5. SEISMIC AND VOLCANIC RISK

Talking book 

The risk not only depends on the intensity and the frequency of the phenomenon, but also on the density and size of the population and the type of infrastructures in the place affected.

Risk = danger (Probability that the phenomenon happens in this place at a certain time.) × vulnerability (Value of the damage that people, or their belongings, can suffer as a consequence of the phenomenon).

So, in an uninhabited area, the risk is zero. However, in an area with medium danger but a very high population density and poorly designed infrastructures, the risk can be the similar to an area with high seismic danger that has buildings designed to resist earthquakes.

Earthquakes and explosive volcanic eruptions are the internal geological phenomena that cause the most victims and material damage.

5.1. Seismic risk

Talking book 

Every year there are close to a million earthquakes; about 150000 are felt by the population and the rest only by seismographs. To evaluate the strength of earthquakes, there are two types of measurements: magnitude and intensity.

• The magnitude of an earthquake measures the energy released in the hypocentre. To express this, we use the Richter scale, which is exponential. For each unit that goes up on the scale, the energy released is multiplied by 30. For example, an earthquake with a magnitude of 6 on the Richter scale releases 30 times more energy than one with a magnitude 5.
• The intensity evaluates the damage caused by the earthquake in a certain area. Nowadays, it is measured on a European macroseismic scale EMS-98, that has twelve levels, from1 (‘not felt') to 12 (‘completely devastating').

An earthquake will only have one value of magnitude, but its intensity will vary depending on the area where it is measured: the maximum is found in the epicentre and decreases further away from it.

The main risk of earthquakes is the movement of the ground and the collapse of buildings, as occurred in 1976 in Tangshan, China, where more than half a million people died. Other historic examples of the effects of an earthquake are the following:

 

 

5.1.1. Seismic risk in Spain

Talking book 

The Iberian Peninsula is situated near the contact region of the African and the Eurasian plates, although it is not a very active area.

As a result, there is a moderate seismic risk, less than in most other Mediterranean countries. This means that there are earthquakes which are felt by the population and more rarely, others that cause considerable damage.

Catastrophic earthquakes in this area cannot be ruled out, such as the one in Arenas del Rey (Granada) in 1884, which caused more than 1 000 deaths, or the most recent one in Lorca (Murcia) in 2011, which caused nine deaths.

5.2. Volcanic risk

Talking book 

The risks associated with volcanic activity are quite varied, as you can see in the following figures and examples:

• The eruption of Laki (Iceland) in 1783-84 released so much ash and gases into the atmosphere that temperatures in the northern hemisphere went down substantially, causing losses of crops and starvation.
• On 27 August 1883 the explosion of Krakatoa in Indonesia produced a tsunami that caused 36000 deaths.
• On 8 May 1902 a pyroclastic flow from the volcano Mount Pelèe destroyed the city of St. Pierre (Martinique Island) killing 28000 people.
• On 13 November 1985 a small eruption in the Nevado del Ruiz volcano (Colombia) melted part of snowy summit causing a lahar that buried 23000 people.
• On 21 August 1986 a cloud of carbon dioxide from a volcano caused the death of 1746 people due to suffocation in Cameroon.
• On 17 January 2002 the streams of lava from the Nyiragongo volcano caused close to one hundred deaths in the city of Goma (Zaire).

5.3. PREDICTING EARTHQUAKES AND VOLCANOES

Talking book 

We are still very far away from being able to predict the day and time that the next earthquake will occur. We do not even have reliable indicators that tell us how close it is. But we do know that where there have already been earthquakes of some intensity every certain amount of time, they will continue to happen with a similar intensity and regularity.

The elastic rebound theory can help us to have an approximate idea of the areas of a fault that are susceptible to displacement soon.

Earthquake prediction is based on measuring the accumulated pressure in the rock near the faults or detecting the microfractures that sometimes appear before the main displacement. These microfractures produce changes in the magnetic and electrical properties of the rocks and allow certain gases to escape, such as radon.

 

Volcanic eruptions can be predicted with a certain amount of accuracy, provided we have the instruments and people monitoring them. This is done by registering the signs that magma is accumulating in the magma chamber and then the factors that allow it to rise towards the surface. When this occurs, a series of micro-earthquakes known as volcanic tremor take place.

Another important source of indicators is the measurement of the amount, composition and temperature of the gases and thermal springs released by the volcano before eruption.

5.4. Seismic and volcanic prevention

Talking book 

Although we cannot stop earthquakes or volcanic eruptions, it is possible to take decisions that reduce the risk or take measures that lessen the damage caused. The risk formula doesn't include the reduction of the danger but it does include the reduction of vulnerability. For example, we can lessen seismic risk by not constructing buildings on poorly consolidated sediments, or by reinforcing them with iron or reinforced concrete for better resistance to the movement of the ground.

The first step is to determine the seismic or volcanic risk in the area and make maps showing the danger. Then, prevention is worked with in three areas in order to reduce the vulnerability or exposure to that phenomenon:

• Territorial planning by consulting seismic hazard maps and avoiding, for example, building in areas of high risk, especially buildings such as hospitals, power plants and schools.
• Earthquake resistant design used in buildings and infrastructures and the reinforcement of existing structures to reduces seismic risk.
• Planning before emergencies increases the capacity of the population to respond and the methods of self-protection against earthquakes and volcanic eruptions. In the case of volcanoes, the most adequate measure is to plan evacuation of risk zones before the eruption. For this, there are three levels of alert, represented in the form of a traffic light.