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

Preliminary evidence for deep-focus earthquakes was first shared with scientists in 1922 by Herbert Hall Turner. Later, in 1928, Kiyoo Wadati showed that earthquakes can happen far below the Earth's surface, proving they are not just shallow events.

Seismic characteristics

Deep-focus earthquakes create very few surface waves. Because they happen far below the Earth's surface, the energy from these earthquakes is less likely to shake the surface strongly. The seismic waves from deep-focus earthquakes travel through the upper mantle and crust only once, which means they become less weakened and mixed up along the way. This results in very sharp peaks in the body waves that are recorded by seismometers.

Focal mechanisms

The way an earthquake releases energy can be shown using special diagrams called beachball diagrams, which come from something called the moment tensor solution. Deep-focus earthquakes, which happen far below the Earth's surface, can have different ways of releasing this energy 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 along their depth. At somewhat shallower depths around 250 to 300 km, the motion is less clear but tends more toward stretching along that same depth direction.

Physical process

Shallow-focus earthquakes happen when rocks suddenly break after storing up energy over time. But for deep-focus earthquakes, which happen more than 300 km underground, scientists are still trying to figure out exactly how they occur.

Some ideas include changes in the minerals' structure, water being released from rocks which makes them weaker, or heat building up in a small area until the rock can’t hold anymore and breaks. Each of these ideas tries to explain how rocks far below the surface can still move and cause earthquakes, even though they usually shouldn’t break the same way as rocks closer to the surface.

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 seen many large earthquakes, such as the M_w  8.3 2013 Okhotsk Sea earthquake. These earthquakes happen because of stresses on the subducted 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 occur up to 675 kilometres below the surface. The Australian plate subducting under the Sunda plate creates earthquakes in Indonesia at similar depths. The subduction of the Pacific plate under the Australian, Tonga, and Kermadec plates causes very deep earthquakes in Papua New Guinea, Fiji, and New Zealand. In the Andes, the subduction of the Nazca plate under the South American plate leads to deep earthquakes in several countries.