Earth’s deepest earthquakes
Normally, Earthquakes are a shallow phenomenon, confined to Earth’s upper ~20 kilometers. There are only a couple places where earthquakes take place any deeper than that; subduction zones.
Based on its physical properties, the outer portion of the Earth can be divided into two layers, the lithosphere and the asthenosphere. The lithosphere is the outermost layer of the planet, including the crust, and it is cold. So cold, in fact, that it is able to break and fracture.
The asthenosphere, deeper in the planet, is warm. Warm enough, in fact, that the rocks are able to move and flow without breaking. Apply a force to a rock this hot and, over geologic time, it will flow like a glacier or like silly putty.
For an earthquake to happen, the rocks have to be cold enough to break. If they can flow, they won’t fracture and there won’t be any earthquakes. The boundary between the lithosphere and the asthenosphere is the deepest depth that an earthquake can happen at, unless cold material is taken down deeper into the mantle.
That’s exactly what happens at subduction zones, where old, cold, oceanic plates are taken down into the mantle. Those rocks have been at the surface for hundreds of millions of years and they take time to warm up. They stay cold up to hundreds of kilometers deep into the Earth, and during this portion of their voyage they continue producing earthquakes. Scientists using seismometers can locate these earthquakes and actually track that they get deeper farther from the subduction zone. This layer of deep earthquakes within sinking plates is known as the Wadati-Benioff zone, after two geophysicists, and is a solid demonstration of how subduction zones work. These earthquakes let us see where the plates are as they sink.
Image credit: Marshak, Essentials of Geology, licensed to me for teaching purposes