How deep are earthquakes?
Published:
Two earthquakes can share the same magnitude and still end completely differently: one flattens a neighbourhood, the other is felt only as a gentle swing of a lamp a thousand kilometres away. The difference is often depth.
What depth actually means
Depth is the distance down to the hypocentre, the point underground where the rupture starts. The point directly above it on the surface is the epicentre, and that is what the map shows as a dot. The energy has to travel from the hypocentre up to you, and every kilometre on the way spreads and weakens the shaking.
Three depth classes
Seismologists sort quakes into shallow (0–70 km), intermediate (70–300 km) and deep (300–700 km). The distribution is heavily lopsided: of the more than 1.5 million quakes in the EarthquakeIndex archive, 89% are shallower than 70 km, just under 10% are intermediate, and only about 1% lie deeper than 300 km. That is why the map's depth filter splits the top class again at 10 km. That is where the differences are felt most clearly.
Why shallow quakes hit hardest
A shallow quake delivers its energy into a small area: the shaking is violent, but local. A deep quake sends its energy through hundreds of kilometres of rock before it arrives: spread across an enormous area and weakened by the time it gets there. The archive shows it plainly: the Christchurch quake of February 2011 was "only" M6.3, but it sat 6 km down and cost 185 lives. The strongest deep earthquake ever recorded, M8.3 in the Sea of Okhotsk in 2013, was 598 km down, was felt as far away as Moscow, and killed no one. And when a shallow quake sits under the seafloor, it can move the water above it: that is how tsunamis are born. So magnitude tells only half the story: see also what magnitude actually measures.
Why do quakes stop at 700 km?
Deep quakes exist only where one tectonic plate dives beneath another. The plate carries its cold, brittle core down with it, and as long as it stays brittle it can snap. Further down the rock becomes so hot that it yields by flowing slowly instead of breaking, and below roughly 700 km earthquakes stop altogether. Our deepest large quake is the M7.8 near the Bonin Islands in 2015, 691 km down: it was felt in all 47 of Japan's prefectures, yet did almost no damage.
Why does it so often say "10 km"?
Depth is the hardest of an earthquake's basic numbers to pin down, especially when the nearest stations are far away. When the data is thin, seismologists fix a default depth, typically 10 km, until better calculations arrive. Roughly one in eight rows in our archive reads exactly 10 km for precisely that reason. Depth is often revised in the hours after a quake, and the detail view shows when EMSC last updated the figure.
Try it on the map
The depth filter on the live map lets you show only the shallow quakes, or only the deep ones. Try it off the west coast of South America or around Tonga: pick 70+ km and the deep quakes line up well inland, while the shallow ones hug the coast and the trench. What you are seeing is the descending plate, drawn out in dots.