About 80 km off the east coast of New Zealand lies the Hikurangi Trench, a subduction zone that plunges 3,000 meters beneath the surface and forms the margin between the vast Pacific Plate and its smaller western neighbor, the Australian Plate. The earthquake potential for this region is well known, but new research indicates the massive quakes that have occurred in the past may occur with greater frequency than once believed, which is bad news for New Zealand. According to the latest findings, they are long overdue.
A report appearing today in the Bulletin of the Seismological Society of America (BSSA) details the latest research on tectonic activity along the Hikurangi margin. Researchers from GNS Science, a research organization that studies the geology of the region around New Zealand, examined core samples taken from a salt marsh at Big Lagoon, which is located on the South Island. By reconstructing the depositional history of the area, they were able to identify two major earthquakes that occurred within the past 1,000 years, the most recent of which struck between 520 and 470 years ago. An earlier event, which probably took place around 850 years ago, was accompanied by a massive tsunami that brought waves more than 360 meters inland.
"Subduction earthquakes are not a 'new' risk for New Zealand, as we have always assumed they can occur, and they are accounted for in our seismic hazard models," reports Kate Clark, a researcher from GNS Science. "This study is significant in that it confirms that risk."
Subduction zones are formed when two plates, like the Pacific and Australian, converge along a tectonic boundary. As the plates move against each other, one plate is thrust beneath the other, and it's this grinding motion that can set off earthquakes, which can then trigger tsunamis.
And what does this mean for the residents of New Zealand and its surrounding islands?
"We have a record of three to five past earthquakes on most of the major upper plate faults in the [New Zealand] lower North Island and upper South Island, but there was previously no evidence of past subduction earthquakes on the southern Hikurangi margin," Clark says. "Subduction earthquakes have the potential to be significantly larger in magnitude than upper plate fault ruptures, affect a much larger spatial area and are much more likely to trigger tsunami."
The devastation could be horrific. Previous researchers have estimated that a magnitude 8.9 Hikurangi quake could result in over 3,000 dead, 7,000 injured and cost $13 billion for New Zealand's capital of Wellington alone.
And a major event could happen sooner than previously anticipated. Previous seismic models indicated past quakes occurred about every 500 to 1,000 years. But GNS's new data point to past frequencies of around 350 years between major events. And since the last major event occurred over 400 years ago, the scientists from GNS are worried. They hope their research can shed light on future events.
"In addition we would like to go further back in time and find evidence of older subduction earthquakes. With a longer record of past subduction earthquakes we can get a better constraint on the recurrence of such earthquakes, which will help to forecast future subduction earthquakes."