Volcanic and Seismic Activity: Processes, Hazards, and Safety

Posted on Dec 15, 2024 in Geology

Item 6: Volcano and Seismic Activity: Legislation

Volcanism

Volcanoes occur in three different tectonic settings:

  • Convergent plate boundaries: 80% of active volcanoes are in these conditions. Magmas generated are usually very viscous, trapping the dissolved gases to achieve high pressures in the magma chamber to be released during an explosive eruption.
  • Stratovolcanoes: These are collections of materials based on successive eruptions of ash layers and lava, generating high-rise buildings. In some cases, their explosions are so great that they remove the top of the cone to produce a central cavity called a caldera.
  • Divergent plate boundaries: Or rift volcanoes. Low viscosity magmas are generated, resulting in low explosive eruptions and effusive nature, especially from the deep Earth. They are low viscosity magmas.
  • Intraplate-Volcanoes: Hot spot volcanoes caused by the rise of molten material from the deep Earth. They are low viscosity magmas.

Classification of Volcanic Eruptions (Least to Most Explosive)

  • Island
  • Hawaii
  • Strombolian
  • Vulcan
  • Vesuvius
  • Plinian
  • Pelean

Volcanic Processes and Risk Factors

  • Eruptions, low viscosity magmas: The gas easily escapes into the atmosphere. The outlet duct sealing is resolved with low-energy rhythmic explosions that cause pyroclastic dispersal in a small area. The risk is linked to the collapse of pyroclastic and lava flow.
  • Magma eruptions with high viscosity: The volcanic pipe is usually plugged. When the gases get released, they do so explosively, breaking the volcano and causing avalanches that can reach high speeds, producing a total destruction of the affected area.
  • Pyroclastic Rain: This fallout may produce conditions in population centers (sunken ceilings, reduced visibility), agriculture (destruction of vegetation), and livestock (pasture contamination).
  • Pyroclastic casting: Or burning clouds. These processes are the most destructive volcanic eruptions. Their high temperatures and high mobility endow them with tremendous lethality, destroying and burning everything in their path.
  • Lava Flows: The lavas are always confined to the valley bottoms, and their discourse is easy to predict.
  • Mud-flows or lahars: A lahar is a twitching mass of rocks and mud caused by water on the slopes of a volcano. Flows are highly mobile and capable of carrying large volumes of materials over long distances. There are times when the volcanoes that are located at high altitude are crowned by ice caps, in case of eruption can be liquefied quickly and generate devastating mudflows.
  • Poisonous volcanic gases: Some gases that occur in volcanic eruptions can be dangerous or toxic to plants and animals. These gases accumulate in depressed areas called valleys of death.
  • Tsunami: Large waves related to volcanic earthquakes generated by volcanic eruptions or by displacement of water during explosive eruptions.

Prediction and Prevention of Volcanic Eruptions

To predict the phenomena, precursor events should be observed, given that a volcanic eruption is not a phenomenon that occurs spontaneously. The most common precursor phenomena are:

  • Earthquakes: The magma ascending causes many small earthquakes that increase as we approach the time of the eruption. These are studied and detected by seismographs.
  • Deformation of the ground: These movements are controlled by a distancemeter.
  • Emission of gases: Before eruptions, there are changes in the volume and type of exhaust gases.
  • Temperature anomalies: Increased temperature detected in hot springs, fountains, and groundwater flows.
  • Animal Behavior: Instinct saves them from many disasters.
  • Surface temperature: Measured through artificial satellites.
  • Magnetic Anomalies: Before eruptions, changes in the local magnetic field are detected.
  • Electrical: The rise of hot magma changes the resistivity of the rocks in the zones.
  • Gravity anomalies.

Seismic Activity

An earthquake is a sudden movement or shaking of the substrate produced by an abrupt displacement of rock masses, usually about 15-30 m deep. These movements occur in response to tectonic forces. Rocks accumulate tensions until they exceed the consistency of rock fractures called faults that occur, extending from the focus or hypocenter. During the break, the edges of the fault rub each other, emitting energy that is emitted by seismic waves. Most earthquakes are located in the active edges of the plates (2/3 in the Pacific Ring of Fire), and some occur within the plates.

Types of Seismic Waves

  • P-waves (Primary): Longitudinal waves traveling at high speed (5-8 km/s).
  • S-waves (Secondary): Vibrate perpendicular to the direction of travel. They move at slower speeds (about 3 km/s).
  • Rayleigh and Love waves: Surface waves, slower and more damaging.

P waves are the first to cause the vibration of a building, and the S waves arrive later and cause the vibration from one side to another. The S waves are more destructive because the buildings are designed to withstand vertical forces but not horizontal. The shocks are expressed in terms of speed and momentum of the ground (ground acceleration). The acceleration of soil expresses the value at which the ground moves horizontally and vertically, the effect of an earthquake. Earthquakes are described in terms of their magnitude and intensity. The scale measures the energy released by an earthquake. It is measured by the Richter scale, where an earthquake of catastrophic scale should be above 5.5. Intensity measures the damage caused by an earthquake in a particular location. The intensity is determined subjectively and not objectively, observing the damage caused by an earthquake on human structures.

