Intermediate-Depth Earthquakes Explained

An intermediate-depth earthquake has a focal depth between 70 and 300 km. Unlike shallow events, which occur in many tectonic settings, these are found almost exclusively in one place: inside slabs of oceanic lithosphere descending into the mantle.

Why they only occur at subduction zones

At 150 km depth, the surrounding mantle is hot enough that rock deforms plastically — it flows slowly rather than snapping. Brittle fracture, which is what an earthquake is, should not be possible.

The exception is subduction. When a slab of cold oceanic lithosphere descends, it carries its low temperature with it and warms only gradually, staying brittle far deeper than the mantle around it. Earthquakes trace this descending slab, forming an inclined plane of seismicity known as a Wadati–Benioff zone.

Mapping these zones was one of the observations that established plate tectonics. Plot earthquake depths across a subduction zone and they get systematically deeper as you move inland from the trench, sketching the geometry of the slab.

What causes rupture at these depths

Simple frictional sliding cannot explain failure under the confining pressure at 100–300 km. Two mechanisms are thought to be responsible.

Dehydration embrittlement is the leading explanation for this depth range. Oceanic crust is hydrated during its long life beneath the ocean, with water chemically bound into its minerals. As the slab heats, those minerals break down and release water, raising pore pressure and allowing the rock to fail even under enormous confining stress.

That same released water rises into the overlying mantle wedge, lowers its melting point and generates the magma that feeds volcanic arcs — so intermediate-depth seismicity and arc volcanism share a common cause.

How they feel at the surface

Intermediate-depth earthquakes behave noticeably differently from shallow ones.

Weaker near the epicentre. The rupture is 70–300 km away from anyone at the surface, so peak shaking directly above is much lower than a shallow event of the same magnitude.

Felt over much wider areas. Because energy radiates from deep down, the shaking footprint is broader and decays more gradually. An intermediate-depth event can be felt across several countries while causing little damage anywhere.

Longer-period shaking. By the time energy reaches the surface, higher frequencies have been preferentially absorbed. The remaining longer-period motion is comparatively harmless to small stiff buildings but can affect tall structures, which resonate at longer periods.

Rarely tsunamigenic. Deformation from a rupture at this depth is spread far too widely to lift the sea floor meaningfully, so intermediate-depth events almost never generate tsunamis regardless of magnitude.

Where to find them

Anywhere a slab is subducting: beneath Japan, the Kuril Islands and Kamchatka; along the western coast of South America, where the Nazca plate descends beneath Chile, Peru and Bolivia; through Indonesia, the Philippines and Tonga–Kermadec; and beneath the Aleutians and Alaska.

The depth distribution of a region's earthquakes is a useful diagnostic. A country recording a large share of intermediate-depth events almost certainly has a subducting slab beneath it. One recording almost entirely shallow events is dominated by crustal faulting instead.