Slow earthquake

A slow earthquake, also known as a silent earthquake, is a special kind of earthquake. Unlike regular earthquakes that happen in just a few seconds or minutes, a slow earthquake can take hours or even months. This makes it feel very quiet, even though it can still move the ground.

Scientists first found slow earthquakes by watching how the Earth's surface changes over long periods. Today, we know that these events often come with small vibrations called tremors. These tremors can be detected by special instruments called seismometers, which help scientists study where and how these slow earthquakes happen.

It's important to know that slow earthquakes are not the same as tsunami earthquakes. In a tsunami earthquake, the ground moves more slowly along the fault, but the energy is still released quickly, like in other earthquakes. Slow earthquakes are different because they spread their energy out over a much longer time. These quiet quakes help scientists learn more about how the Earth moves and changes.

Causes

Earthquakes happen when stress builds up in rocks until they break. Slow earthquakes are different. They happen over hours or months, not seconds. Scientists believe this is because of special cracks in rocks and movements in the Earth's crust. These cracks and movements depend on how strong the rocks are and how much stress they can handle. Slow earthquakes are often found in places where one part of the Earth’s crust slips under another, like in subduction zones. They can also happen along other types of faults, such as the San Andreas fault.

Locations

Slow earthquakes can happen in many places around the world where the Earth's plates move. These places include areas like Cascadia, California, Japan, New Zealand, Mexico, and Alaska. Learning where slow earthquakes occur helps scientists understand more about regular earthquakes and predict future ones better.

Types

Scientists have found different kinds of slow earthquakes. These happen more slowly than regular earthquakes. Types include low frequency earthquakes (LFE), very low frequency earthquakes (VLF), slow slip events (SSE), and episodic tremor and slip. Each type releases energy over hours or months, making them quieter than regular earthquakes.

Low frequency earthquakes

Low frequency earthquakes (LFEs) are special seismic events. They have longer waves than regular earthquakes and often happen during slow earthquakes. These earthquakes can be linked to places where tectonic plates meet, especially in areas where one plate slides under another.

Scientists first identified LFEs in 1999 in Japan. They noticed these events happened away from volcanoes and were different from usual earthquake tremors. LFEs are now known to often accompany slow slip events, which are movements of tectonic plates that happen slowly over time. Researchers study LFEs to understand if they might signal larger earthquakes in the future. These events help scientists map deep parts of the Earth where plates meet.

Very low frequency earthquakes

Very low frequency earthquakes (VLFs) are a special kind of earthquake. They last about 20 seconds and have a size of about 3 to 3.5. VLFs are rich in low frequency energy and often happen with another type called low frequency earthquakes (LFEs).

VLFs are mainly found in two places: within the offshore accretionary prism and deeper at the plate interface. These earthquakes have been seen in places like the Cascadia subduction zone in western North America, and in Japan at the Nankai trough and Ryukyu trench. Like LFEs, VLFs move along the Earth's surface during certain events. They are caused by reverse fault mechanisms, similar to LFEs.

Slow slip events

Slow slip events (SSEs) are long movements that happen where tectonic plates slide under each other. These movements can last for weeks or even years, so they are called "slow." In places like Cascadia, these events happen about every 14.5 months. In Shikoku, Japan, they happen about every 6 months.

During these slow earthquakes, the movement travels along the edge of the subduction zone at a speed of about 5 to 10 kilometers per day. This movement is linked to tremors, which are small shakes that instruments can detect.

Episodic tremor and slip

Main article: Episodic tremor and slip

Slow earthquakes can happen in patterns called "episodic tremor and slip" or ETS. These are different from normal earthquakes because they last for weeks instead of just seconds.

For example, every five years, a slow earthquake happens under Wellington, the capital of New Zealand. The first one was measured in 2003, and they have happened again in 2008 and 2013. Each of these events lasts about a year.