The third dimension · Cross-sections
Earthquake Depth Map & Cross-Sections
Every earthquake happens at a depth, but almost every earthquake map throws that number away. This one keeps it. Tint quakes by depth on the globe, or cut a cross-section into the Earth and look at hypocentres arranged in three dimensions — which is the only way some of the most important structures in geology become visible at all.
Hypocentre vs. epicentre
An earthquake begins at a point underground called the hypocentre (or focus). It has three coordinates: latitude, longitude and depth. The epicentre is just the point on the surface directly above it — a two-dimensional shadow of a three-dimensional fact.
Conventional earthquake maps plot epicentres. That is a reasonable simplification for a paper map, but it means two earthquakes that appear to be in the same place can have originated 15 km and 300 km down, in completely different tectonic settings, produced by completely different physics. On a globe you can rotate and slice, that distinction is immediately obvious.
The three depth classes
| Class | Depth range | Setting & behaviour |
|---|---|---|
| Shallow | 0 – 70 km | The large majority of all earthquakes. Includes essentially every continental fault-zone quake. For a given magnitude these are the most destructive, because the energy has the least rock to cross before reaching the surface. |
| Intermediate | 70 – 300 km | Almost exclusively inside descending slabs at subduction zones. Felt across a wider region than a shallow quake of the same size, but usually less violently at any one point. |
| Deep | 300 – ~700 km | Confined to subducting slabs. Rarely damaging, but detectable at great distance and scientifically valuable — they are our most direct probe of where a slab has gone. |
Why earthquakes stop at about 700 km
Earthquakes require brittle failure: rock has to store elastic stress and then break, suddenly. As a slab descends it heats up and the surrounding pressure climbs. Past a certain point, rock stops behaving brittly and begins to deform ductilely — flowing instead of fracturing. Stress is released continuously rather than in sudden slips, so no seismic waves are radiated.
That transition sets a hard floor on the earthquake catalog at roughly 700 km. Set the depth filter to look only at the deepest events and the map empties out almost everywhere, surviving in a handful of long ribbons — the Tonga–Kermadec arc, South America beneath the Andes, the Sea of Okhotsk, Indonesia. Those ribbons are slabs.
Try this: search a wide box over South America or Tonga with no magnitude floor, turn on depth tinting, then rotate until you are looking along the trench rather than across it. The scatter of dots resolves into a plane tilting down and away.
The Wadati–Benioff zone
That plane has a name. In the 1930s Kiyoo Wadati in Japan, and later Hugo Benioff in California, noticed that earthquake depths were not randomly distributed near ocean trenches — they increased systematically with distance inland. Plotted in cross-section the hypocentres fell on a dipping surface descending beneath the continent.
The Wadati–Benioff zone was one of the decisive pieces of evidence for plate tectonics. It is, quite literally, a map of one plate sliding beneath another, drawn by the earthquakes happening inside it. Dip angles vary — some slabs descend steeply, others at a shallow angle for hundreds of kilometres before steepening — and those differences drive where volcanoes appear and how wide the hazard zone is at the surface.
It is also close to invisible on a flat map, where it is just "more dots inland". This is the single clearest reason to look at earthquakes in three dimensions. See how it relates to plate boundaries →
Depth and shaking
Depth is a large part of why magnitude alone does not predict damage. A magnitude 6.5 at 10 km can be devastating directly above; the same magnitude at 250 km may be felt over a million square kilometres and break nothing. Seismic waves lose amplitude as they spread, so the distance from hypocentre to your feet matters as much as the energy released.
This is why USGS reports depth alongside magnitude, and why shaking-intensity products like ShakeMap model it explicitly rather than working from magnitude alone.
Common questions
How deep can earthquakes be?
The deepest recorded earthquakes reach roughly 700 kilometres below the surface. Below that depth earthquakes effectively cease, because the descending slab has warmed enough that rock deforms by flowing rather than by brittle fracture, so it no longer stores and releases stress the way a shallow fault does.
Are shallow earthquakes more dangerous than deep ones?
Generally yes, for the same magnitude. A shallow earthquake releases its energy close to the surface, so the shaking arrives with less attenuation and is concentrated over a smaller, more intense area. A deep earthquake of the same magnitude spreads weaker shaking over a much broader region.
What is the difference between a hypocentre and an epicentre?
The hypocentre, also called the focus, is the actual point underground where the rupture begins — it has a latitude, a longitude and a depth. The epicentre is simply the point on the Earth's surface directly above it. Standard earthquake maps plot epicentres, which is why depth usually disappears from view.
What is a Wadati–Benioff zone?
A Wadati–Benioff zone is the dipping plane of earthquakes that marks a subducting tectonic plate descending into the mantle. Plotted in three dimensions, the hypocentres line up on a surface that grows deeper with distance from the ocean trench, tracing the slab's path downward.