An earthquake is caused by a sudden slip on a fault. A fault is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Stresses in the earth’s outer layer push the sides of the fault together, pressure builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.
Earthquakes tend to be concentrated in narrow zones. There are 7 major crustal plates on earth, about 50 miles (80 km) thick, all in constant motion relative to one another, moving at the speed at which your fingernail grows, at between 10 and 130 mm (from less than one half to 5 inches) per year.
Number of earthquakes per year
It is estimated that there are several million earthquakes in the world each year. Many of these earthquakes go undetected because they occur in remote areas, mostly under the sea, or have very small magnitudes.
Seismicity of the Earth 1900-2013 – USGS
The USGS Earthquake Info Center locates about 20,000 earthquakes each year (about 50 per day). On average, about 60 earthquakes per year are classified as significant, with up to 19 classified as major.
A significant earthquake is one of magnitude 6.5 or higher or one of lesser magnitude that causes casualties or considerable damage. Major earthquakes have a magnitude larger than 7.0.
In August 1999, an earthquake registering 7.4 on the open ended Richter Scale hit Turkey, killing more than 15,000 people. The next month, an earthquake registering 7.6 hit Taiwan. Later the same month, an earthquake registering 7.4 hit Mexico. In November, Turkey was hit again by an earthquake that registered 7.2.
Large earthquakes that originate from the sea bed result in tsunamis, causing huge waves to travel 500 mph (800 km/h) or more toward coasts. In 2004, the largest ever such tsunami killed more than 227,000 people in 14 countries along the Indian Ocean. It was estimated to have released more energy than 20,000 atomic bombs.
On January 12, 2010 an earthquake of magnitude 7.0 occurred 10 miles off the coast from Port-Au-Prince, Haiti, killing more than 200,000 people. The earthquake did not, however, came as a surprise to seismologists who predicted the fault slip just a week earlier. Governments and people often ignore the predictions of earthquakes.
The largest earthquake ever recorded occurred in Chile in 1960: it had a magnitude of 9.5. See the list of largest and deadliest earthquakes.
2 basic types of earthquakes
There are 2 basic types of earthquakes:
- A strike-slip earthquake occurs when the rock on one side of a fault slides horizontally past the other.
- In a dip-slip earthquake, the fault is at an angle to the surface of the earth and the movement of the rock is up or down.
Did you know? There are more earthquakes in Alaska than in California.