Seismic waves.

Step 1 of 6

Two kinds of wave

An earthquake is rock breaking suddenly along a fault, and the energy released goes out in every direction as waves. In the Unit on waves we saw longitudinal waves, in which the material moves along the direction of travel, and transverse waves, in which it moves at right angles to it. Here too there are two kinds: P waves, compression waves, and S waves, shear waves.

In the Scene the Earth is cut in half through the quake, and the waves cross its inside. Time since the quake is a timeline, from the moment the quake strikes until the waves have crossed the Earth. Seismometers stand round the rim, every $10^\circ$⁠, and light up when the wave reaches them.

Step 2 of 6

Arrival times

The picked seismometer is drawn with an arc from the quake, and its seismogram, the record of its shaking over time, is beside the Scene. Time since the quake is at the end, after $28$⁠ minutes, and Wave type changes only what is drawn inside the Earth: the seismogram shows both waves. The Seismometer is locked.

Step 3 of 6

The gap grows

Both waves leave the source at the same moment. Predict what will happen to the gap between them at a station farther away.

Step 4 of 6

How far

At a distance $d$⁠, the P wave arrives after $\dfrac{d}{v_P}$⁠ and the S wave after $\dfrac{d}{v_S}$⁠, so the gap

$\Delta t = t_S - t_P = d\left(\frac{1}{v_S} - \frac{1}{v_P}\right)$$\Delta t =$$t_S - t_P =$$d\left(\frac{1}{v_S} - \frac{1}{v_P}\right)$⁠

is in proportion to the distance. For a nearby quake, when the waves travel in the crust, the speeds are almost constant, and the formula gives the distance directly.

Step 5 of 6

Reading distance from the gap

For a distant quake the waves dive deep, where the rock is faster, so instead of a formula a graph of arrival times is used. The Scene works it out from the structure of the Earth.

Step 6 of 6

Three stations

From the gap each station knows how far away the source is, but not in which direction.