Slow earthquake

Most earthquakes shake the ground very fast.

But some earthquakes are different. They are called slow earthquakes. They release energy very slowly. They can last for many months.

These slow shakes happen deep underground. They often happen near places where plates move. Scientists use special tools to find them. These small shakes help us learn about the Earth.

They help us see where big shakes might happen next.

Most earthquakes shake the ground very quickly. They last for only seconds or minutes. But some events are different. We call them slow earthquakes. They release power over hours or even months.

These events happen deep underground. They often occur where plates meet. Some are called low frequency earthquakes, or LFEs. These are small shakes that happen over long periods. They are often part of slow slip events. These are times when plates slide slowly.

Scientists study these to learn about our world. LFEs can happen in places like Japan or Mexico. They also happen in Cascadia. This is a place near North America.

LFEs are hard to find. They are very quiet. Scientists use special tools to hear them. They use a method called correlation. This means they match new data to old patterns. This helps them find the tiny shakes. Studying them helps us guess where big earthquakes might happen next.

Most earthquakes happen very quickly. They shake the ground for only a few seconds or minutes. But there is another kind called a slow earthquake. These events release energy over many hours or even months.

They are much quieter than regular earthquakes. Some scientists call them silent earthquakes. However, they are not truly silent. They often involve tiny tremors that we can detect with special tools.

How do these slow events work? It all starts with stress building up in the rocks. Rocks can only take so much pressure before they break. When they break, they release energy as seismic waves. These waves move through the Earth like ripples on a pond.

In a slow earthquake, the movement happens because of tiny bumps called asperities. These bumps are on the faces of the cracks. If the stress is low but the rock is strong, the slip happens slowly. This creates a long, steady release of energy instead of one big snap.

Scientists have been studying these for a long time. The Japan Meteorological Agency first classified low frequency earthquakes in 1999. These are a specific type of slow earthquake. Before this, people thought these tremors only happened near volcanoes.

In 2002, researchers found continuous tremors near the Philippine Sea plate. By 2007, they realized these were actually swarms of low frequency earthquakes. Now we know they are a huge part of tectonic tremors. They happen in many places, not just near volcanoes.

These earthquakes happen in many parts of the world. You can find them in Japan, Mexico, New Zealand, and Alaska. They also happen in Cascadia, which is near the coast of North America.

In Cascadia, these events happen deep underground. They often occur between 25 and 47 kilometers deep. Scientists look at these locations to understand bigger earthquakes. Studying the tremors helps them estimate where future earthquakes might occur. It is a very important way to study the Earth's movement.

Low frequency earthquakes are hard to find because they are so subtle. They do not have the strong, sharp waves that regular earthquakes have.

Scientists use a method called correlation to find them. They compare new data to a template of known earthquake patterns. This helps them pull the signal out from the background noise. These small events are like a warning sign. Some math shows that a large earthquake is more likely during a slow slip event. Understanding these quiet shakes helps us stay prepared for the big ones.

A slow earthquake, often called a silent earthquake, is a unique geological event. Unlike typical earthquakes that release energy in seconds or minutes, these events last for hours or even months. They are discontinuous, meaning they occur in bursts rather than one single motion. While they are much quieter than standard earthquakes, they are not truly silent. Most slow earthquakes are accompanied by tremors and fluid flow. Scientists can locate these tremors by using seismometer data filtered to specific frequencies, typically between 1 and 5 Hz.

To understand how they work, we must look at how stress builds in the Earth. Earthquakes happen when stress increases in a region until it reaches a maximum limit. When the rocks can no longer withstand this stress, a rupture occurs. This rupture causes a drop in the shear stress of the system. This sudden change generates seismic waves that move through the Earth like ripples on water. Slow earthquakes are caused by processes called stick-slip and creep. These processes happen between brittle fracture and ductile fracture.

A key part of this mechanism involves asperities. Asperities are tiny bumps or protrusions on the faces of rock fractures. Mathematical models show that slow earthquakes occur under specific conditions. If the initial stress minus the sliding frictional stress is low, and the specific fracture energy is high, slow earthquakes happen regularly. This means the strength of the crustal material relative to the stress allows for a slow release. These events are often found in subduction zones, such as those in Chile, Cascadia, or SW Japan. They also appear on strike-slip plate boundaries like the San Andreas fault.

Scientists have identified several distinct types of slow earthquakes. Teruyuki Kato identifies types including low frequency earthquakes (LFE), very low frequency earthquakes (VLF), and deep-low-frequency earthquakes. Other types include slow slip events (SSE) and episodic tremor and slip (ETS). Low frequency earthquakes are particularly important. These are seismic events with waveforms that have much longer periods than ordinary earthquakes. Tectonic LFEs usually have low magnitudes, specifically less than magnitude 3. They often occur during long-lived slow slip events at subduction interfaces.

The history of these discoveries changed how we view the Earth. The Japan Meteorological Agency first classified LFEs in 1999. Before this, scientists thought these tremors only happened near volcanoes due to flowing magma. However, researchers found seismic signatures away from volcanic areas. In 2002, Japanese researchers detected low-frequency continuous tremor near the Philippine Sea plate. By 2007, they realized these were actually LFE swarms. We now know that LFEs are the largest part of tectonic tremor.

Detecting these events is a major technical challenge. LFEs lack the distinct, impulsive body waves found in regular earthquakes. Their P-wave arrivals are often so small that they are nearly impossible to detect with classical techniques. Instead, scientists often rely on S-wave arrivals. To find them, researchers use advanced seismic correlation methods. They compare seismic records to a "template" made of known LFE waveforms. By stacking similar waveforms, they can reduce background noise and find the subtle signal.

One of the most important areas of study is the Cascadia subduction zone. This zone spans from northern California to Vancouver Island. In Cascadia, LFEs occur at depths between 25 and 47 kilometers. This area is known as a transition or transient slip zone. It sits between the locked zone and the stable-slip zone. This region is also called a Low Velocity Zone because of its specific seismic properties. Some researchers believe high pore fluid pressures in this zone help trigger the earthquakes.

Studying slow earthquakes is vital for predicting larger hazards. While no megathrust earthquake has been accompanied by a slow slip event yet, they are still a concern. Slow slip events can actually increase stress in the seismogenic zone. This happens because they force the locked interval to accommodate down-dip movement. Some calculations suggest the probability of a large earthquake during an SSE is 30 to 100 times greater than normal. By mapping these tremors, seismologists can better estimate the risks of future large earthquakes.