Depth of focus (tectonics)

What Is Depth of Focus?

Depth of focus is how deep an earthquake happens inside the Earth. Scientists measure this depth to learn more about our planet. Earthquakes can happen close to the surface or very deep down. This helps us understand how the Earth moves and changes.

Different Kinds of Earthquakes

Earthquakes that happen less than 70 km below us are called shallow-focus earthquakes. Some happen between 70 km and 300 km deep, and these are called mid-focus or intermediate-depth earthquakes. In special places called subduction zones, where ocean plates move under other plates, deep-focus earthquakes can happen even deeper, from 300 km up to 700 km.

Why Do Deep Earthquakes Happen?

Scientists are still learning why deep earthquakes happen. They think minerals like olivine might change shape in a process called a phase transition. This could cause these deep earthquakes. These earthquakes often happen at places where ocean plates meet continents, called convergent boundaries, along lines known as Wadati–Benioff zones.

Discovering Deep Earthquakes

A long time ago, people thought all earthquakes happened close to the surface. But in 1922, a scientist named H.H. Turner from England showed that earthquakes could happen deep underground. In 1931, scientists proved this by studying earthquake waves. This helped us learn more about how our Earth works. Sometimes, when scientists cannot tell exactly how deep an earthquake happened, they use a "fixed depth" to guess.