Ancient earthquakes point to modern dangers

By Amanda James 11 November 2010
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The traces of ancient earthquakes under the seafloor in New Zealand are helping predict future tremors.

SCIENTISTS ARE LISTENING IN on the sounds of the ocean floor to reveal the locations of ancient earthquakes below the seabed.

The information will be used to help predict the risk of future earthquakes around the ‘Ring of Fire’ region of New Zealand – a geologically active zone where the Pacific plate meets other tectonic plates. Armed with this information, engineers can develop better building codes to help cities withstand earthquakes, such as the recent 7.1 magnitude quake that shook Christchurch in September.

“All of this is really about how the past is the key to the future,” says Dr Mark Stirling, a seismic hazard expert at the Institute of Geological and Nuclear Sciences in Lower Hutt on New Zealand’s North Island.
The research was presented in September at the European Science Foundation Conference, held in Austria.

Finding faults

Using sonar to scan layers underneath sea floor, the scientists have been pinpointing where past earthquakes occurred and figuring out the likelihood of a damaging quake unleashing its fury from undersea faults close to New Zealand’s coast.

“We have found evidence for eight large prehistoric earthquakes on the Wairau Fault, six on the Cloudy Fault, and five on the Vernon Fault, which have all been big earthquakes in the past 18,000 years,” says Dr Philip Barnes, a researcher for the National Institute of Water and Atmospheric Research (NIWA) in Wellington.

The mapping of the evolution of earthquakes along a continental fault, stretching back 18,000 years, is a geological first, he says.

To find faultlines under the sea, scientists tow an instrument from the back of a boat, which emits bursts of sound. These penetrate down to 50 m below the seabed and bounce back to the boat, building a profile of the layers of the sea floor. This is then used to figure out the location of faults by looking for displacements between the layers – deeper layers indicate older geological activity. 

By merging this information with similar records from land, the scientists can study the relationship between different faults. They found, for example, that a newly discovered submarine fault matched up with the Wairau fault on the New Zealand mainland.

A history of the frequency, timing and location of earthquakes can be developed for each fault. This kind of comprehensive record enables scientists to observe what happened over multiple earthquake cycles and help determine how often earthquakes occur.

According to Mark, the research is particularly important in the Cook Strait. “Now there’s a continuous representation of plate boundary across from the South Island to the North that matches up with what we’ve got from onshore,” he says. This creates a continuous chart for the fault that runs between the two islands.

Building codes

The research is also being used to help to improve building codes in New Zealand by contributing to its national seismic hazard model, which is a representation of the risk of earthquakes occurring around the nation. It takes into account the historical records of both terrestrial and marine earthquakes, and allows engineers to gauge the lifespan of a building or dam, for example, by looking at long-term geological hazards.

“What the engineers effectively want to know is what the expected ground-shaking levels will be at certain locations within a certain period of time – information that can be found in the seismic hazard model,” says Mark. 

“Engineers will often build something to withstand the ground shaking levels expected with a 1-in-10 chance in 50 years,” says Mark, which means they may use more caution when choosing a location for a public building, such as a hospital or school.