Updated: 07 November 2007

Earthquakes

Introduction

Earthquakes occur throughout the world, but the vast majority occur along narrow belts which are a few tens to hundreds of kilometres wide. These belts mark boundaries on the planet's surface that are very active geologically.

What is an earthquake?

An earthquake occurs when rocks break under the stress created by the constant movement and collision of the thin jigsaw-like tectonic plates found at the surface of the Earth.

Rock breakage is called faulting and causes a release of energy when stored stress is suddenly converted to movement. Vibrations known as seismic waves are produced - they travel outwards in all directions at up to 14 kilometres per second. At these speeds, it would take the fastest waves only 20 minutes or so to reach the other side of the Earth by going straight through its centre - that's a distance of almost 13,000 kilometres. The waves distort the rock they pass through, but the rock returns to its original shape afterwards.

The epicentre is the point on the Earth?s surface directly above the source of the earthquake. The source, also known as the focus, can be as deep as 700 kilometres. Earthquakes do not occur deeper than this because rocks are no longer rigid at very high pressures and temperatures - they can't store stress because they behave plastically.

Smaller events occur more frequently - in fact, most earthquakes cause little or no damage. A very large earthquake can be followed by a series of smaller aftershocks while minor faulting occurs during an adjustment period that may last for several months.

Earthquakes can also cause tsunamis, which are giant ocean-waves that can cross an ocean and may cause extensive damage in coastal regions. In areas where there are steep slopes, vibrations may cause landslides.

Where do earthquakes occur?

No part of the Earth's surface is safe from earthquakes. But some areas experience them more frequently than others.

Earthquakes are most common at plate boundaries, where different tectonic plates meet. The largest events usually happen where two plates are colliding - this is where large amounts of stress can build up rapidly.

About 80 percent of all recorded earthquakes occur at the circum-Pacific seismic belt which:

extends from New Zealand northward through Fiji, Papua New Guinea, the Philippines, and Japan
along the eastern coast of Russia
east to the southern edge of Alaska
south along the western coast of Canada, North America and South America.

Intraplate earthquakes occur less commonly and do not follow recognisable patterns. They take place in the relatively stable interior of continents, away from plate boundaries. This type of earthquake generally originates at more shallow levels.

How are earthquakes detected?

A seismograph records the vibrations caused by earthquakes and produces a seismogram.

An earthquake's epicentre can be located accurately if three or more seismographs detect the event. The latitude, longitude, depth and timing are needed to uniquely define an earthquake.

The arrival time of seismic waves depends on the type of vibration being carried. P waves (primary waves) arrive first and deform rocks by changing their volume, done by rapidly expanding and compressing them. S waves (secondary waves) reach us next and deform rocks by changing their shape, or shearing them.

Surface waves arrive last and pass around the Earth rather than through it.

The epicentre is found by comparing the arrival times of the P and S waves at a seismograph. The greater the time difference, the further the instrument is from the epicentre.

How do we measure the size of an earthquake?

Earthquake sizes are compared by measuring the maximum heights of the seismic waves at a distance of 100 kilometres from the epicentre. The range in possible heights is used to construct the Richter scale.

The scale divides the size of earthquakes into categories called magnitudes. The magnitude of an earthquake is an estimate of the energy released by it.

For every unit increase in magnitude on the Richter scale, there is roughly a thirty-fold increase in the energy released by an earthquake. For instance, a magnitude 2 earthquake releases 30 times more energy than a magnitude 1 earthquake. The difference in the energy released between earthquakes of magnitudes 3 and 1 is 900 times (30 x 30).

A magnitude 8.6 earthquake releases energy equivalent to about 10,000 atom bombs of the kind that destroyed Hiroshima in World War II.

In populated areas, the effects seen during an earthquake depend on many factors, such as the distance of the observer from the epicentre. Generally, magnitudes of:

less than 3.4 are recorded only by seismographs
3.5-4.2 are felt by some people who are indoors
4.3-4.8 are felt by many people and windows rattle
4.9-5.4 are felt by everyone, while dishes break and doors swing
5.5-6.1 cause slight building damage with plaster cracking, and bricks falling
6.2-6.9 cause much building damage and houses move on their foundations
7.0-7.3 cause serious damage with bridges twisting, walls fracturing, and many masonry buildings collapsing
7.4-7.9 cause great damage and most buildings collapse
more than 8.0 cause total damage with waves seen on the ground surface and objects are thrown in the air

Note that events between magnitudes of roughly 2.0 and 3.4 may be felt within a few kilometres of the epicentre.

It's likely that earthquakes do not exceed magnitudes of about 8.6 - there is a limit to the amount of energy that rocks can store before breaking.

Did you know

On average, the Earth experiences each year:

one earthquake of magnitude 8
ten earthquakes of magnitude 7
one hundred earthquakes of magnitude 6