# Plate Tectonics and Earthquakes

The world's major tectonic plates.
An earthquake is caused by a sudden slip on a fault. A fault is a fracture or zone of fractures between two blocks of rock. During an earthquake, the rock on one side of the fault suddenly slips with relative to the other. The faults are most commonly found around the edge of plates which are continental-size blocks of rocks that comprise the outermost part of the earth.

These plates are constantly moving (albeit very slowly) at rates up to four inches per year (10 cm/yr) although most rates of travel are considerably less. Also, the rate of travel varies at different locations within each plate. At the boundary where plates are colliding, one plate is forced under the other plate forming deep trenches. Where plates are moving apart, mountain ridges form.

Since large geological forces are at work near these plate boundaries, it stand to reason that these boundaries are where the majority of earthquakes occur.

How these tectonic plates move relative to each other determines the type of fault that exists at their junction point. There are three basic types of faults. The junction where plates are moving away from each other produces a "normal" fault. It is not normal in the sense of it being common as it is not the most common. The word "normal" refers to the usually very steep fault plane between two blocks of earth.

In a normal fault, the two blocks are pulling away from one another causing one of the fault blocks to slip upward and the other downward with respect to the fault plane.

If the two blocks of earth are moving toward each other, the resulting fault is called a reverse fault. This is where one block of earth is forced up and over (or one is forced under) the opposing block. In either case, there is a change in the height of one or both blocks of earth.

A third fault is called the strike-slip fault. In strike-slip faults the opposing blocks of earth move horizontally opposite to each other. There is no (or very little) vertical movement.

There are also combinations of these basic fault movements as the land can move both horizontally and vertically. However, there is no way to telling the type of fault movement until well after the event is over.

While any of these three faults can produce extensive damage on land, the reverse fault is sources of most tsunamis.

The scale by which earthquakes are rated is called the Richter Scale. This scale was developed in 1935 by Charles Richter as a way to measure the size of waves produced by earthquakes as recorded by seismographs. This scale provides a value of the magnitude of an earthquake.

Magnitude, Scale, Physical Effects, and Number of Occurrences Each Year
Magnitude
Class
Richter
Scale
Effects Yearly
Occurrence
Great ≥8 Can cause serious damage in areas several hundred miles across. ≤1
Major 7.0-7.9 Can cause serious damage over larger areas. 18
Strong 6.0-6.9 Can be destructive in areas up to about 100 mi (160 km) across in populated areas. 120
Moderate 5.0-5.9 Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings. 800
Light 4.0-4.9 Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. ~6,200
Minor 3.0-3.9 Often felt, but rarely causes damage. ~49,000
Micro ≤3.0 Generally not felt, but recorded. ~3,300,000

Richter Scale values are logarithmic meaning that with each increase in whole value the amplitude of the ground motion increase by ten. For example, a magnitude 5.0 earthquake is ten times as powerful as a magnitude 4.0 earthquake. For a 6.0 magnitude earthquake, it is ten times more powerful than a magnitude 5.0 quake but is 100 times stronger than a 4.0 magnitude event.

This logarithmic increase in released energy at the 'strong' and 'great' earthquake levels means that minor increases in magnitude indicate huge jumps in released energy. According to the U.S. Geological Survey, the December 26, 2004 Sumatra earthquake measured 9.1 on the Richter Scale. Three months later, March 28, 2005, another 'great' earthquake occurred on the same fault line as with the earlier quake and measured 8.7 on the Richter Scale.

Despite the 0.4 difference in magnitude, due to the Richter Scale's logarithmic values, the December 9.1 magnitude earthquake was 2½ times MORE powerful than the March 2005 earthquake (and over 125,000 times as powerful as a magnitude 4.0 quake).

Next: Tsunami Generation