What Causes Earthquakes?

Most earthquakes are caused by tectonic activity associated with plate margins and faults. Earthquakes are also caused by volcanic eruptions.

An earthquake is a sudden tremor or movement of the earth's crust, which originates naturally at or below the Earth's surface. Most earthquakes are related to stresses built up along the edges of the huge moving lithospheric plates that make up the Earth's surface. The lithosphere is the outer part of the Earth, consisting of the crust and upper mantle and it is approximately 100 km (62 miles) thick.

Shock waves created in the Earth by an earthquake are similar to the waves created when a stone is thrown into water. After the stone hits the water, a series of concentric waves move outwards from the center. There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. The origin of an earthquake is called the focus. Since this is often well below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth's surface directly above the focus, called the epicenter.

The immediate cause of most shallow earthquakes is the sudden release of stress along a fracture in the earth's crust called a fault. This results in movement of the opposing blocks of rock past one another. Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if conditions are favorable.

Explosive volcanic eruptions can cause earthquakes. Earthquakes are very common in areas of volcanic activity where they either proceed or accompany eruptions. Given that not all volcanoes are prone to violent eruption, and ones that are, remain inactive for long periods of time, it is not surprising earthquakes caused by volcanic eruptions are rare. When a volcano explodes, it is likely that the associated earthquake effects will be confined to an area 10 to 20 miles around its base.

The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly upon contact with the air. This tends to block the volcanic vent and block the further escape of pressure. The only way in which such a blockage can be removed is by the build up of pressure to the point at which the blockage is literally exploded out of the way. The weakest part of the volcano will be the part which gives way, sometimes leading to a sideways explosion as in the Mt. St.Helens eruption.

When extraordinary levels of pressure develop, the resultant explosion can produce an earthquake of considerable magnitude. When Krakatoa (located in Indonesia, between Java and Sumatra) exploded in 1883, the explosion was heard over 5,000 km away in Australia. The shockwaves produced a series of tsunami (large sea waves), one of which was over 36 meters high.

By contrast, volcanoes producing free flowing basic lava rarely cause earthquakes. The lava flows freely out of the vent and down the sides of the volcano, releasing pressure evenly and constantly. Since pressure doesn't build up, violent explosions do not occur.

Earthquakes are also caused by tectonic activity associated with plate margins and faults. The majority of earthquakes across the world are tectonic earthquakes.

Tectonic earthquakes are triggered when the crust becomes subjected to strain, and eventually moves. The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to move slowly, they can either drift towards each other, away from each other or slide past each other. Many of the earthquakes which we feel are located in the areas where plates collide or try to slide past each other.

The process of Elastic Rebound Theory explains how this works and can be demonstrated with a green twig or branch. Holding both ends, the twig can be slowly bent. As it is bent, energy is built up within it. A point will be reached where the twig suddenly snaps. At this moment, the energy within the twig has exceeded the elastic limit of the twig. As it snaps, the energy is released, causing the twig to vibrate and to produce sound waves.

An example of plates sliding past each other occurs along the San Andreas Fault in California. Here, two plates, the Pacific Plate and the North American Plate, are both moving in a northwesterly direction, but one is moving faster than the other. The San Francisco area is subjected to hundreds of small earthquakes every year as the two plates grind against each other. Occasionally, as in 1989, a much larger movement occurs, triggering a far more violent earthquake.

Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move, called foreshocks, or a period of less frequent shocks as the two rock masses temporarily 'stick' and become locked together. Surveys in San Francisco have shown that railway lines, fences and other longitudinal features very slowly become deformed as the pressure builds up in the rocks, and then become noticeably offset when a movement occurs along the fault. Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses 'settle down' in their new positions. Aftershocks can cause problems for rescue services, bringing down buildings already weakened by the main earthquake.

Earthquakes are three dimensional events, the waves move outwards from the focus, but can travel in both the horizontal and vertical plains. This produces three different types of waves which have distinct characteristics and can only move through certain layers within the Earth.

Primary Waves (P-Waves) are very similar to sound waves. They are high frequency, short-wavelength, longitudinal waves which can pass through solids and liquids. The ground is forced to move forwards and backwards as it is compressed and decompressed. This produces relatively small displacements of the ground. P-Waves can be reflected and refracted, and under certain circumstances can change into secondary or S-Waves.

S-Waves travel more slowly than P-Waves and arrive at any given point after the P-Waves. Like P-Waves, they are high frequency, short-wavelength waves, but instead of being longitudinal they are transverse which means they move in all directions away from their source, at speeds which depend upon the density of the rocks through which they are moving. They cannot move through liquids. On the surface of the Earth, S-Waves are responsible for the sideways displacement of walls and fences, leaving them 'S' shaped.

Surface Waves (L-Waves) are low frequency transverse vibrations with a long wavelength. They are created close to the epicenter and can only travel through the outer part of the crust. They are responsible for the majority of the building damage caused by earthquakes. This is because L-Waves have a motion similar to that of waves in the sea. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground.

Trending Now

© High Speed Ventures 2011