A trembling or shaking of the ground caused by the sudden release of energy stored in the rocks below the surface, radiating from a fault along which movement has just taken place. Generally, only seconds. Strong ground shaking during a moderate to large earthquake typically lasts about 10 to 30 seconds.
Earthquakes can occur at any time of the year and at any time of the day or night.
In an open field, where nothing can fall on you. If you are indoors, when you feel the ground start to shake, take cover immediately under a table or sturdy piece of furniture, placing a barrier between falling objects and yourself. Do not attempt to use the stairs or an elevator or run out of the building. A seismometer is an sensor placed in the ground to detect vibrations of the earth. A seismograph is an instrument that records these vibrations.