Deep-focus earthquake

A deep-focus earthquake in seismology (also called a plutonic earthquake) is an earthquake that happens deep inside the Earth. Unlike most earthquakes that occur near the surface, these earthquakes start more than 300 kilometers below the ground. They are quite rare and usually happen in places where tectonic plates come together, called convergent boundaries.

These deep earthquakes are linked to the movement of old ocean floor, known as lithosphere, which dives beneath another plate in a process called subduction. They happen along a special slanting zone far below the surface, named the Wadati–Benioff zone. Studying these earthquakes helps scientists understand how the Earth’s layers move and change over time.

Discovery

Scientists first learned about deep-focus earthquakes in 1922 from Herbert Hall Turner. In 1928, Kiyoo Wadati showed that earthquakes can happen very deep inside the Earth, far below the surface. This proved that earthquakes are not just shallow events.

Seismic characteristics

Deep-focus earthquakes make almost no surface waves. They happen far below the Earth's surface, so they do not shake the ground very much. The seismic waves from these earthquakes go through the upper mantle and crust only one time. This makes the waves less mixed up. The body waves recorded by seismometers have very sharp peaks.

Focal mechanisms

The way an earthquake releases energy can be shown using special diagrams called beachball diagrams. These come from something called the moment tensor solution.

Deep-focus earthquakes happen far below the Earth's surface. They can release energy in different ways, depending on where they occur in the moving plates of the Earth.

At very deep levels, more than 400 km down, the earthquakes usually show a squeezing motion. At depths around 250 to 300 km, the motion is less clear but tends more toward stretching.

Physical process

Shallow-focus earthquakes happen when rocks break after storing up energy. Deep-focus earthquakes happen more than 300 km underground. Scientists are still learning how they occur.

Some ideas include changes in the minerals, water making rocks weaker, or heat building up until the rock breaks. These ideas help explain how rocks deep below the surface can move and cause earthquakes.

Deep-focus earthquake zones

The border of the Pacific plate and the Okhotsk and Philippine Sea plates is one of the most active deep-focus earthquake regions in the world. This area has had many large earthquakes, like the 2013 Okhotsk Sea earthquake. These earthquakes happen because of pressure on the Pacific plate as it moves deeper into the Earth.

Other active regions include the borders of the Philippine Sea plate and Sunda plate, where the deepest earthquakes can happen up to 675 kilometres below the surface. The Australian plate moving under the Sunda plate creates earthquakes in Indonesia at similar depths. The Pacific plate moving under the Australian, Tonga, and Kermadec plates causes very deep earthquakes in Papua New Guinea, Fiji, and New Zealand. In the Andes, the Nazca plate moving under the South American plate leads to deep earthquakes in several countries.