All earthquakes start with P waves  These P waves move fast, causing the ground to vibrate up and down, causing less damage  The next one is S waves, which move very slowly, causing the ground to sway back and forth, left and right, which is destructive Great  Early warning systems measure the effects of P waves along the ground to predict the effects of S waves, and then issue warnings  However, scientists working on such systems have discovered that in fact, people don't have much time for them
When a major earthquake occurs, people first feel up and down bumps, and then there is strong horizontal shaking, which is caused by S waves
 Be prepared for an earthquake