Effects Associated with Earthquakes

  • Appearance of surface fractures as a result of the movement of faults.
  • Appearance of topographic scarps and subsidence.
  • Landslides, rock falls, and avalanches.
  • Soil Liquefaction.
  • Changes in the water table.
  • Tsunamis, floods, and fires.

Premonitory Events of Earthquakes

  • Reducing the speed of seismic waves months before the earthquake.
  • Deformation of the substrate.
  • Changes in the composition of spring waters.
  • Emergence of microseismic series before the main earthquake.
  • Changes in the tidal regime.
  • Changes in the electrical resistivity and magnetic susceptibility of materials in the area.
  • Appearance of luminosities.
  • Anomalies in the behavior of certain animals.

Seismic Legislation

These regulations include primarily:

  • Zoning of the territory with the characteristics of maximum earthquake.
  • Methods of calculating static and dynamic forces.
  • Recommendations on building types and reinforcements.

Seismic Activity in Spain

  • Far East-Northeast: Gulf of Cadiz, Gibraltar Strait. Collision zone between the African and Iberian plates.
  • Pyrenees: Suture zone with the Eurasian Iberian subplate.
  • Canary Islands: The effect of volcanic islands.

Item 4: Erosion, Slope Evolution, Stability, and Landslides

Erosion: It is a natural process that is part of the geomorphic cycle linked to the evolutionary dynamics of the planet’s surface. The water erosion process begins with the impact of rain on the ground. This water removes finer particles of soil. Then there is an opening rill erosion low flows in areas where the runoff. When you increase the amount of water in the furrows, they become more profound.

Evolution of the slopes: All natural sloping surfaces are influenced by gravity and can be modified by downslope mass movements. Landslides are all processes influenced by gravity and causing the carriage beneath the rock slope, regolith, or soil. It considers any move by sliding slope.

Stability and instability: In the triggering of landslides, various external and internal factors attend to certain critical values that facilitate the occurrence of the process:

  • External factors increase the destabilizing forces without exerting actions remobilized ground resistance, vibration, climate change.
  • Internal factors: decreasing the resistance forces due to changes specific to the weathering of soil material, the material falls progressive.

Soil Stability:

  • Cohesive soil: clay and limonite compact. When the thickness is powerful, it leads to slippage.

Landslides:

There are different classifications for landslides. The most common criteria are:

  • Shape, size, and nature of the body mass moved.
  • Speed, shape, and motion path.
  • Damage caused.
  • Types and parts of the movement.
  • Other.

Item 5: Coastal Processes, Dynamics, Erosion, Accumulation, Types of Coast, Engineering Problems

Coastal Processes

Processes relating to all interactions between the mass of water and land (coastal areas). The coastal environment is very complex, however, actions that affect it are fairly homogeneous.

Coastal dynamics

The materials received by the coastal strip undergo continuous replacement through reprocessing of the strip. The responsible actions of these actions are the waves, tides, and coastal currents. These actions also affect the coastline itself is being constantly modified by the action of the coast. The current lines of coast are the result of multiple processes, mainly of rises and dips in sea level in addition to human activity.

The Seas

The stationary waves are ripples in the water that entail energy transport, often caused by wind. They are classified according to their energy, frequency, and hydrodynamic characteristics. The waves striking the coast experience a series of changes:

  1. Reflection: When the wave encounters a vertical obstacle, it is forwarded to the sea.
  2. Diffraction: When the wave encounters a sharp edge that emerges from the bottom, its energy is transmitted laterally to the crest of the wave.
  3. Refraction: When the wave moves into shallow water, friction on the bottom causes a delay of it and will die more slowly.

The effect of refraction concentrates the wave energy incident on the headlands erosion appearing on them, whereas in the bays dominated sedimentation.

The Tides

Are periodic oscillations of the average level of the water due to gravitational interactions Earth-Moon-Sun.

Types:

  1. Micromareales oscillations < 2 meters. (Mediterranean)
  2. Mesomareales à 2-4 meters (Biscay)
  3. Macromareales à > 4 meters (Cantabrian-Atlantic)

The importance of tides is due to the leverage effect exerted on the waves as well as tidal currents as a result of the rise and fall of water.

Forms of Erosive and Accumulation

Erosion:

  1. Cliffs
  2. Abrasion platforms
  3. Estuaries
  4. Rias

Accumulation:

  1. Beaches
  2. Arrows and bazaars
  3. Dune plains
  4. Deltas

Types of coast

  1. Coasts of estuaries: formed by the flooding of a river valley (Galicia)
  2. Fjord: Norway
  3. Barrier island: Mar Menor
  4. Sandy or stabilized beaches
  5. Tidal plains (Santoña)
  6. Delta
  7. Reefs
  8. Cliffs

Engineering problem

Fill anthropic estuarine intertidal zones:

  1. Bahia Santander have lost 76% of its original cost and 50% of its surface.
  2. The 22 estuaries and salt marshes of Cantabria have reduced their area by 52%.