Deep-focus earthquake

Most earthquakes happen near the surface. But some happen very deep.

These are called deep-focus earthquakes. They happen far under the ground. This can make the ground stay still. It does not shake as much at the top.

These shakes happen when parts of the earth move. One part of the earth slides under another. This can happen in many places. It happens near the Pacific Ocean. It also happens near the Andes mountains. These deep shakes are a big mystery.

Most earthquakes happen near the surface. But some happen very deep.

These are called deep-focus earthquakes. They happen more than 300 km below the ground. They usually occur where one tectonic plate slides under another. This area is called a subduction zone. These deep shakes create very few surface waves. This means the ground at the top may not shake much.

Scientists are still studying why they happen. At such great depths, rocks should be soft. They should bend instead of breaking. One idea is called dehydration embrittlement. This is when water is let out of rocks. This water makes the pressure change. This helps the rock slip.

Another idea is a phase transition. This is when a mineral changes its shape. It might become more dense, or packed tightly. This change could cause an implosion.

Many places have these deep shakes. The most active area is near Fiji and New Zealand. The deepest earthquake ever recorded was in Vanuatu. It was 693 km deep!

Caption: A map shows where many deep earthquakes happen.

Most earthquakes happen close to the surface where we live. However, some earthquakes happen much deeper in the Earth. These are called deep-focus earthquakes. They happen at depths greater than 300 km. They almost always occur at convergent boundaries. This is where one tectonic plate slides under another. Scientists call this process subduction. These deep quakes happen along a special dipping zone. This area is known as the Wadati–Benioff zone.

Deep-focus earthquakes work differently than shallow ones. They create very few surface waves. This happens because their depth keeps the energy from concentrating at the top. When the waves travel upward, they pass through the mantle and crust only once. This means the waves do not lose much strength. They also do not bounce around as much. Because of this, the waves arrive at recording stations with very sharp peaks. It is like a clear sound rather than a muffled one.

People have been studying these deep quakes for a long time. In 1922, Herbert Hall Turner shared early evidence about them. Later, in 1928, Kiyoo Wadati proved they really exist. He showed they happen well beneath the lithosphere. The lithosphere is the hard outer layer of our planet. His work showed that earthquakes do not only happen in shallow spots. This discovery changed how we understand the deep Earth.

There are many places where these deep shakes occur. The most active zone is between Papua New Guinea, Fiji, and New Zealand. In this area, earthquakes of 4.0 or higher happen almost every day. The strongest deep-focus quake ever recorded was the 8.3 magnitude Okhotsk Sea earthquake in 2013. The deepest one ever found was in Vanuatu in 2004. That small 4.2 quake was 693 km deep!

Scientists are still trying to solve the mystery of how they happen. At such great depths, the heat and pressure should make rocks soft. Instead of breaking, they should bend like plastic. One idea is called dehydration embrittlement. This is when water is released from minerals inside the rock. This extra water increases the pressure and allows the rock to slip. Another idea is transformational faulting. This happens when minerals change into a denser shape due to stress. Scientists are still learning which idea is correct.

A deep-focus earthquake, sometimes called a plutonic earthquake, is a seismic event with a hypocenter depth exceeding 300 km. These earthquakes are significant because they occur in the deep interior of our planet. Most earthquakes happen in the shallow crust, but these occur much further down. They happen almost exclusively at convergent boundaries. This is where one tectonic plate slides beneath another. These events occur along a dipping, tabular zone known as the Wadati–Benioff zone. This zone follows the subducted oceanic lithosphere as it sinks into the mantle.

The seismic characteristics of these events differ greatly from shallow earthquakes. Deep-focus earthquakes produce minimal surface waves. Because of their extreme depth, the energy is less likely to concentrate at the Earth's surface. When seismic waves travel from the focus to a recording station, they pass through the heterogeneous upper mantle and variable crust only once. This path results in less attenuation, which is the loss of energy, and less reverberation. Consequently, the body waves arrive with very sharp peaks, making them distinct to seismologists.

Scientists have worked for decades to understand the focal mechanisms of these quakes. A focal mechanism describes the pattern of energy radiation. This is often represented by a moment tensor solution, which can be shown using beachball diagrams. These mechanisms vary depending on the depth within the subducting plate. At depths greater than 400 km, down-dip compression is the dominant force. At depths between 250 and 300 km, the stress regime is more ambiguous. However, at that specific depth, it often appears closer to down-dip tension.

Determining how these earthquakes actually happen is a major challenge in seismology. In shallow areas, earthquakes result from brittle fracture and frictional slip. However, at depths greater than 300 km, the high pressure and temperature should prevent brittle behavior. Instead, the rock should undergo plastic deformation, meaning it flows like warm wax. Several theories attempt to explain this mystery. One proposal is solid-solid phase transitions. This suggests an implosion occurs when material changes to a higher-density, lower-volume phase. For example, metastable olivine might suddenly transition to a spinel structure at 410 km depth. This theory is largely discredited because it lacks a significant isotropic signature in seismic data.

Another theory is dehydration embrittlement. This process involves the dehydration of mineral phases that contain high water content. As these minerals release fluids in-situ, the pore pressure within the rock increases. This rise in pressure reduces the effective normal stress in the slab. This allows slip to occur on pre-existing fault planes at much greater depths than usual. However, many scientists believe this mechanism is less significant beyond 350 km. This is because most dehydration reactions likely reach completion between 150 and 300 km. A third idea is transformational faulting, or anticrack faulting. This occurs when a phase transition happens in response to shear stress in a fine-grained zone. Rapid shearing along these planes of weakness could then trigger an earthquake.

Finally, some researchers study shear instability, also known as thermal runaway. This occurs when heat produced by plastic deformation is generated faster than it can be conducted away. This creates a positive feedback loop of heating and material weakening. This process is called strain localization. While mathematical models use this to simulate deep earthquakes, it has not been documented in nature or the laboratory. The exact process of deep-focus earthquakes remains an outstanding problem in the field of deep-earth seismology.

Deep-focus earthquakes occur in several major zones around the world. The most active is the region from Papua New Guinea to New Zealand. Here, the Pacific plate subducts under the Australian, Tonga, and Kermadec plates. This area is so active that earthquakes of magnitude 4.0 or higher occur almost daily. The deepest earthquake ever recorded was a small 4.2 magnitude event in Vanuatu in 2004, reaching a depth of 693 km. The strongest deep-focus earthquake on record was the 8.3 magnitude Okhotsk Sea earthquake in 2013. Other notable regions include the Andes, where the Nazca plate subducts under the South American plate. This has caused massive events, such as the 8.2 magnitude earthquake in Bolivia in 1994 at a depth of 631 km.