Earthquake Depth Explained

Every earthquake report carries a focal depth alongside the magnitude, and it is routinely ignored. It shouldn't be. Depth frequently determines whether an earthquake is a minor curiosity or a disaster.

The three depth categories

Seismologists divide earthquakes into three depth ranges, and the boundaries reflect genuine differences in physics rather than arbitrary convention.

Shallow: less than 70 km. About 75% of all earthquakes, and the overwhelming majority of damaging ones. These occur within the crust and the uppermost mantle. Because the rupture is close to the surface, the energy arrives concentrated over a relatively small area, producing intense shaking directly above the focus.

Intermediate: 70–300 km. These occur within subducting slabs of oceanic lithosphere descending into the mantle. Shaking near the epicentre is generally weaker than a shallow event of the same magnitude, but it is felt across a wider area because the waves radiate from deeper down.

Deep-focus: 300–700 km. Rare and confined almost entirely to subduction zones. They can be very large — magnitude 8 deep-focus events have been recorded — yet cause remarkably little surface damage. The 2013 magnitude 8.3 Sea of Okhotsk earthquake, 609 km deep, was felt across an enormous area of Asia but caused essentially no damage.

Why depth changes everything

Seismic energy spreads out as it travels, and its intensity falls roughly with the square of the distance travelled, plus additional losses to attenuation in the rock. That geometry is unforgiving.

Consider a magnitude 6.0. At 10 km depth, someone standing directly above the focus is 10 km from the source. At 300 km depth, that same person is 300 km from the source — thirty times further away. The energy reaching them is a tiny fraction of what the shallow event delivered.

This is why the pairing of magnitude and depth matters so much:

• A magnitude 5.5 at 8 km can damage buildings in a town directly above it.
• A magnitude 6.5 at 250 km may be felt over a thousand kilometres yet break nothing.

There is a second consequence. Because deep events radiate from far below, the shaking they produce is dominated by longer-period waves by the time it reaches the surface. Long-period motion is comparatively harmless to small stiff buildings but can be problematic for tall structures, which resonate at longer periods.

Why deep earthquakes are a puzzle

Deep-focus earthquakes should not exist. At 500 km depth, the confining pressure is enormous and the temperature is high. Under those conditions rock does not snap; it deforms plastically, flowing slowly like extremely stiff putty. Brittle fracture, which is what an earthquake is, should be impossible.

The resolution lies in subduction. When a slab of cold oceanic lithosphere descends into the mantle, it takes its low temperature with it and warms only slowly. The slab therefore remains brittle to far greater depths than the surrounding mantle. Deep earthquakes trace out these descending slabs, forming inclined planes of seismicity known as Wadati–Benioff zones — one of the key pieces of evidence that established plate tectonics.

Two mechanisms are thought to enable rupture at depth. Dehydration embrittlement occurs when hydrous minerals in the slab break down and release water, raising pore pressure and allowing failure. Transformational faulting occurs when metastable olivine abruptly converts to a denser phase, producing a sudden volume change that can nucleate a rupture.

Below roughly 700 km, seismicity stops almost entirely. Slabs have either warmed enough to deform ductilely or have stalled at the mantle transition zone.

Depth and tsunami risk

Depth is one of the primary filters warning centres apply when assessing tsunami potential. To generate a tsunami, an earthquake has to physically displace the sea floor vertically, lifting or dropping the entire water column above it.

A deep earthquake cannot do this. By the time deformation from a rupture at 200 km propagates to the sea floor it is smeared out over such a wide area that the vertical displacement is negligible. This is why tsunami criteria typically require a shallow submarine event, generally less than about 70 km deep, combined with sufficient magnitude.

How depth is determined

Depth is calculated from the arrival times of seismic waves at multiple stations, and it is the hardest of the four basic parameters to constrain accurately. Location in latitude and longitude is usually well determined; depth much less so, particularly when the nearest station is far from the epicentre.

Analysts improve depth estimates using depth phases — waves such as pP that travel upwards from the focus, reflect off the Earth's surface, then travel down and out to distant stations. The delay between the direct P arrival and the pP arrival is a direct function of focal depth.

You will sometimes see depths reported as exactly 10 km or exactly 33 km. These are conventional default values assigned when the data cannot constrain depth properly, not genuine measurements. A depth quoted to a decimal place, such as 42.7 km, generally reflects a real determination.