Depth of focus (tectonics)

In seismology, the depth of focus or focal depth is the depth at which an earthquake occurs. Earthquakes happening less than 70 km below the surface are called shallow-focus earthquakes. Those between 70 km and 300 km deep are known as mid-focus or intermediate-depth earthquakes. In places where oceanic plates sink under other plates, called subduction zones, deep-focus earthquakes can happen much deeper, ranging from 300 km up to 700 km.

Scientists are still learning why deep-focus earthquakes happen. At such deep pressures and temperatures, the Earth's rock usually doesn't break in the same way as closer to the surface. One idea is that minerals like olivine change their shape in a process called a phase transition, which might cause these earthquakes. These deep earthquakes often happen where ocean plates meet continents at convergent boundaries, following lines known as Wadati–Benioff zones.

Discovery

The idea that earthquakes could happen deep underground was first shown in 1922 by H.H. Turner from Oxford, England. Before this, people thought all earthquakes happened close to the surface. In 1931, scientists studied earthquake waves and proved that some earthquakes occur far below the ground, which helped them understand more about how these deep earthquakes happen.

Fixed depth

Sometimes, when there isn't enough information to find out exactly how deep an earthquake happened, scientists use a "fixed depth." This means they choose a depth that is usually close to the real one. For example, many earthquakes under the ocean have a fixed depth because there aren't many seismic stations nearby to measure them. The United States Geological Survey now uses a fixed depth of 10 km (about 6 miles) for most shallow earthquakes, because this is often very close to the true depth. In the past, they used a fixed depth of 33 km (about 21 miles).