How deep are earthquakes? Shallow, intermediate and deep quakes explained
An earthquake's depth is the distance from the surface down to the point where the rupture started, called the hypocentre or focus. It ranges from a few hundred metres to about 700 km.
The three depth classes
Seismologists sort earthquakes into three groups by focal depth:
| Class | Depth | Where they happen |
|---|---|---|
| Shallow | 0–70 km | Everywhere earthquakes occur: plate boundaries, mid-ocean ridges, faults inside continents. About three quarters of all the seismic energy on Earth is released here. |
| Intermediate | 70–300 km | Almost only inside plates that are sinking into the mantle at subduction zones, plus a few unusual places such as the Hindu Kush and Vrancea in Romania. |
| Deep | 300–700 km | Only in the oldest, coldest sinking plates: under Tonga and Fiji, Japan and the Bonin Islands, Indonesia, and South America. |
In the 3D map the colour follows the same idea: yellow quakes are shallow, and the colour moves through orange and red to purple as the depth increases.
See it on the map → Spin to Tonga and look under the islands: the shallow yellow quakes near the trench turn purple as they go down.Why the depth matters for shaking
Seismic waves lose energy as they travel. A shallow M6 directly under a town can destroy buildings, while an M6 at 500 km is often barely felt, because the waves have travelled 500 km through rock before they reach anyone. Deep quakes also spread their weaker shaking over a much wider area.
This is why news reports always give the depth next to the magnitude: two quakes with the same magnitude can have very different effects.
Why are there no earthquakes below 700 km?
Rock normally breaks in earthquakes only while it is cold and brittle. Below a few tens of kilometres, rock is hot enough to flow slowly instead of snapping. Sinking plates are the exception: they are colder than the mantle around them and stay brittle much deeper. Below about 700 km even the coldest slabs are too warm and the minerals have changed into denser forms, so earthquakes stop.
How depth is measured
Seismic networks time the arrival of waves at many stations. The earlier the waves arrive at stations near the quake compared with those far away, the shallower the quake. For shallow quakes the depth is often uncertain by several kilometres, and some catalogues show a fixed default depth (often 10 km) when it could not be solved. Clusters of quakes at exactly 10 km on the map usually mean "shallow, depth not well known".