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Cause of Earthquakes and Tremors
Earthquakes and tremors are caused by two basic forces. The largest are caused by the interaction of tectonic plates. This typically occurs along two types of plate boundaries. Convergent boundaries represent areas where plates are colliding. Transform boundaries represent areas where plates are sliding past one another. Friction causes the plates to stick. When the pressure overcomes this friction, the plates violently shift, producing earthquakes. The second force which causes earthquakes and tremors is volcanic activity. Here, rising magma is the trigger.
Seismic Waves
When an earthquake or tremor occurs, it results in the release of energy waves that propagate outward from the origin. There are two categories of waves: body and surface. Body waves travel through the earth and are the first to arrive. They do the least damage. The next waves to arrive are surface waves. As the name suggests, they propagate along the surface and do more damage. The strongest surface wave is the Rayleigh wave. These seismic waves move the ground in a motion similar to an ocean wave. These waves are much larger than the others and account for the majority of earthquake damage.
Measuring Earthquake Intensity
The intensity of an earthquake or tremor is measured by the Richter scale. This scale provides a method to compare earthquakes, based upon the amount of ground movement they create. It is a logarithmic scale, meaning each increase in a whole number corresponds to a tenfold increase in intensity. Quakes registering less than 2.5 are generally referred to as tremors, and are not felt. Quakes between 2.5 and 6 can cause slight damage. Quakes between 6.1 and 6.9 are called strong earthquakes and can result in moderate damage. Quakes between 7 and 7.9 are called major earthquakes and cause serious damage. Quakes over 8.0 on the Richter scale are called great earthquakes and can cause catastrophic damage.
Earthquake Facts
The largest earthquake in the world occurred on May 22, 1960, in Chili and measured 9.5 on the Richter scale. The largest earthquake in the U.S. occurred on March 28, 1964, near Prince William Sound, Alaska. It measured 9.2 on the Richter scale. An estimated 500,000 earthquakes occur around the world each year. Of these, around 100,000 can be felt and 100 cause damage. Southern California experiences around 10,000 quakes a year. Of these, most are only tremors, with less than 20 registering more than magnitude 4. Alaska, not California, is the most seismically active state. The state experiences a magnitude 7 quake almost every year and a quake greater than magnitude 8 around every 14 years.