Why do earthquakes occur at different depths?
Earthquake depth marks where rupture begins in the crust or within a sinking slab and varies with tectonic setting.
Formation process and geographic mechanism
At divergent and transform boundaries earthquakes mostly occur in shallow, cold brittle crust. In subduction zones the oceanic slab forms an inclined earthquake belt extending hundreds of kilometers into the mantle. Ordinary brittle fracture is unlikely at great pressure, so mineral transformations, dehydration and sudden instabilities may contribute. Molten material does not directly produce the earthquake.
Spatial outcomes, measurement and management
A shallow event releases energy nearer the surface and may cause stronger local shaking than a similar deep event. Deep earthquakes can be felt over broad areas but do not rupture the surface. Damage also depends on magnitude, rupture direction, distance, ground and buildings. Location and depth are calculated from wave arrival times at multiple stations.