Slow earthquake

A slow earthquake, also known as a silent earthquake, is a special kind of earthquake that behaves differently from the ones we usually think about. Unlike normal earthquakes that happen in just seconds or minutes, a slow earthquake releases its energy over a much longer time — sometimes taking hours or even months! This slow release makes it quieter and harder to notice right away.

Scientists first found out about slow earthquakes by using long-term measurements of how the ground changes shape. Today, we know that most slow earthquakes are linked to the movement of fluids deep underground, and they often come with tremors — small shaking motions that can be picked up by special sensors called seismometers.

Even though they are called "silent," slow earthquakes are not completely quiet. They do make sounds, just at frequencies that are harder to detect without the right tools. It's important to understand these slow earthquakes because, even though they are gentle, they can still affect the land and give scientists clues about what is happening deep inside the Earth. They are different from tsunami earthquakes, where the earthquake moves more slowly but still releases its energy quickly, sometimes causing big ocean waves.

Causes

Earthquakes happen when stress builds up in rocks until they break, sending waves through the Earth. Slow earthquakes are different because they release energy over hours or months instead of seconds. Scientists think these slow earthquakes happen when small cracks form in the Earth's crust. These cracks grow slowly because of the way rocks are stressed and how they slide past each other. This process is influenced by tiny bumps along the cracks, called asperities, which are found in places where one tectonic plate slides under another, like in parts of Japan, Cascadia, and Chile, as well as along other types of faults such as the San Andreas fault.

Locations

Faulting happens all over Earth, including on convergent, divergent, and transform faults, usually near where tectonic plates meet. Recent studies of slow earthquakes have looked at places like Cascadia, California, Japan, New Zealand, Mexico, and Alaska. By studying where these slow earthquakes happen, scientists can learn more about how regular earthquakes behave and predict future ones in those areas.

Types

Scientists have found different kinds of slow earthquakes. These include low frequency earthquakes, very low frequency earthquakes, slow slip events, and episodic tremor and slip. Each type releases energy slowly over time, unlike regular earthquakes that happen quickly.

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 caused by the movement of tectonic plates, and scientists study them to better understand possible bigger earthquakes in the future.

LFEs were first identified in 1999 by the Japan Meteorological Agency. They were discovered in places where tectonic plates are moving beneath others, called subduction zones. These events are different from regular earthquakes because they don’t have the same strong, quick seismic signals. Scientists detect LFEs using special methods that can find very small signals in the seismic data. In places like Japan and Cascadia, LFEs are often linked to slow movements of the Earth’s plates and can help scientists learn more about the deep structure of these areas.

Very low frequency earthquakes

Very low frequency earthquakes (VLFs) are a special type of earthquake that last about 20 seconds and have magnitudes around 3 to 3.5. They are rich in low-frequency energy, with vibrations between 0.03 and 0.02 Hz. These earthquakes often happen together with another type called low frequency earthquakes (LFEs), but not the other way around.

VLFs are mainly found in two places: within the offshore accretionary prism and at the plate interface deeper down. These locations give them the names shallow VLFs and deep VLFs. They have been observed 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 earthquake events. They are caused by reverse fault mechanisms, similar to LFEs.

Slow slip events

Slow slip events (SSEs) are long-lasting movements along the boundaries of tectonic plates, linked to slow earthquakes. These events can last for weeks and sometimes even years. In some places, like the Cascadia region, they happen about every 14.5 months, while in other areas, such as Shikoku in Japan, they occur roughly every 6 months. These movements can trigger tremors, which scientists can detect using special instruments.

During these events, the movement travels along the plate boundaries at speeds of about 5 to 10 kilometers per day. This travel pattern helps explain why tremors and other seismic activities happen in a specific order in certain regions.

Episodic tremor and slip

Main article: Episodic tremor and slip

Slow earthquakes can happen in patterns and are somewhat predictable. These patterns are called episodic tremor and slip, or ETS. Unlike normal earthquakes that happen in seconds, ETS events can last for weeks. Some of these slow earthquakes have been linked to bigger, damaging earthquakes in areas closer to the surface of the Earth.

In Wellington, New Zealand, a slow earthquake happens every five years. It lasts about a year and was first measured in 2003. It has happened again in 2008 and 2013, releasing energy similar to a large earthquake.