Shallow Earthquakes Explained

A shallow earthquake is one whose focus lies less than 70 km below the surface. It is by far the most common category, accounting for roughly three quarters of all recorded events — and it accounts for nearly all of the damaging ones.

Why depth this small matters so much

Seismic energy spreads outwards from the focus in three dimensions, and its intensity falls off rapidly with distance travelled. That geometry does most of the work.

Someone standing directly above a rupture at 10 km depth is 10 km from the source. If the same rupture happened at 200 km, they would be twenty times further away, and the energy reaching them would be a small fraction of the shallow case.

The practical consequence is that magnitude alone is a poor predictor of damage. A magnitude 5.5 at 8 km can wreck buildings in a town above it, while a magnitude 6.5 at 250 km may be felt across a thousand kilometres and break nothing.

Shallow events also concentrate their energy over a smaller surface footprint. The area of severe shaking is tighter than for a deep event, but within that area the intensity is far higher.

Where shallow earthquakes happen

Almost everywhere earthquakes happen at all, but some settings produce them exclusively.

Transform boundaries such as the San Andreas Fault and the North Anatolian Fault are essentially vertical faults confined to the brittle crust, so every earthquake they produce is shallow — typically 5–20 km. This is why these faults are so dangerous despite not producing the very largest magnitudes.

Mid-ocean ridges produce continuous shallow seismicity, though the events are usually small because the young, hot crust is thin and cannot store much strain.

Continental collision zones such as the Himalaya generate large shallow earthquakes across broad regions rather than along a single narrow line.

Subduction zones produce shallow events on the plate interface itself — and these are the ones that reach magnitude 9 and generate ocean-crossing tsunamis, because only a shallow rupture can deform the sea floor enough to displace the water column.

Shallow earthquakes and tsunamis

Shallow depth is a precondition for tsunami generation. Warning criteria generally require a submarine earthquake less than about 70 km deep, because deformation from a deeper rupture is spread too thinly by the time it reaches the sea floor to lift a meaningful volume of water.

Depth alone is not sufficient — the fault motion also has to be vertical, which is why shallow thrust earthquakes generate tsunamis while shallow strike-slip earthquakes usually do not.

The damage record

Look at the deadliest earthquakes in history and the pattern is unmistakable. The 2010 Haiti earthquake was magnitude 7.0 at roughly 13 km depth, close to Port-au-Prince, and killed well over 100,000 people. The 2023 Turkey–Syria sequence ruptured at similarly shallow depth beneath populated areas.

Compare a deep event of comparable or greater magnitude: the 2013 Sea of Okhotsk earthquake was magnitude 8.3 — far more energetic — at 609 km depth, and caused essentially no damage anywhere.

Depth is not a footnote to the magnitude. For anyone assessing what an earthquake actually did, it is the second number you should read, and often the more important one.