Slow-slip earthquakes, like those in NZ, can relieve pressure on dangerous faults
New research directly relevant to New Zealand shows slow-slip earthquakes can reduce stress at tectonic plate boundaries, but it's not clear how that affects the risk of big quakes and tsunamis.
The research carried out at the Nankai Trough subduction zone under the sea east of Japan is similar to work being planned for the Hikurangi Margin east of the North Island.
"The Nankai Trough results are important for understanding the risk of large earthquakes and tsunamis generated at offshore plate boundary zones worldwide," GNS geodetic scientist Dr Laura Wallace said.
Wallace was a member of the team that carried out the Japanese research and is co-leader of the team doing the New Zealand study.
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"The big gap in understanding is why they happen. We don't understand that yet."
The relationship between slow-slip events and earthquakes was also not fully understood, Wallace said.
It was known slow-slip events relieved stress that had built up on plate boundaries, but they also re-distributed stress to other parts of the plate boundary and could trigger big earthquakes in rare cases.
The Nankai Trough is where the Philippine Sea Plate dives beneath the Eurasian Plate on which Japan sits. The fault beneath the seafloor that underlies the trough has been the source of devastating earthquakes and tsunamis.
The last big quake on the subduction fault was in 1944. It had a magnitude of 8.1 and triggered an 8-metre-high tsunami that hit Tokyo.
The Nankai Trough researchers found up to 50 per cent of tectonic energy at the subduction zone was released during slow earthquakes that happen under the seabed in that area every 12 to 18 months.
The findings came from two boreholes 11km apart, drilled into the ocean floor in 2009 and 2010. Monitoring equipment was packed into the boreholes, which were part of a network of instruments, including sensors on the seafloor.
A paper on the study, published in Science, said outstanding questions about slow-slip events included how they were related to large, damaging earthquake ruptures. In some cases they might precede and trigger damaging interplate quakes.
Similarly, little was known about the role of slow-slip events in long-term plate motion, particularly in accumulating and releasing strain near the trench where seafloor deformation was intimately linked to the generation of tsunamis.
Nankai Trough study leader Dr Demian Saffer of Pennsylvania State University said questions remained about the 50 per cent or so of tectonic energy not being dissipated by periodic slow earthquakes at the trough. One possibility was that it could 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," he said.
It was too simplistic to conclude that slow-slip events automatically reduced the risk of big quakes and tsunamis, because the research showed the part of the subduction interface studied was capable of storing strain.
Saffer is the other co-leader of the work being done in this country in 2018. That will involve drilling boreholes and installing instruments into the seafloor about 60km off the Gisborne coast.
One of the aims off Gisborne is to collect samples, from beneath the seafloor, of rocks that are the same as those in areas where slow-slip events are happening.
"That can tell us something about what it is about the rock properties that might cause slow-slip events to happen," Wallace said.
The slow-slip events offshore from Gisborne were the shallowest of such events to have been well-documented. They were within a couple of kilometres of the seafloor, while other well-documented events were at least 20km beneath the Earth's surface.
Understanding slow-slip events provided an opportunity to improve the ability to forecast earthquake probabilities, Wallace said.
Other work was under way to learn more about the history of earthquakes in the subduction zone, which was not well understood.