BACKGROUND INFORMATION: When the earth is shaking – reasons, measurement and threatened areas

by Ruth Bücker, 2010/03/14

The term earthquake is used to describe any tremor of the Earth’s surface. These tremors can have a variety of different reasons. Earthquakes happen, for example, if subterranean caves collapse or due to volcanic activities.

Those earthquakes which are strongest and most frequent, however, are caused by another event. They are called tectonic earthquakes and in order to understand their origin, it is necessary to first have a closer look at the structure of the Earth.

This is the structure of the Earth

The Earth consists of several layers. A solid core consisting of iron and nickel forms the innermost layer of the Earth. This core is surrounded by a layer of liquid substance which is estimated to be about 2000 kilometres thick. The next layer is called the Earth’s mantle. It consists mainly of solid rock and is about 2900 kilometres thick.

The outermost and thinnest layer of the Earth’s mantle is called the Earth’s crust, which can be described as a mosaic of tectonic plates of different sizes. Due to the currents flowing in the subjacent layer – the Earth’s mantle – the pattern of tectonic plates shifts continuously. And although they shift only by few centimetres a year, the frictions created by these movements can have immense consequences.

Why does the Earth shake?

If the tectonic plates move towards or past each other (which is a normal event), these movements, and irregularities in the rock, can cause the earth plates to get stuck. Frictions and pressure between the tectonic plates increase and if it becomes too strong, a sudden shift of the plates along the already existing fault line leads to an abrupt jerk.

This picture clearly shows the wave-like movement of the Earth. Photo: Reuters

In the case of substantial shifts, the Earth’s surface trembles noticeably. If the event of plates getting stuck takes place below the sea floor, the discharge of friction can cause a tsunami.

The built up friction discharges and the energy which is released during this event is radiated from the seismic centre into all directions by means of wave-like seismic vibrations. The place on the Earth’s surface located above these discharges of friction is called the epicentre of the earthquake. This is how an earthquake happens.

How is the strength of an earthquake measured?

Today, the most common way of measuring the strength of an earthquake is to classify the earthquake according to the so-called Richter scale. The measured strength – the magnitude – describes seismic energy which is measured at the epicentre of the earthquake and it is reported in points.

Starting at zero, the Richter scale is basically not limited upwards. Humans generally do not feel earthquakes of a lower magnitude than three. From magnitude four onwards, objects start moving noticeably. Earthquakes are considered to be strong starting with magnitude six on the Richter scale.

Classification on the Richter scale is reported in points. Every additional point on the scale means that an earthquake is stronger by ten times. Hence, an earthquake of magnitude two does not cause the earth to shake two-times as strong as an earthquake of magnitude one, but ten-times as strong.

Why are some countries and regions hit particularly often?

Understanding how tectonic earthquakes are caused, the explanation of why earthquakes happen particularly frequent in some regions of the Earth becomes obvious. The countries most frequently hit by strong earthquakes are located above the borders of adjoining tectonic plates.

The Caribbean Plate, which Haiti is located on, is an extremely threatened region, since the plate adjoins to even several other tectonic plates. In the north and east frictions are created due to the North American Plate, while the large South American Plate creates pressure from the south. In the west there are the Coca Plate and the Nazca Plate causing frictions to arise between them and the Caribbean Plate and this friction is frequently released in the form of earthquakes.

Also Chile and Turkey – countries only lately hit by strong earthquakes – are located in such threatened regions. The fact that these countries and their inhabitants’ basis of life are destroyed again and again by earthquakes is therefore not surprising. But being aware of the danger does unfortunately not improve the situation of the people.

It is regions like Chile and Haiti where people urgently need sustainable support. In this phase after the earthquake they need help in particular with rebuilding their houses in order to being safer when the next earthquake will hit. Thank you very much for supporting our work.

If countries are located above fault lines, earthquakes are particularly frequent and strong – as is the case in Haiti. Cartography: Lars Mohri

This article contains information from focus-online, wissen.de, mdr.de, survival4u and the US Geologist Service.

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