On February 16, 1861, a magnitude-8 earthquake struck Sumatra, an Indonesian island in the Indian Ocean. The earthquake shook the western side of the island, triggering a tsunami. That massive wall of rushing water poured onto shore, destroying towns and claiming thousands of lives.
Recently, though, a team of scientists discovered that another earthquake preceded this deadly event. That quake started in 1829 — and didn’t stop for 32 years!
Yet no one felt a single shake.
This kind of slow-moving earthquake is called a “slow-slip event.” They’ve also been called “silent earthquakes” because not even instruments such as seismometers (Size-MAH-meh-turs) can detect them. Scientists have only begun discovering them because of advances some two decades ago in GPS (global positioning system) technology.
Studying earthquakes helps scientists figure out what’s going on under Earth’s surface. Every earthquake, small or big, can teach scientists something about how much the ground will shake in the future. Unfortunately, no one can predict when an earthquake will hit. Scientists just have to prepare to study any earthquake that strikes.
But slow-slip events like the one in Sumatra change the game for scientists. These silent quakes happen frequently the world over. Sometimes, a slow-slip event happens just before a regular earthquake. This means the two types of Earth movements might be related. By investigating slow-slip events, scientists hope to better understand regular quakes — and possibly how to predict them.
Silent quakes
Earth’s surface is made of a collection of tectonic plates: huge masses of land, like the ground under your feet. These plates lie on top of a deep layer of gooey, hot rock that allows them to slide around. Sometimes the plates run into each other. Other times, they slide away from each other. Sometimes they just slide past each other.
As these plates play a slow-moving game of bumper cars, they sometimes get stuck together. The rocks then push and push against each other, creating stress. When the rocks suddenly unstick, or snap, they release all that stress as an earthquake. This is similar to what happens when you bend a stick. As you start bending the stick, stress builds in the center. Once there’s too much stress, the stick snaps.
Explainer: Understanding plate tectonics
When this happens underground, that stress release sends waves of energy, called seismic waves, through the ground. On land, we can see and feel that as the Earth shaking. Seismometers can record those waves — even halfway across the planet.
These quakes are probably the type you think of when you hear the word “earthquake.” Framed pictures falling off walls, vases shattering, the ground rumbling. And while these quakes can be scary and even deadly, they typically last less than a minute. Slow-slip events are quite different. They can last days, weeks or longer. As researchers are now figuring out, sometimes these silent earthquakes can last decades.
“We love slow-slip events because they give you all of the excitement of earthquakes, just in slow motion,” says Rishav Mallick. He studies geodesy (Jee-AH-deh-see), or the precise three-dimensional shape of land at any given point on Earth. He works at the Nanyang Technological University in Singapore.
In a slow-slip event, the rocks still slide past each other but very slowly, explains Laura Wallace. She’s a geodetic scientist who splits her time between two scientific institutions. One is at GNS Science, Te Pū Ao, in New Zealand. It’s in a city called Lower Hutt. Her other institution is the University of Texas at Austin. During a slow-slip event, rocks move so slowly that the “energy gets dissipated very, very slowly,” she says. “You don’t feel the shaking.”
Rocks can slip just as far as they would in a regular earthquake. It just takes them far longer. If the rocks slipped the same distance in seconds, such a quake might register as magnitude 7, Wallace says. That would result in some pretty strong shaking. It’s strong enough to damage buildings or even kill unlucky people.
Scientists detect many slow-slip events along structures called subduction zones. In a such a zone, an oceanic plate dives beneath a continental plate. Along the U.S. Pacific Northwest coast, the oceanic Juan de Fuca plate dives under the North American plate. This has formed a subduction zone. This tectonic-plate boundary is called the Cascadia Subduction Zone (CSZ). It runs from Vancouver, Canada to northern California.
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