 Look out! Earthquake! What should you do during a big tremor? This is Dan. He recently arrived in this country to start a job. Suddenly his office starts to shake and the building sways violently. Dan's from a country that doesn't have earthquakes and he panics. Luckily his co-worker Rena is there to show him what to do. First, get under a desk for protection. If there's time, open a door to secure an exit. In a high-rise building, tremors could continue for 10 minutes or more. Although the shaking may make you feel sick, the building is unlikely to collapse, so stay calm. Dan is impressed by how cool Rena is. But he wonders why high-rise buildings sway with such long movements. Rena explains. The shaking movement due to an earthquake is called seismic ground motion. The number of seconds for each back-and-forth movement is its period. Depending on the length of the period, the movement is either short or long-period ground motion. The thicker the ground's sedimentary layers, the longer the period. Because cities are usually built on planes composed of very thick sedimentary layers, they often experience long-period ground motion. Each building has its own period, which depends on the type of construction and height. Higher buildings have longer periods, and these are more likely to be the same as the ground motion. When a building's period is the same as that of the ground, this causes resonance, which greatly magnifies the shaking. That's why, even though the ground outside is only moving slightly, a tall building might be shaking violently. Dan now understands what's happening. The only reason desks and lockers don't move is that they are all securely fastened down. After the shaking stops, Dan asks Rena to lunch. Not so fast, says Rena. Don't assume it's safe to go outside so quickly. If you're near the sea or a river, a tsunami might be coming. And after a major tremor, aftershocks make it dangerous to be near damaged structures, older buildings, or cliffs that might collapse. Rena tells Dan he should train to be ready for an earthquake. In earthquake-prone countries, communities hold disaster drills, and many facilities provide earthquake simulations. The latest facilities can simulate earthquakes in high-rise buildings and many other scenarios. To find out what's available in your area, just look online. Thanks to Rena, Dan is now ready for life in an earthquake zone. The next weekend, he takes his family to an earthquake simulator. Now they'll know exactly what to do in a big quake. To stay safe during a major earthquake, the most important thing is knowledge. Official online disaster mitigation sites provide all the information you need. Remember, don't rush. Stay calm and help others.