In most cases, the earthquakes are due to movements along fault lines between the different tectonic plates that form the terrestrial surface region, which move apart. Throughout geologic time, Earth has been subject to strains responsible for the construction of mountain and continental drift. Under the action of these stresses the rocks deform. The failure of rock material occurs after they have exceeded their limits of endurance, causing vibrations or seismic waves, which propagate within the earth. It is these vibrations that you feel when an earthquake occurs. Earthquakes can also be generated in the movement of flaws within the tectonic plates. Volcanic activity and the movement of molten material at depth may be other causes of earthquakes.

The point on the surface of the land situated vertically above the epicenter is the area where the earthquake is felt with greater intensity. The movements of the land around the epicenter, are caused by seismic waves when they reach the surface. These depend on the depth, the characteristics (geological, topographical, etc.) and the magnitude of the earthquake.

When the seismic activity is generated in the ocean, an earthquake may be accompanied by tsunamis, causing much destruction to coastal structures or river (boats, houses, bridges, etc.).. In Portugal, the occurrence of tsunamis resulting from seismic activity has been recorded mainly in the Algarve, in the Peninsula of Setúbal and in Lisbon.

How long does an Earthquake last?

The duration of an earthquake varies from seconds to a couple of minutes. However the 2011 Japan earthquake lasted for around 5 minutes, this was recorded as 9.0 on the Richter scale. The main earthquake can be followed by rearrangements of rocky material that gives rise to weaker earthquakes called aftershocks.

Can anyone predict Earthquakes?

Although many scientists are doing research, it is still not possible to predict earthquakes. However, you can try to minimize their effects by identifying areas of highest risk, by building stronger foundations, promoting the education of the population, particularly with regard to security measures to be taken during an earthquake, and making contingency plans.

Earthquakes in Portugal

You may not be aware, but earthquakes are a regular event in Portugal, with a quake around the 3.0 mark occuring at least every couple of weeks. If you want to see where they have taken place then the Instituto Português do Mar e da Atmosfera has a very good page, IPMA Seismic Page

Portugal is close to the Azores - Gibralta Fault line which marks the boundary between 2 continental plates. The "Great Lisbon Earthquake" of 1755 took place on this boundary line. This was the first earthquake every studied scientificaly for it's effects over a large area, and lead the birth of modern Siesmology and Earthquake Engineering.