How Earthquakes Happen

earthquakeThe globe is divided into tectonic plates, which move in relation to each other. They are floating in different directions with different speeds on the molten rock layer and they can bump each other. Well, any interaction between the tectonic plates causes earthquakes, which are nothing but jerks, jolts or shortcuts between the tectonic plates.

An earthquake is one of the most destructive natural phenomena on Earth. When such a disaster starts, lots of shock waves begin to start from its epicenter, which is the point located above the hearth of the earthquake. The first waves are called primary waves or P waves. They are longitudinal waves that propagate like sound waves: produce back and forward movements in the direction where they’re spreading. Primary waves are followed by secondary waves, also called S waves. Under their effect, rocks will hitch perpendicularly on their travel direction. There is also a third type of waves – the surface waves, which cause the undulation of the soil and accelerates the damaging effect of the secondary waves.

The severity of an earthquake can be expressed in many ways, through its magnitude and through its intensity. Although these two parameters are very different, they are very often confused. The magnitude of an earthquake is usually expressed on the Richter scale  and it’s a way of measurement of the earthquake’s strength or energy, released from the combustion chamber in the form of seismic waves. It can be determined by instrumentation, using the maximum amplitude and the frequency of oscillations, measured on seismographs.

On the other hand, the earthquake’s intensity is usually expressed on Mercalli modified scale, which is a subjective measurement that describes how strong a shock was felt at a given location. It’s based on the observed effects an earthquake produces to people, buildings, land, etc..

Richter magnitude scale

It was named after Dr. Charles F. Richter of the California Institute of Technology and it’s the best known way of measuring any kind of magnitude. Richter invented this scale in 1935, as a mathematical tool to compare measurements of earthquakes. The scale is logarithmic, so a record level of seven (for example) indicates a ground movement 10 times greater than that corresponding to an earthquake with a six magnitude; it also has an energy of about 30 times higher. Earthquakes of magnitudes smaller than two are called micro-earthquakes are not felt by people and are recorded only by local seismographs. Earthquakes with magnitude greater or equal to 4.5 are strong enough to be recorded by sensitive seismographs across the globe, being also felt by people, those with a magnitude greater than six are considered large earthquakes, while those with a magnitude larger than eight degrees are considered major disasters. Although the Richter scale has, theoretically, no upper limit, it has a limit – the one of the largest earthquake that happened so far: 9,5 degrees. It happened in Chile, in 1960 and it caused the death of over 5.700 people.

The modified Mercalli scale

The effects of an earthquake on the Earth’s surface are represented numerically by the term called “seismic intensity”. Although over the last few centuries there were created different intensity scales for assessing the effects of earthquakes, the modified Mercalli scale is the most used in USA, while in Europe it’s used an adjusted version of it: the European scale of macro-seismic intensities.

The modified Mercalli scale is the result of the first Mercalli scale made in 1902 mixed with the modifications brought by other seismologists. This scale has 12 levels of increasing intensity from imperceptible movements to catastrophic failures and it’s usually represented by Roman numerals. It doesn’t have a mathematical basis and it’s arranged arbitrarily based only on observed effects. The assessment of the intensity of an earthquake can be made only after eyewitnesses reports and after studying and interpreting results of field research. Intensity scale is more useful to people who work in the field than because intensity refers to the actual effects of the surface.