Seismology and Seismic Risk
Seismology
Earthquakes occur in fracture zones subjected to compressive and shear rifting. The slow movements of the plates cause the rocks forming their edges to deform slowly as they support the effort. This strain accumulates, releasing elastic energy abruptly when rocks break. This elastic rebound produces seismic waves.
In the seismic focus, or hypocenter, two types of waves originate that propagate through the Earth:
P-Waves
These travel at high speed and are the first ones recorded by seismographs. P-waves are longitudinal waves, moving in the same direction as the wave propagates. They are transmitted in both solid and liquid media, thus crossing the entire planet. Their speed is higher when the stiffness of the material they pass through is higher.
S-Waves
These are slower than P-waves. S-waves are transverse waves that propagate perpendicular to the motion of particles. They spread in solid media but not in liquid media, disappearing when they reach the Earth’s core.
Upon reaching the surface, P and S waves generate surface waves, which cause the damage. The earthquake’s epicenter is the point on the Earth’s surface located vertically above the seismic focus, or hypocenter, and where P-waves arrive first.
Seismic Risk
Seismic risk relates to destructive processes:
- Collapse of buildings and infrastructure
- Failure of services
- Fires
The thixotropic property, present in inconsistent materials saturated with water, causes them to completely lose their rigidity during a shock, leading to collapsing buildings and landslides.
Seismic shocks can trigger unstable configurations of land or buildings, causing landslides, avalanches, and falling blocks.
When an earthquake occurs on the seabed, high-energy waves arise in the water body. Upon reaching the shore, these waves cause rapid oscillations of several meters in sea level (tsunamis).
Earthquake Intensity
Earthquake intensity is an estimate of the damage caused and is measured by the Mercalli scale.
Earthquake Magnitude
Earthquake magnitude is a measure of the energy released in the earthquake. It is measured on the Richter scale and is expressed as a dimensionless number. This scale has no upper limit and is logarithmic, meaning that magnitude increases exponentially.
Seismic Hazard
Seismic hazard is one of the risks arising from internal natural geological processes. High-magnitude earthquakes often cause thousands of casualties and material losses, especially in areas with high exposure and vulnerability.
Seismic Forecasting
Seismic forecasting consists of developing hazard maps and earthquake warning systems.
Mitigation Measures
Measures are taken to reduce exposure and vulnerability to avoid or minimize damage if an earthquake occurs:
Structural Measures
- Adequate building foundations
- Strengthening of structural elements
Non-Structural Measures
- Closing valves on gas and water pipes to prevent accidents
- Securing electrical systems
- Emergency supplies
Contingency Measures
- Specific training of firefighters, army, police, and civil protection to respond effectively in case of disaster.
Seismic Prediction
Seismic prediction involves knowing in advance the location, timing, and magnitude of an earthquake.
Seismic Risk in the Iberian Peninsula
- Norpirenaica Fault: Represents the collision and suture line between the Iberian and Eurasian plates, parallel to the Pyrenees.
- Crevillente Fault: Parallel to the Sierra Nevada, representing the collision and suture zone between the Iberian and Alboran plates.
- Azores-Gibraltar Fault: A fracture originating from the Atlantic Ridge, bordering North Africa and moving into the Mediterranean. It represents the boundary between the Eurasian and African plates.
- Valencia Groove: A rift zone fracturing the European continent, passing through the Mediterranean coast of Spain near Valencia. This groove structure has higher seismicity due to its compressive and shear nature. Volcanism is linked to the relaxation of the lithosphere along the Valencia Groove.
Volcanic and Seismic Risks in the Canary Islands
The Canary Islands are an active volcanic archipelago. The volcanism is associated with the relaxation produced in a fracture zone covering the Atlas area in North Africa, extending deep into the Atlantic. The seismicity is linked to the instability of volcanic structures.