EARTHQUAKES

Made by Matteo Di VitaClass Form3.02

What causes an earthquake? An Earthquake is a sudden tremor or movement of the earth's crust, which

originates naturally at or below the surface. The word natural is important here, since it excludes shock waves caused by French nuclear tests, man made explosions and landslides caused by building work.

There are two main causes of earthquakes. Firstly, they can be linked to explosive volcanic eruptions; they are in fact very

common in areas of volcanic activity where they either proceed or accompany eruptions.

Secondly, they can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes world wide are of this type.

Terminology An earthquake can be likened to the effect observed when a stone is thrown

into water. After the stone hits the water a series of concentric waves will move outwards from the center. The same events occur in an earthquake. There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. Geologists and Geographers call the origin of the earthquake the focus. Since this is often deep below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth surface directly above the focus. This point is called the epicentre.

The strength, or magnitude, of the shockwaves determines the extent of the damage caused. Two main scales exist for defining the strength, the Mercalli Scale and the Richter Scale.

Some earthquakes facts

The largest recorded earthquake in the United States was a magnitude 9.2 that struck Prince William Sound, Alaska on Good Friday, March 28, 1964 UTC.

The largest recorded earthquake in the world was a magnitude 9.5 (Mw) in Chile on May 22, 1960.

The earliest reported earthquake in California was felt in 1769 by the exploring expedition of Gaspar de Portola while the group was camping about 48 kilometers (30 miles) southeast of Los Angeles.

Before electronics allowed recordings of large earthquakes, scientists built large spring-pendulum seismometers in an attempt to record the long-period motion produced by such quakes. The largest one weighed about 15 tons. There is a medium-sized one three stories high in Mexico City that is still in operation.

The average rate of motion across the San Andreas Fault Zone during the past 3 million years is 56 mm/yr (2 in/yr). This is about the same rate at which your fingernails grow. Assuming this rate continues, scientists project that Los Angeles and San Francisco will be adjacent to one another in approximately 15 million years.

Japan tsunami appeal The massive earthquake and tsunami that struck Japan on 11 March

caused widespread destruction and suffering. Almost 28,000 people have been reported dead or missing and tens of thousands are still living in evacuation centres.

Since the disaster began, the Japanese Red Cross has been providing people in need with first aid and emergency healthcare, as well as distributing relief items.

As the operation moves out of the relief and into the recovery phase, the Japanese Red Cross is helping to fit out 70,000 temporary homes with an appliance package consisting of key domestic items such as rice cookers, microwave and kettles. These items will benefit around 280,000 people in the three hardest hit prefectures of Miyagi, Fukushima and Iwate.

The completion and equipping of these prefabricated homes will be a major step towards easing the pressure on evacuation centres and allowing people to return to some semblance of normal life.

In the unlikely event that we receive more donations to the Japan Tsunami Appeal than the Japanese Red Cross and International Red Cross and Red Crescent Movement can reasonably and efficiently spend, any surplus funds will be used to help us prepare for and respond to humanitarian disasters both here in the UK and overseas.

Japan`s earthquake

Here are some pictures of earthquakes

Earthquakes can cause tsunamis so let me show you a bit some tsunamis.

Next page

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 their own distinct characteristics and can only move through certain layers within the Earth. Lets take a look at these three forms of shock waves.

Primary Waves (P-Waves) are identical in character to sound waves. They are high frequency, short-wavelength, longitudinal waves which can pass through both 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 S-Waves.

Secondary 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. 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 epicentre 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. Together with secondary effects such as landslides, fires and tsunami these waves account for the loss of approximately 10,000 lives and over $100 million per year.

The End