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At what depth do earthquakes occur? What is the significance of the depth?

 

Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles).

The strength of shaking from an earthquake diminishes with increasing distance from the earthquake's source, so the strength of shaking at the surface from an earthquake that occurs at 500km deep is considerably less than if the same earthquake had occurred at 20 km depth.

Also, the depths of earthquakes gives us important information about the Earth's structure and the tectonic setting where the earthquakes are occurring. The most prominent example of this is in subduction zones, where plates are colliding and one plate is being subducted beneath another. By carefully plotting the location and depth of earthquakes associated with a subduction zone, we can see details of the zone's structure, such as how steeply it is dipping, and if the down-going plate is planar or is bending. These details are important because they give us insight into the mechanics and characteristics of the deformation in the subduction zone.

The deepest earthquakes occur within the core of subducting slabs - oceanic plates that descend into the Earth's mantle from convergent plate boundaries, where a dense oceanic plate collides with a less dense continental plate and the former sinks beneath the latter. The plate boundary contact between two such plates generate very large, shallow subduction zone earthquakes such as the Sumatra 2004 M9.1 event, and the 2011 M9.0 Japan earthquake, and is only active to relatively shallow depths - approximately 60 km. However, because oceanic slabs are relatively cold with respect to the surrounding mantle in deeper subduction zone environments, faults within the core of these slab remain brittle and can generate earthquakes to depths of as much as 700 km (e.g., Pacific Plate beneath Japan and Kamchatka, and beneath Tonga).

As the slab descends into the mantle, rheology changes (viscosity characteristics) cause the plate to bend and deform, and generates these earthquakes. The trend of such events can be seen in cross-sections of subduction zones, and are known as "Wadati-Benioff Zones". 

Within continents, and along continental plate boundary transform faults such as the San Andreas, faults are only active in the shallow crust - perhaps to depths of approximately 20 km.

Accurately determining the depth of an earthquake is typically more challenging than determining its location, unless there happens to be a seismic station close and above the epicenter. So generally, errors on depth determinations are somewhat greater than on location determinations.

(Contributed by Gavin Hayes & Tony Crone)

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    What is an earthquake and what causes them to happen?

    An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.In California there are two plates - the Pacific Plate and the North...

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    What is an earthquake and what causes them to happen?

    An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.In California there are two plates - the Pacific Plate and the North...

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    Foreshocks, aftershocks - what's the difference?

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    Full moon appearing over red rocks in Grand Canyon.

    Can the position of the moon or the planets affect seismicity? Are there more earthquakes in the morning/in the evening/at a certain time of the month?

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    Can the position of the moon or the planets affect seismicity? Are there more earthquakes in the morning/in the evening/at a certain time of the month?

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    What is surface faulting or surface rupture in an earthquake?

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    What does it mean that the earthquake occurred at a depth of 0 km? How can an earthquake have a negative depth; that would mean it’s in the air. What is the geoid, and what does it have to do with earthquake depth?

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    Why do so many earthquakes occur at a depth of 10km?

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    Why do so many earthquakes occur at a depth of 10km?

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    Scientist pointing to graphic on a screen while children watch

    Where can I find earthquake educational materials?

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    Where can I find earthquake educational materials?

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    Learn More
    link
    Research has identified 17 areas in the central and eastern United States with increased rates of induced seismicity.

    Can we cause earthquakes? Is there any way to prevent earthquakes?

    Earthquakes induced by human activity have been documented at many locations in the United States and in many other countries around the world. Earthquakes can be induced by a wide range of causes including impoundment of reservoirs, surface and underground mining, withdrawal of fluids and gas from the subsurface, and injection of fluids into underground formations. While most induced earthquakes...

    link

    Can we cause earthquakes? Is there any way to prevent earthquakes?

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    What is the difference between aftershocks and swarms?

    Aftershocks are a sequence of earthquakes that happen after a larger mainshock on a fault. Aftershocks occur near the fault zone where the mainshock rupture occurred and are part of the "readjustment process” after the main slip on the fault. Aftershocks become less frequent with time, although they can continue for days, weeks, months, or even years for a very large mainshock.A swarm, on the...

    link

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