Quiet quakes let off some steam

Insights could improve understanding of NZ zones

Silent, slow-slip earthquakes like those recently recorded near Hawke’s Bay and Kapiti could partly act as stress relievers for under-pressure fault zones.

That finding has come not from studying New Zealand’s quake-making geology but from investigating a subduction zone east of Japan, where up to half of tectonic energy was being released in slow-slip quakes under the seabed.

A GNS Science geophysicist, Dr Laura Wallace, who was part of the study just published in the journal Science, says the insights could markedly improve our understanding of New Zealand’s own shallow subduction zones.

Unlike the short and shaky quakes Kiwis are accustomed to, slow-slip quakes aren’t felt and can occur over months.

Scientists have only been able to detect them with the advent of GPS equipment that can reveal sub-centimetre changes in land movements.

In Japan, a team of international scientists analysed information from two boreholes 11km apart and drilled into the ocean floor at the Nankai Trough subduction zone in 2009 and 2010.

The sub-seafloor fault that underlies the trough — where the Philippine Sea Plate dives beneath the Eurasian Plate on which Japan sits — has been the source of past devastating quakes and tsunamis. In 1944, a magnitude 8.1 quake in the fault sent an 8m-high tsunami surging toward Tokyo.

The Nankai finding hints that slow-slip earthquakes may reduce tsunami risk by periodically relieving tectonic stress. Dr Demian Saffer, Pennsylvania State University

The boreholes — part of a network of sensors and instruments — were packed with monitoring instruments which have revealed new information about the earthquake behaviour of plate boundaries.

They studied eight different slow-slip earthquakes over the course of six years, finding that around 50 per cent of tectonic energy dissipated with the events.

But the project’s leader, Dr Demian Saffer of Pennsylvania State University, said it remained unclear how the rest of the energy was being spent.

It could be taken up by “silent creep”, he said, but it could also be accumulating for release during the next big earthquake on the subduction zone.

“The Nankai finding hints that slow-slip earthquakes may reduce tsunami risk by periodically relieving tectonic stress, but it is probably much more complicated than acting as a simple relief valve,” Saffer said. It was too simplistic to conclude that slow-slip events automatically reduce the risk of big quakes and tsunamis, he said, because the research also showed that the slipping part of the subduction interface was capable of storing strain.

But Dr Wallace, of GNS, said the study could be important for understanding more about our own subduction zones.

Here, the Hikurangi subduction zone — where the Pacific Plate dives beneath the North Island off the East Coast — has the potential to be the country’s single biggest geological hazard.

As part of the same International Ocean Discovery Programme, the Hikurangi margin will next year be probed with similar exploratory boreholes off the Gisborne coast, where slow-slip quakes occur every 12 to 18 months.

Using the US research ship JOIDES Resolution, the boreholes will be drilled about 60km off the coast, ultimately helping create an offshore earthquake observatory.

Wallace said the Japan insights opened up the possibility that similar boreholes planned off the Gisborne coast might reveal types of tectonic plate behaviours that had not been observed before.

“It might open a new view of what is happening in the plate boundary east of Gisborne,” Wallace said.

“It’s exciting because the project has the potential to markedly improve our understanding of shallow subduction zones not just in New Zealand, but internationally as well.”