Earthquake
Earthquakes are a complex set of ground motions caused by sudden dislocation of rock along a break in the Earth’s surface called a fault. The sudden movement releases energy stored in the rock, which travels up to the surface in waves that shake buildings and other infrastructure. These waves are called seismic waves.
Scientists are working hard to figure out whether there are warning signs that an earthquake is coming, but for now the best early warning signal comes from a seismograph. It takes between three and five seconds for an earthquake to reach the nearest stations, and another few seconds for computers to analyze the data and issue a warning. That’s a long time in which to lose your footing or be struck by falling debris.
The early warnings that are issued are based on the difference between the arrival times of P and S waves recorded by a seismograph station. P waves are the first to arrive at a location, and S waves follow later. By measuring the time it takes for the two waves to reach the seismograph station, scientists can determine how far away the earthquake is. They can also figure out how big the quake was by dividing the distance by the number of seconds it took for the P and S waves to arrive at the station.
If you feel a strong quake, stay where you are. Don’t run outside (you could be hit by falling debris), or into a building (you might get trapped inside). If you are in a vehicle, don’t drive off the road. Avoid standing near power lines, water lines or streetlights.