Seismic Risk: Understanding Earthquakes, Causes, and Prevention
Seismic Risk
Earthquakes are destructive geological phenomena, often following floods. Between 1959 and 2003, they caused 800,000 fatalities and heavy material losses.
Basic Concepts: Types of Seismic Waves
Earthquakes are sudden movements or tremors of the Earth’s crust and surface, manifesting internal tectonic activity. Tectonic movements cause internal P and S waves, which extend from a hypocenter. Upon reaching the Earth’s surface, they transform into surface L and R waves, causing devastating earthquakes.
Causes of Earthquakes
Earthquakes can have many causes: explosions, landslides, injection of fluids into the subsoil, reservoir filling, mining activities, and volcanic activity (the second most important cause). However, the most important cause is tectonic activity: the movement and sliding of lithospheric plates, resulting in drag and fault zones.
Magnitude and Seismic Intensity
Earthquakes are measured using two scales:
- Magnitude: The amount of energy released by an earthquake (in ergs), measured on an exponential scale of nine degrees (1 to 9). Each grade is 1000 times greater in magnitude than the previous two units. Proposed by Richter in 1935.
- Intensity: The number and severity of earthquake effects on buildings and terrain. Measured on a scale of 12 degrees (I to XII) using the Mercalli scale (defined in 1902, later amended as the MM scale). Currently, the Medvedev-Sponheuer-Karnik (MSK) scale is used. It has practical applications as the effects on the ground and buildings are evident, although it is less rigorous as it depends on building construction quality.
The frequency of earthquakes on Earth is as follows:
- Magnitude > 8 (major): 1/year
- Magnitude 7-7.9 (large): 18/year
- Magnitude 6-6.9 (strong): 120/year
- Magnitude 5-5.9 (moderate): 800/year
- Magnitude 4-4.9 (light): 6,200/year
- Magnitude 3-3.9 (small): 49,000/year
- Magnitude < 3 (minor): 9,000/day
Spatial Location of Earthquakes
Seismic risk is concentrated along the margins of lithospheric plates, especially active faults, although intraplate earthquakes are associated with smaller volcanoes and faults. The most seismically active areas are the edges of lithospheric plates, including the Pacific Ring of Fire, the Alpine-Himalayan belt, and mid-oceanic ridges. On continents, seismicity is more diffuse than in oceans. However, detailed studies show epicenters are concentrated along fault lines.
Risk Factors: Hazard, Exposure, and Seismic Vulnerability
Seismic hazard is associated with the magnitude, nature, and depth of the hypocenter. Earthquakes are classified as surface (less than 70 km deep), intermediate (70-300 km), and deep (over 300 km). The affected area is larger for deeper earthquakes, but the intensity (effects) is less for the same magnitude because P and S waves attenuate before reaching the surface.
Exposure to earthquakes depends on the existence of densely populated areas near plate edges or risk areas, which has increased in recent years: 50+ cities with over 2 million inhabitants are within 200 km of plate edges.
Vulnerability is greater in developing countries than in developed ones because the latter can afford earthquake-resistant construction.
Earthquake Prediction and Prevention
Predicting earthquakes is an unsolved problem, as reliable predictions are not yet possible, although earthquakes do not occur randomly in space or time. Prediction is based on:
- Location and seismic study of historical areas (spatial prediction)
- Recurrence of earthquakes (temporal prediction) and study of causative faults using seismographs and theodolites.
According to the expansion theory, rock deformation and microcracks cause precursor phenomena: increased microsecond time units, decreased electrical conductivity or magnetic properties of rocks, decreased Vp, and groundwater input into cracks. This water entry causes ground elevation, increased soil electrical conductivity, increased Vp, and radon emissions into groundwater.
Prevention measures include:
- Structural Measures:
- Applying building codes for earthquake-resistant structures, prepared for vertical (w) or horizontal (L wave) motions.
- Periodically “lubricating” blocked fault lines to facilitate sliding.
- Nonstructural Measures:
- Risk maps
- Spatial planning
- Civil protection measures
Earthquake risks include:
- a) Damage to buildings
- b) Landslides, avalanches, or shifting earth
- c) Rupture of dams and water, gas, or oil pipelines
- d) Tsunamis
- e) Disappearance of aquifers
- f) Submarine landslides
- g) Human casualties
Seismic Risk in Spain
The Iberian Peninsula (IP) is located on the Eurasian plate, with its western part coinciding with the southern edge of this plate and the African plate. Some authors consider the IP to be a separate Iberian microplate. The southern and southeastern areas of the Iberian Peninsula (Granada, Malaga, Almeria, Murcia, Alicante), along with the Pyrenees, have the highest incidence of seismic activity and the most destructive earthquakes, characterized by intermediate-frequency earthquakes.
Seismic Risk in Murcia
Compared to the rest of Spain, the Murcia Region is in a medium-high seismic zone, considering the IP as having moderate seismicity. The annual rate of earthquakes in Murcia is double the average for the Peninsula. Earthquakes are related to regional faults. Faults in the Murcia Region can be grouped into a conjugate system with NE-SW and NW-SE directions.
Seismotectonic zones in the Murcia Region:
- Guadalentín Valley: The most important, delimited to the north by the Alhama de Murcia-Librilla-Totana-Lorca fault, a strike-slip fault parallel to the road. The most relevant earthquake linked to this fault occurred in Lorca in 1976, with an intensity of 4.2.
- Vega Alta del Segura: Numerous normal faults exist, with up to eight earthquake epicenters located along the Vega Alta. The most recent was in 1980 in Archena-Torres de Cotillas. Notable earthquakes include Ceutí (1912) with a magnitude of 8 and Ojós (1950) with a magnitude of 6.5.
- Vega Media and Baja del Segura: Located east of the Guadalentín, it has very similar geological characteristics.
- Fortuna-Mula: A seismotectonic belt with a fault of the same name. The fracture is supported by:
- Extrusive volcanic rocks south of Mula and between Fortuna and Abanilla.
- Seismic epicenters between Fortuna and Abanilla.
- Hot springs in Baños de Mula and Fortuna.