**Earthquakes and Explosions cause Seismic Waves:📡**

When there is an earthquake somewhere, it produces seismic waves that travel out through the Earth. We can detect these using seismometers.

Seismologists work out the time it takes for the shock waves to reach each seismometer.

There are two different types of waves you need to learn in I/GCSE Physics= **P WAVES AND S WAVES.**

P-WAVES:

- They can travel through solids and liquids. They travel faster than S-waves.

S-WAVES:

- They can only travel through solids. They're slower than P-waves.

**Refraction and Triangulation 🤯**

When the waves reach a boundary between different layers of the Earth, some get reflected.

The waves also change speed as the density changes. This causes them to change direction, which is refraction.

Seismometere readings can be used to work out the distance to an earthquake's epicentre, the point on the Earth's centre directly above the earthquake.

- As the waves travel at different speeds, you'll see two distinct tremors. The first one is the P-wave and the second the S-wave.
- The time difference between the two waves is used to calculate how far away the explosion/earthquake was.
- Then you can draw a circle on a map, centred on the location of your seismometer, with the distance you calculated as its radius.
- This is a distance arc, and three or more seismometers distance arcs from other places will cross over to find the epicentre.

**This is called TRIANGULATION.**

References: https://volcanodiscovery.de/uploads/tx_tpkeywords/p_image.gifv]

https://i.ytimg.com/vi/iTD5DO5MMgA/maxresdefault.jpg

Drafted by Catrina (Physics)