Earthquake depth is the distance from the surface to the hypocenter, the point where the rupture begins. It is one of the most useful context variables in an earthquake catalog because it affects how far seismic waves must travel before reaching the surface. Depth does not determine impact by itself, but it helps explain why similar magnitudes may produce very different patterns of shaking.
Shallow earthquakes
Shallow events occur close to the surface and can produce strong local shaking because the waves travel a shorter path before reaching buildings and people. Many damaging continental earthquakes are shallow. A shallow depth, however, is not a guarantee of severe damage: magnitude, distance from populated areas, building quality and local geology remain decisive.
Intermediate and deep earthquakes
In subduction zones, earthquakes can occur hundreds of kilometers below the surface within a descending slab. Deep earthquakes may be felt over broad areas because their waves travel efficiently through the mantle, yet they often cause less intense surface shaking near the epicenter than a comparable shallow event. The deepest cataloged earthquakes approach about 700 km.
Depth uncertainty matters
Depth can be one of the less certain parameters in a rapid solution, especially where station coverage is sparse. Catalog values may be fixed to a conventional depth initially and later refined. A precise-looking number should therefore be read as an estimate produced by a seismic location model, not as a direct physical measurement made at the source.
Frequently asked questions
What counts as a shallow earthquake?
Definitions vary slightly, but events shallower than roughly 70 km are commonly described as shallow.
Are deep earthquakes harmless?
No. They can be widely felt and occasionally damaging, although their great depth often reduces the strongest surface motion.
Why are many deep earthquakes found near subduction zones?
A cold oceanic plate can remain brittle enough to generate earthquakes as it descends into the mantle.