Why mysterious ‘slow-motion’ quakes hold big secrets

Unusual slow-burning quakes that silently unfold deep beneath New Zealand could allow scientists to better understand the violent, quick-fire shakes that Kiwis know much better.

Tracking what are called slow-slip earthquakes — known to shift faults over days or months without any perceptible shifts in the earth — has only been made possible with recent advances in GPS technology.

These events play out when faults grind incredibly slowly against each other, like an earthquake in slow motion.

Over the course of weeks, one might release the same amount of energy as a typical big quake like the 7.1 event that shook Canterbury in 2010.

Because they occur deep in the earth and release energy so slowly, they’re marked by very little deformation at the surface, even though they might affect an area of thousands of square kilometres.

In the two decades since their discovery, they have been observed in a handful of places, including Japan, Mexico, the northwest coast of the US — and here. Scientists have watched them play out every one to two years near Gisborne, at a relatively shallow depth beneath the seabed, and usually driving a spate of localised quake activity.

The most recent one, which was considered among the biggest on record and lasted for weeks, triggered a swarm of East Coast quakes, among them a magnitude 5.1 jolt that struck near Mahia in May.

In a study published in major journal

Nature, a team of US scientists have discovered intriguing similarities between slow-slip earthquakes and normal ones, with potentially big implications.

Caltech geoscientist Professor JeanPhilippe Avouac said there had been much uncertainty surrounding slow-slip events.

“You can’t study them using traditional seismological techniques because the signal they create is too faint and gets lost in the noise from human activities as well as from natural geological processes like ocean waves, rivers, and winds.”

But his team has managed to get a clearer picture of slow-slip events where the North American tectonic plate slides southwest over the Pacific Ocean plate using a network of 352 GPS stations.

By analysing a decade of data, they were able to catalogue more than 40 slowslip events of varied sizes, and then characterise their features more precisely.

One key finding was that slow-slip events obey the same “scaling” laws as regular earthquakes. These laws described the “moment” of a slip event on a fault.

In practical terms, that meant that a big slip across a broad area yielded a longlasting earthquake.

It had long been known that the moment of an earthquake was proportional to the amount of time the earthquake lasted.

In 2007, a team from the University of Tokyo and Stanford suggested that slowslip events appeared to be different, with the moment seemingly directly proportional to time.

But now, the US scientists believed magnitudes of slow-slip events were also proportional to their duration, like regular earthquakes.

Since these events behaved similarly to regular earthquakes, studying them could shed light on their more destructive cousins, Avouac said.

GNS Science geoscientist Dr Laura Wallace said as the study was focused on one location, the model would need to be applied to other areas to fully test it.

“If correct, it really does highlight a greater similarity between earthquakes and slow-slip events.”

One key finding was that slow-slip events obey the same ‘scaling’ laws as regular earthquakes.