Earthquake is the shaking of Earth’s crust. The crust is the planet’s outermost layer that covers everything from mountains to oceans. It sits on top of a thicker, denser layer called the mantle. Faults are places where the layers of rock stick together or bump into each other. When one of these faults slips, immense pressure and friction build up until they break. The sudden release of this energy causes an earthquake.

Most earthquakes last a few seconds, but the shaking from the largest quakes can last 45 to 90 seconds or more. During that time, it is important to drop, cover and hold on. The safest place to be is under a table or bed. Stay away from windows and furnishings with glass; they may shatter. Be prepared for aftershocks, which can be much stronger than the main quake and could cause additional damage.

The most dangerous effects of an earthquake are the ones that occur in unprepared areas. They include falling debris, fires and liquefaction. Liquefaction is when water-filled mud or sand in the ground loses its solidity. This makes the ground swell, buckle and sink in certain areas.

Modern early warning systems can give you a few tens-of-seconds of advance notice of an earthquake by detecting seismic waves right after it happens. The alert is delivered via phone, TV or radio. If you hear a loud ocean roar, it could mean a tsunami is approaching. If so, move to higher ground or inland, avoiding river valleys.