Swarm of NZ quakes triggered by major 'slow slip' event

A swarm of East Coast earthquakes – one of them measuring magnitude 4.5 - has just been triggered by one of the largest "slow slip" events yet observed in New Zealand.

Slow slip events are silent, slow-burning earthquakes that can displace faults over days or months, particularly around the sprawling plate boundary to the east of the country that is the Hikurangi Subduction Zone.

By contrast, in a typical earthquake, fault movement occurs over a matter of seconds, unleashing an instant surge of energy.

Slow slip events are known to occur every one to two years near Gisborne, at a relatively shallow depth beneath the seabed, and have been linked to an increase in localised earthquake activity.

The current event, which began several days ago, had already generated a swarm that so far included 60 earthquakes around the Mahia Peninsula, most of them too small to feel.

Scientists regard the region as one of the best places in the world to study the events – and this one happened to be the first captured by state-of-the-art sensors only just installed there.

These have so far recorded up to 3cm of eastward displacement at GeoNet continuous GPS sites, which in turn had been caused by up to 10 to 15cm of movement on the Hikurangi boundary.

This event was now on track to be as large as the biggest previous slow slip event, recorded off Gisborne in March 2010, said Dr Laura Wallace of GNS Science.

"We tend to have moderate to small slow slip earthquakes in that area about every one to two years, but a larger one like this one happens about every four to six years.

This was based on the last 17 years of observations.

The black arrows show the horizontal displacement of the GeoNet GPS sites over the last week, while the yellow to red colors show the areas of the Hikurangi Subduction Zone fault that have been slipping. There has been up to 10 to 15 cm of movement on the plate boundary offshore Gisborne in the last week. Source / GNS Science

"Right now, we have instruments out in on the seafloor off Poverty Bay that are waiting to capture the data from events like this."

Those instruments included seismometers, seafloor pressure sensors that measured upward and downward movement, and flow meters that measured the flow of the fluids coming out of the sea floor.

"We expect a big response to that because, during the straining of the crust in a slow slip, the rate of flow can change," she said.

This graphic shows the locations of the quakes in the swarms and their magnitudes. Image / GNS Science

"Our scientists can then study these land movements and changes offshore, to better understand the processes at subduction zones and the relationship of slow slip events to earthquakes."

A large slow-slip event was later found to have proceeded the Tohoku 9.1 earthquake and tsunami, which killed nearly 16,000 people when it struck the coast of Japan on March 11, 2011.

But Wallace pointed out that their relative frequency around New Zealand made this event essentially normal behaviour.

"Remember that these are quite common in New Zealand, and our scientists have been anticipating this."

The Hikurangi Subduction Zone is a major offshore fault is where the Pacific Plate dives – or subducts – westward beneath the North Island, and poses New Zealand's largest geological hazard.

Scientists believe the subduction zone has the potential to unleash "megathrust" earthquakes larger than magnitude 8, such as those which created tsunamis that devastated Indonesia in 2004 and Japan in 2011.

This map shows the location of the swarm. Image / GNS Science

Over the past decade, the system has been the centre of a massive scientific collaboration between the world's leading earthquake experts.

Already, scientists have put in place two sub-seafloor earthquake observatories - making New Zealand only the fourth country in the world to have such capabilities.

The technology could help pave the way for offshore instrumentation needed for earthquake and tsunami early warning systems.