Most recently on 12th January 2010:

The magnitude 7.0 quake struck 16 kilometres from the Haitian capital city of Port-au-Prince. There have been numerous aftershocks. The earthquake which has also brought down the presidential palace was the strongest to hit the country in 200 years and was felt as far away as Cuba. The UN peacekeeping headquarters in Haiti has collapsed under the strain of a massive earthquake and everyone inside is expected to have died.

What are they?

Earthquakes are caused when two of the massive tectonic plates that make up the solid surface of our Earth rub together violently. These tectonic plates normally move past each other, relatively easily, but from time-to-time they get stuck. This is when strain builds up in the faults (rocks) that haven’t moved, eventually the pressure is overcome and the tectonic plates move violently – causing the earthquake. If you think about pushing your hands together very tightly and then trying to move one forward, you get a big movement and heat generated when your hand does move – this is similar to the two tectonic plates finally slipping.

Earthquakes are mainly caused by the movement of geological faults, but can also be caused by:

• volcanic activity
• landslides
• mine blasts
• nuclear experiments

The epicenter of an earthquake is the point at ground level that is directly about the initial point of rupture. This initial point of rupture is called the focus or hypocenter.

Effects of Earthquakes

The earthquake produces seismic waves – these are what cause the shaking. It is the seismic waves that do the damage. Earthquakes are measured on the Richter scale, an earthquake of magnitude 3 or lower is hardly noticeable and a magnitude 7 earthquake will cause serious damage over large areas.

Human impacts:

• Loss of life
• Building collapse
• Loss of amenity/basic necessities – water, electricity, shelter
• Fires
• Disease
• Road / bridge damage

Effects of earthquakes include:

• Ground shaking – the severity of this depends on many things, but most commonly the distance from the epicentre and focus of the earthquake. The ground shaking can cause massive damage to structure (buildings etc) on the land that is shaking.
• Land rupture - in some cases the ground near the epicentre can literally “open up” and fissures can also be seen at some distance from the epicentre.
• Land slippage – you can sometimes see how the ground has moved by looking at things that used to be in straight lines eg fences, and seeing how they now seem to have a “kink” in them – this is indicative of ground slippage.
• Landslides - these can happen at any time, though the shaking of the earth from and earthquake makes them possible as the hillsides can become unstable.
• Fires - these are caused by gas mains and water pipes rupturing in the main earthquake. More deaths and damage were caused in the 1906 San Francisco earthquake by fires than by the initial earthquake.
  

Image via Wikipedia

• Tsunami – these are massive waves caused by an sudden movement of a large volume of water. They are commonly caused by submarine earthquakes or submarine landslides. Tsumamis are not commonly associated with earthquakes under magnitude 7.5. The Indian Ocean Earthquake of 2006 killed nearly 230,000 people over 11 countries with waves over 30m tall.
• Soil liquefaction – this is when the soil appears to turn from a solid to a liquid due to the shaking of the ground. Solid structures built on this soil can tilt and in some cases can become submerged. In the 1964 earthquake in Alaska this happened to many buildings that eventually collapsed.
• Floods - these occur because the total volume of water within a water body (lake, river) exceeds the amount that it can hold, this water then floods the surrounding area. The flood can be caused by the water body filling with other material eg landslip or by dams that hold the water in an artificial environment breaking.

Other Effects

• Volcanic activity: earthquakes can occur either before or after volcanic activity, as earthquake swarms can show magma flowing through volcanoes. Such earthquakes occurred before the 1980 Mt St Helens eruption.
  
• Image via Wikipedia

• Earthquake clusters: there can often be a number of much smaller earthquakes after the main shock, they cause little if any additional damage.
• Aftershocks: these are earthquakes that occur after a previous quake (the main shock.) An aftershock is always in the same region as the original earthquake but it is of a smaller magnitude. Aftershocks are caused by the crust around the displaced fault adjusting to the effects of the main shock.
• Earthquake swarms: these are a sequence of earthquakes hitting the same area within a short period of time. They are different to aftershocks as no one event has the larger magnitude and therefore there in no one main shock. A good example of an earthquake storm occurred in Yellowstone National Park in 2004.
• Earthquake storms: these occur when a series of earthquakesoccur on a fault in clusters, each one is triggered further down the fault by the release of pressure at the initial earthquake epicenter. These can occur over a period of years with some of the later earthquakes being as severe as the initial earthquake.

Prediction

Although it is not yet possible to predict earthquakes, scientist are looking for areas of “strain” in fault lines to look for where the next quake may occur.

Buildings and bridges are built to be quake proof, through a variety of engineering methods – in order to reduce the amount of damage caused.

You might like my other science articles:

Lunar Eclipse: August 5-6, 2009

Hurricanes, Tropical Storms, Cyclones, and Typhoons

The Perseids Meteor Shower – August 2009

Viewing Venus and The International Space Station on August 17th

Bubonic Plague: Symptoms, Treatment & Transmission