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Slow earthquakes rumble across the Pacific Northwest




Just 20 years ago, the discovery of “slow” earthquakes spread seismic hazards in the form of a storm. Today, scientists are still excited about these sneaky earthquakes.

Written by Meghomita Das, colleague of Palomar (@meghomita)

Citation: Das, M. , 2022, Slow Earthquakes Resound across the Pacific Northwest, Temblor,

The most notable earthquakes in history were sudden and violent episodes of shaking that struck without warning. These, and all earthquakes, accommodate the slow collision of Earth’s tectonic plates. They can destroy buildings and create tsunamis, leaving little time to take cover.

Off the coast of the Pacific Northwest lies the intersection of the North American and Juan de Fuca tectonic plates. The subduction zone separating the plates sinks deep into the US and Canadian coasts, and earthquakes are an irregular visitor to the region. In 2001, this area was hit by an earthquake measuring 6.8 on the Richter scale, which was felt by residents along the coast.

But in the depths of the Pacific Northwest, another special type of shock known as a “slow earthquake” strikes periodically. These events go unnoticed by all the sensitive ground motion detectors, and leave no devastation in their wake – like the silent whispers of an earthquake. These slow earthquakes play an important role in the earthquake cycle and revealing their secrets is key to understanding the risks of future earthquakes in the region.

Off the Pacific Northwest coast, the Cascadia Subduction is quietly rolling in. Credit: Luke Flint, via Unsplash

Quiet earthquakes detected

Slow earthquakes have been observed all over the world – from Japan to Mexico – at different depths and in different tectonic environments. They are considered slow because, although they release the same amount of energy as a regular earthquake, they are triggered by many small individual events that occur over the course of days to months or even years.

These earthquakes were first detected in Japan in 2001, when scientists noticed repeating noise-like signals that appear as a dense wobble in seismic data. Scientists called these signals “tremor.” Prior to this observation, tremors were often associated with volcanoes about to erupt. However, the signals in Japan were far from any volcano. They occurred in southwestern Japan, within the subduction zone between the Philippine and Pacific tectonic plates.

Meanwhile, researchers on the other side of the Pacific have noticed similar tremor signals at their seismic stations between Seattle, Washington and Victoria, British Columbia. These Cascadia tremors occurred every 13-16 months between 1997 and 2003 and were accompanied by slow, imperceptible ground motions or “slow slips”. Because the tremor and slip occurred at regular intervals and both tremor and the movement of the earth appeared, the researchers called this phenomenon episodic tremor and slow slip.

Understanding the slow earthquake

“[These quakes are] “It’s like eating crunchy peanut butter,” says Kylie Condit, a structural and metamorphic geologist at the University of Washington.

Regular earthquakes are like gnawing at a handful of peanuts. Pieces crack while you bite your teeth. Conversely, a smooth peanut butter dough would be equivalent to the smooth motion of tectonic plates sliding over each other. Slow earthquakes in the middle. Peanut chunks float in smooth peanut butter dough that slips in your mouth. The gentle movement of your teeth as you apply the paste is like a slip signal. When you bite your teeth into a piece of peanut, that crack you feel is equivalent to the signal of a tremor. The contrasting textures that are soft and crisp together create a hint of an occasional shake and slip.

One of the goals of the Cayley research group is to uncover the cause of slow earthquakes.

Before slow earthquakes were discovered, scientists believed in a binary concept of earthquake risk: earthquakes or no earthquakes. “There was nothing in between,” says Joanne Gomberg, a geophysicist with the USGS. “But realizing that slow earthquakes exist and also release energy like regular earthquakes is exciting and shows that there is still a lot to learn.” The discovery led to collaborative efforts in various fields to understand its importance and expand the field of seismology.

To find the causes of slow earthquakes, scientists needed to understand their behavior. Slow earthquakes do not shake or damage buildings. They are not dangerous. But their slowness also means that they can hide in plain sight. Detecting a device requires highly sensitive equipment – both on land and at sea. In certain parts of the world, such as Japan and Cascadia, they are cyclical, allowing scientists to predict and monitor their timing.

Earthquake risk in the Pacific Northwest. Credit: Temblor

The causes and ramifications of a slow slip event, such as whether it could cause or dissipate a larger, sudden earthquake, remain obscure. Within subduction zones, pressure builds up along one plate boundary as one plate is pushed under the other. During a regular earthquake, such as the 9.1-magnitude Tohoku earthquake, this stress is released when slip occurs on that boundary, or on a nearby fault.

Since slow earthquakes also release stress, they may make a larger earthquake less likely – since stress does not build up as much. On the other hand, the stress produced during a slow earthquake can be transmitted to a different part of the fault system and lead to a larger earthquake. But more research is needed for scientists to determine the role these events play in earthquake risk in these regions.

Seattle skyline. Credit: Dnak, via Wikimedia Commons (CC BY 2.0)

“Because slow earthquakes occur regularly and contain many small events, [we have] This is the opportunity to study the mechanisms behind them more closely and use this knowledge to better understand the mechanisms behind large earthquakes,” says Condit. As scientists seek to understand the mystery behind slow earthquakes, we can observe their presence as they quietly rush across the Pacific Northwest.

Megumita Das is Palomar’s classmate at Temblor. She is a Ph.D. Candidate at McGill University in Montreal, Canada, where she studies ancient earthquake signals and slow slip events ( Palomar Holdings sponsors a science writing fellow to cover important earthquake news across the United States




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