Study of slow-motion tremor in tectonic plates a first for scientists.

For the first time scientists have observed a slow-motion earthquake unfold deep beneath the sea - an insight that has big implications for New Zealand's ever-present threat of tsunamis.

The world-first discovery, published this week in Science, has boosted our understanding of earthquakes at what are called offshore subduction zones, where one tectonic plate dives below another.

Scientists are interested in what happens at these underwater plate boundaries as they generate the world's largest earthquakes and tsunamis.

These included the undersea megathrust quakes that triggered 2004's Boxing Day tsunami, which killed more than 230,000 people, and the Tohuku tsunami that struck Japan in 2011 and left more than 15,000 people dead.


The study focused on the Hikurangi subduction zone, off the North Island's east coast, where a two-week-long underwater "slow-slip" quake was observed in 2014.

Slow-slip events are similar to earthquakes, but instead of releasing strain between two tectonic plates in seconds, they do it over days or weeks without being felt.

Using a network of seafloor pressure recorders, an international team of scientists was able to find the event had resulted in up to 20cm of movement along the "megathrust" interface between the two colliding plates.

Despite that seemingly small amount, it was equivalent to three to four years of typical, steady motion at the megathrust zone.

Had it unfolded suddenly, a magnitude-6.8 earthquake would have erupted.

The event also occurred in the same location as a 7.2 earthquake in 1947 which generated a large tsunami that caused widespread damage along the Gisborne coast.

Crucially, this showed that both slow-slip and normal quakes could occur on the same part of a plate boundary.

"The link has been difficult to document in the past because most slow-slip monitoring networks are strictly land-based and are located far from the offshore trenches that host tsunami-generating earthquakes," said project leader Laura Wallace, of the University of Texas at Austin.

The revelation that the same spots could see catastrophic seismic rupture increased the need to closely monitor slow-slip quakes with similar technology to that being used off Japan, she said.

While earthquakes were unpredictable events, the link between the two types could help in forecasting the likelihood of damaging earthquakes and tsunamis.

"To do that we will have to understand the links between slow-slip events and earthquakes much better than we currently do."

While the sluggish speed of land displacement by slow-slip quakes was well short of what was needed to kick off tsunamis, the events could cause big changes in sea levels.

Waikato University researcher Dr Willem de Lange said the variations observed in sea-level rise at Wellington suggested they could temporarily push up the rate as the shoreline subsides in response to the events.

The same behaviour might have occurred before the 2004 and 2011 tsunamis, Dr de Lange said, and the events measured off Gisborne might indicate an "increasing potential" for a major tsunami in the future.