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Events

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Lessons / Restless Earth

Restless Earth · 4 min · 4 steps

How deep do earthquakes go?

Plays here

Goal. Read earthquake depths and use them to trace a plate sinking into the mantle.

Steps

  1. Each earthquake dot is now coloured by depth: red for shallow (less than 70 km), yellow for intermediate (70 to 300 km), blue for deep (more than 300 km). Tap a dot to read its exact depth.

  2. Predict

    Near Tonga the Pacific Plate sinks westwards under the trench. Where do you expect the deepest earthquakes?

    • East of the trench, under the open Pacific
    • West of the trench, towards Fiji
    • Right on the trench
    Show the answer

    West of the trench, towards Fiji. West of the trench: the further the plate has sunk, the deeper its earthquakes.

    Near Tonga the Pacific Plate dives westwards. If this week's map has earthquakes here, look at their colours: they get deeper the further west you go, down to about 700 km under Fiji. Seismologists traced sinking plates this way.

  3. Near Taiwan most earthquakes are shallow, because the plates collide close to the surface. North-east of Taiwan, under the Ryukyu Islands, earthquakes get deeper as the Philippine Sea Plate sinks northwards, when there are any this week.

  4. Shallow earthquakes do the most damage, because the shaking starts close to the surface. A deep earthquake of the same magnitude is felt over a wider area, but much more weakly.

The common mistake

Myth

Earthquakes happen deep in the Earth's core.

Fact. Almost all earthquakes happen in the top 700 km, in the crust and in plates sinking into the mantle. Below about 700 km no earthquakes have been recorded: the rock there flows slowly instead of breaking.

Check yourself

Question 1

Why do earthquakes get deeper the further you go from a trench, on the side the plate sinks towards?

  • They follow the plate as it sinks into the mantle
  • The ocean is deeper there
  • Deep earthquakes happen at random
Show the answer

They follow the plate as it sinks into the mantle. The sinking plate stays colder and stiffer than the mantle around it, so it can still break as it goes down, and its earthquakes trace its path.

Question 2

About how deep do the deepest earthquakes go?

  • About 7 km
  • About 700 km
  • About 7,000 km
Show the answer

About 700 km. The deepest are about 700 km down, inside sinking plates.

Question 3

Of two earthquakes with the same magnitude, which usually does more damage?

  • The shallow one
  • The deep one
  • They always do the same
Show the answer

The shallow one. The shaking from a shallow earthquake starts close to the surface, so it is much stronger above it.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
EIa-Vc-4
US Next Generation Science Standards
MS-ESS2-3
England national curriculum
KS3 Geography: geological timescales and plate tectonics