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19 major earthquakes every year
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 80 km (50 miles) thick, all in constant motion relative to one another. They move at between 10 and 130 mm (from less than one half to 5 inches) 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 or have very small magnitudes. The USGS Earthquake Info Center locates 12 000 to 14 000 earthquakes each year (about 35 per day). On average, about 60 earthquakes per year are classified as significant, with 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, and 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.
The largest earthquake ever recorded occurred in Chile in 1960: it had a magnitude of 9.5.
Did you know that there are more earthquakes in Alaska than in California?
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.