These significant seismic hazards are close to Wellington.

“This is the first long-timescale – 18 000 years – submarine earthquake record derived directly from the growth history of a major active continental fault,” says Dr Barnes.

The scientists are using instruments that are towed in the water to provide recordings of sound reflected from sea-floor layers indicating where the faults are. From the details in the recording, they can work out when, and where, there have been earthquakes on the faults.

“We have found eight prehistoric earthquakes on the Wairau Fault of more than magnitude seven; six on the Cloudy Fault; and five on the Vernon Fault, which have all been big earthquakes in the past 18 000 years. We verified our results on the Wairau Fault by comparison with onshore records from the same fault. The long-time-scale records enable evaluation of the earthquake behaviour of the faults over multiple earthquake cycles,” says Dr Barnes.

By merging this information with similar records from on land, the scientists can learn how the different faults interrelate with each other. A history of ‘when, where and how often’ can be developed for each fault. NIWA are working with earthquake geologists at GNS Science and Victoria University of Wellington to provide new data for identifying earthquake hazards in the wider Wellington region.

For example, Dr Barnes says, “If you know a fault has an earthquake on it every two thousand years on average, and it’s been close to that elapsed time since the last one, then it could be reasonable to interpret that it is due for another one.”

“Another aspect to consider is the variability of the recurrence between earthquakes on the same fault. These types of comparisons help the experts estimate the likelihood of an earthquake occurring on a fault within a given timeframe.”

This information contributes to New Zealand’s seismic hazard model, which feeds into structural design standards for New Zealand building codes.

This work was funded by the Foundation for Research, Science and Technology with support from the Earthquake Commission (EQC), Accident Compensation Corporation (ACC), and Wellington City Council (WCC).

For further comment, please call:

Dr Philip Barnes

NIWA Principal Scientist Ocean Geology

Tel: 04 386 0372

Cook Strait Faults
Research subject: Earthquakes