The type of wave that causes the ground to shake in a motion parallel to the direction of the wave, moving straight away from the epicenter, is called a P-wave or primary wave.

Here's why:

* P-waves are compressional waves. This means they travel by compressing and expanding the material they pass through. Imagine a slinky: you push one end, and the compression travels along the slinky. P-waves do something similar with the Earth's rocks.

* This compression and expansion motion results in a push-pull movement of the ground, parallel to the direction of the wave.

* P-waves are the fastest type of seismic wave, which is why they are the first to arrive at a seismograph after an earthquake.

Let me know if you'd like to learn more about other types of seismic waves!