Through the Earth · QuakeLab

Here Comes an Earthquake

The place underground where rock suddenly breaks is the focus (hypocentre); the point on the surface directly above it is the epicentre. The distance between them is the focal depth.

Magnitude measures how much energy an earthquake releases — one earthquake has one magnitude. Intensity measures how strongly the ground is shaken and damaged — the same earthquake has different intensities in different places.

Set the focal depth and magnitude, press "Trigger earthquake", then click the surface a few times to place towns and read their intensity.

P Waves and S Waves

An earthquake sends out two kinds of body waves at once. A P wave (longitudinal) is like pushing a spring: particles vibrate back and forth along the direction of travel. An S wave (transverse) is like shaking a rope: particles vibrate across the direction of travel.

Predict which arrives first, then click the edge of the cut face or the surface to place seismic stations, press play, and see which wave starts first on the drum.

Locating with Three Stations

An earthquake has struck somewhere; the epicentre is unknown. You have three seismometers.

Place three stations on the map and press "Record". On each seismogram mark the onset of the P and S waves; the S−P time multiplied by 8.4 km/s is the distance from the station to the focus. Draw a circle around each station: the circles meet at the epicentre.

The Shadow Zone Mystery

In 1914 Gutenberg noticed that stations more than 104° from the epicentre received no S waves, and between 104° and 143° hardly even P waves. This belt is the shadow zone.

Place stations at different distances and watch the travel-time chart fill up. Then use the "hypothesis" switch on the right to compare a hypothetical Earth with the real records.

Cutting the Earth Open

Sudden changes in seismic wave speed reveal two key discontinuities inside the Earth: the Moho (on average about 33 km under continents) and the Gutenberg discontinuity (about 2900 km). They divide the Earth into the crust, the mantle and the core; the core is further split at about 5150 km into a liquid outer core and a solid inner core.

Drag the depth cursor on the velocity profile on the right to find the three boundaries; click a layer on the cut face to open its specimen card.

Challenge: Unknown Worlds

A probe has set down seismometers on four unknown planets. Their interiors are covered: you can only see the waves on the surface and the stations' records.

Place up to 6 stations on each planet. From where S waves arrive, where the shadow zones are, and whether faint P waves appear inside them, work out each planet's interior and estimate the size of its liquid core.