Physics World  July 2017
(Shutterstock / Naypong)

Slipping slowly

“Slow earthquakes” are tiny, indiscernible tremors that last for weeks, but could these strange events precede or even cause the more familiar “fast” earthquakes? Sophia Chen investigates

About once every 14 months, something strange happens in the earth beneath the north-western US and British Columbia in Canada: it begins to move. The region, known as the Cascadia subduction zone, consists of a 1000 km fault where several tectonic plates meet. As the rock 35–55 km below the surface slips, it also shakes back and forth a few times per second. This year, the slipping started on 22 February and lasted for a few weeks.

But unlike an earthquake, the motion arrives without drama or destruction. On the Earth’s surface, you feel nothing. Even geophysicists with their seismometers failed to notice the motion for decades. “The motion was so small that it was thought to be just noise from wind and waves,” says geophysicist Heidi Houston of the University of Washington, US, who researches the slip in Cascadia.

This slipping occurs along subduction zones – where the edge of one plate slides under another – all over the world. The phenomenon was first discovered at roughly the same time in 1999 by two independent groups of researchers in Japan and Canada after they had installed better and denser sensor networks in their respective regions. Since then, these “slow earthquakes” or “slow slip events” have fundamentally changed geophysicists’ understanding of the forces deep inside the Earth’s crust. The Earth’s interior remains largely a mystery, but investigating slow earthquakes inches geophysicists closer to their Holy Grail: an accurate earthquake forecast. These quiet temblors may help to develop a picture of how some of the largest earthquakes form.

Tiny tremors

Tectonic plates move across the Earth’s surface, shifting by an average of two to five centimetres a year, with radioactive decay in the mantle producing the heat that drives this motion. This might compress the plate, which stores increasing elastic potential energy.

Before 1999 researchers thought that a compressed plate could release that stored energy in only two ways. The plate could decompress either rapidly in seconds, as in an earthquake, or steadily, through a constant creep. But the discovery of slow slip revealed that this wasn’t the whole picture. “That was the first time we realized that faults have this whole spectrum of behaviour,” says Laura Wallace, a geophysicist at GNS Science in New Zealand, who studies slow slip events off the country’s coast.

Slow slips transfer stress from tectonic plates not in seconds but over the course of days, weeks or even years. And their tenacity pays off. Some of the slow events that have been detected end up releasing the same amount of energy as regular magnitude-7 earthquakes. What’s more, in many regions, the slipping is episodic: for example, it occurs every 14 months in Cascadia and every four years in Mexico.

The simplest model of a slow earthquake consists of two plates, one tucked beneath the other at a subduction zone (figure 1). The shallowest part of the fault is locked and stationary, building up potential energy except during earthquakes. At depths of around 35–55 km, slow slip occurs sporadically. At further depths, geophysicists think that the rock slips steadily throughout the year.

Friction, which depends on the composition of the slipping rock, dictates the motion. But measuring the rock’s frictional properties is hard because it’s usually too deep to directly sample or image. Instead, researchers infer these properties: they calculate the rock’s elasticity, for example, based on the speed that seismic waves zip through the crust, which they measure using a network of seismometers. But these elasticity calculations are simplistic: the rock is anything but uniform, containing pockets of water that researchers have yet to characterize in detail.

Researchers are still not entirely sure what drives the motion of slow slip events, which respond to much weaker forces than regular earthquakes. We know, for example, that the gravitational pull from the Sun and the Moon influence slip. Researchers have observed that during slow slip, the intensity of the accompanying tremor in Cascadia fluctuates at frequencies that correspond to the ocean tides. “It turns out those tiny stresses, which are too tiny to affect regular earthquakes, are pretty strongly related to slow slips,” Houston says.

One of the most useful tools for studying slow slip are the networks of GPS sensors that have been built over the last decade, which are sensitive enough to detect the Earth’s surface moving by mere millimetres. In Japan and Cascadia, GPS measurements reveal that slow slip migrates from one region to the next at about 10 km per day. In addition, by measuring surface displacement, researchers can then model the deeper slow slip that is responsible for it. The models vary between regions based on the geometry of the specific fault. For example, on Mexico’s south-western coast, where some of the largest slow slip events occur, 5 mm of surface displacement detected at specific GPS sites corresponds to about 3 cm of displacement at the slip interface. In Cascadia, though, researchers model 5 mm of surface displacement as 2 cm of slip.

Building these networks can be challenging. GPS sensors generally can only be installed on land but many subduction zones are underwater. For example, Japan’s 2011 To̅hoku earthquake, which created a tsunami that killed more than 15,000 people and led to the Fukushima nuclear accident, began 70 km off its coast. Consequently, researchers are exploring new ways to take measurements in the ocean. Wallace’s team, for example, has installed pressure gauges on the seabed off the coast of New Zealand. “If the sea floor moves up in a slow slip event, for example, you have less water above you,” she says. The gauges can measure the fluctuations in water pressure resulting from the sea floor moving up and down during slow slips.

Deep physics One of the 24 seabed pressure gauges installed in the Hikurangi subduction margin off the north-east shore of New Zealand. (Yoshihiro Ito / Kyoto University)

A deeper understanding

The most tantalizing mystery about slow slip events is why they often precede large “megathrust” earthquakes that occur in subduction zones. We know, for example, that a decade-long slow slip event occurred before the 2011 To̅hoku earthquake. Similarly, researchers later found slow slip took place weeks before the 8.1-magnitude Iquique earthquake in Chile in 2014.

The most tantalizing mystery about slow slip events is why they often precede large “megathrust” earthquakes in subduction zones

In one instance, a slow slip event actually triggered a large 7.3-magnitude earthquake that struck Mexico’s south-western coast in April 2014. In the post-quake analysis, Mathilde Radiguet, a geophysicist at Université Grenoble Alpes in France, and her colleagues spotted the slow slip while mapping the seismic activity in the region. Over two months, the slip migrated along the fault to the point where the earthquake began. “The slow slip made the stress increase,” Radiguet says. “When the stress reached a certain threshold, the rocks broke, and the rupture started.”

Radiguet’s team published its analysis two years after the quake, but it is intriguing to wonder if geophysicists could eventually use slow earthquakes to forecast the large ones. Maybe one day. “We are far from using slow slip as forecasting,” Radiguet says, adding that it is impossible to generalize the effects of slow slip between regions. Slow slip happened to trigger the Mexico earthquake because it migrated to a specific part of the fault, but had it migrated in another direction, it could have reduced tension on the fault or even prevented the earthquake. “They’re a double-edged sword,” Wallace says. “Slow-slip events help reduce the likelihood of earthquakes in some places.”

Why is it slow? How exactly does it deform the earth? Many basic scientific questions about slow slip linger, and Radiguet says that one straightforward task ahead is to install more sensors, particularly in the oceans. The resulting research will be timely, given that seismologists expect at least one earthquake above magnitude 8 to take place somewhere in the world each year. “We do not understand how an earthquake initiates,” she says. “Slow slip probably plays a role.”

A massive earthquake is overdue at the Cascadia subduction zone. The average time between large earthquakes in the region is about 240 years, and the last earthquake in the region, which caused a tsunami on the other side of the Pacific in Japan, occurred 317 years ago. Meanwhile, around April of next year, imperceptible to human senses, the ground below will begin to slowly slip once again.