Shallow vs Deep Earthquakes in a Depth Explorer

Shallow earthquakes start less than 70 km below the surface, and deep earthquakes start between 300 and 700 km down, with an intermediate class from 70 to 300 km in between, following the depth zones used by the USGS. Shallow quakes do the most harm because the rupture lies close to the ground people live on, and shaking fades with distance from its source.

2010 Maule, Chile (M8.8, 22 km), depth from Live Atlas22 km, Shallow (focus less than 70 km deep).Straight above the epicenter the focus is 22 km away; a town 100 km from the epicenter is about 102 km from the focus.Straight above the epicenter, this focus is 1.1 times as far as a focus 20 km deep.Live Atlas calls shallow earthquakes the most dangerous. Shaking weakens with distance from the source, and here the source is close to the surface. Source: USGS, Live Atlas.
Cross-section with the focus at 22 km22 km, Shallow (focus less than 70 km deep). Straight above the epicenter the focus is 22 km away; a town 100 km from the epicenter is about 102 km from the focus.ShallowIntermediateDeep070100200300400500600700kmContinental faults such as the SanAndreas, active to about 20 kmContact between converging plates,active to about 60 kmDeepest earthquakes, about 700 kmEpicenterTown, 100 km away22 km102 km
Schematic, depth to scale, horizontal distance not to scale

Depth classes from the USGS (0 to 70, 70 to 300, 300 to 700 km). Reference depths from the USGS, Maule depth from Live Atlas. Distances use straight-line geometry, √(depth² + 100²).

Depth classes and reference depths as a table
DepthWhat it marksSource
0 to 70 kmShallow, focus less than 70 km deepUSGS, Live Atlas
70 to 300 kmIntermediate, 70 to 300 kmUSGS
300 to 700 kmDeep, 300 to 700 kmUSGS
20 kmContinental faults such as the San Andreas, active to about 20 kmUSGS
60 kmContact between converging plates, active to about 60 kmUSGS
700 kmDeepest earthquakes, about 700 kmUSGS

Drag the slider above from the surface down to 700 km. The explorer names the depth class for the chosen focus, draws it under an epicenter, and works out how far that focus lies from a point straight above it and from a town 100 km away along the surface. Dashed lines mark the depth limit of continental faults, the base of the contact between converging plates and the floor of the deepest quakes. A preset drops the focus to 22 km, the depth Live Atlas gives for the 2010 Maule earthquake in Chile.

Why Do Shallow Earthquakes Shake Harder?

Closeness to the source does most of the work. The USGS explains that shaking weakens as the distance from the source grows, so an earthquake 500 km down produces considerably weaker shaking at the surface than the same event would at 20 km. Directly above the focus, 500 km is 25 times farther than 20 km. For a town 100 km from the epicenter, the explorer gives about 102 km of travel from a 20 km focus and about 510 km from a 500 km one.

The ruptures behind the largest subduction earthquakes are shallow as well. On land, faults such as the San Andreas break only within roughly the top 20 km of crust, and the interface where one plate slides beneath another, the source of giants like the 2004 Sumatra and 2011 Japan earthquakes, stays active to about 60 km.

What Happens Below 70 km?

Every earthquake deeper than 70 km occurs inside a slab of lithosphere sinking into the mantle, according to the USGS. These slabs stay colder than the rock around them, which keeps faults inside them brittle enough to break as far down as 700 km, beneath Japan, Kamchatka and Tonga. Plotted in cross-section, their quakes trace the dipping plate, a pattern called a Wadati-Benioff zone.

Seismologists once assumed that every earthquake had a shallow focus. H.H. Turner of Oxford found the first evidence against that view in 1922, and seismogram studies confirmed deep-focus events in 1931.

How Do Seismologists Measure the Depth of an Earthquake?

By timing an echo. Part of the energy leaves the focus upward, bounces off the surface near the hypocenter and reaches distant stations as a phase called pP, a little after the direct P wave. That delay barely changes with distance but grows quickly with depth, so published tables turn it into a focal depth, and a second echo, sP, arrives roughly half that interval after pP. Deep events also give themselves away by small surface waves rather than the large ones of a shallow rupture. Even so, the USGS notes that depth carries larger errors than location unless a station sits close above the epicenter.

Questions About Earthquake Depth

Is the Epicenter the Same as the Focus?

No, though the two share a vertical line. The focus, or hypocenter, is the point at depth where rupture begins, and the epicenter is the spot on the surface straight above it, the one maps show, as the Live Atlas earthquake glossary puts it.

How Deep Can an Earthquake Be?

Around 700 km. The USGS uses 0 to 700 km as the range for its depth classes, and a USGS FAQ describes the zone where earthquakes can occur, the crust and upper mantle, as reaching about 800 km.

Sources