Seismic and Volcanic Risks: Prediction and Prevention

Posted on Nov 28, 2024 in Geology

Seismic Risk

Earthquake Dynamics

An earthquake occurs when built-up tension within the Earth’s crust is suddenly released. This energy radiates from the hypocenter as P and S waves, traveling through the Earth’s interior. Upon reaching the epicenter, these waves generate surface waves, which are the primary cause of destruction.

Measuring Earthquakes

Two key concepts are used to assess earthquakes:

  • Seismic Intensity: A qualitative measurement using the Mercalli scale (MSK) to categorize the observed effects of an earthquake.
  • Seismic Magnitude: A quantitative measurement of the energy released by an earthquake, typically represented by the Richter scale.

Earthquake Prediction Methods

Predicting seismic activity involves analyzing historical earthquake data and identifying seismic precursors.

  • Historical Tremor Analysis: Studying past earthquakes helps determine earthquake frequency and periods of quiescence. This data informs the creation of hazard maps, which illustrate expected exposure levels, and damage maps, reflecting the impact of previous earthquakes.
  • Seismic Precursors: These are variations in physical properties observed in the vicinity of a fault before an earthquake. Examples include ground elevation changes, fluctuations in electrical conductivity and local magnetic fields, decreases in the Vp/Vs ratio, increased radon emissions, a rise in microseismic activity, and unusual animal behavior.

Earthquake Preventative Measures

These measures aim to minimize exposure and vulnerability in high-risk areas:

  • Land-use planning and zoning regulations.
  • Construction of earthquake-resistant buildings.
  • Civil protection measures and public awareness campaigns.
  • Insurance provisions to mitigate financial losses.

Volcanic Risk

Volcanic Eruptions

Most volcanic eruptions occur at subduction zones and diverging mid-ocean ridges. The Volcanic Explosivity Index (VEI) quantifies a volcano’s potential hazard. Eruption types include:

  • Hawaiian: Tranquil and fluid eruptions.
  • Strombolian: More explosive with pyroclastic emissions.
  • Vulcanian: Primarily pyroclastic ejections with minimal lava flow.
  • Plinian: Highly explosive and violent, potentially producing pyroclastic flows.

Volcano Prediction Methods

Similar to earthquake prediction, analyzing a volcano’s eruption history is crucial. This helps establish recurrence intervals. Volcanic precursors include:

  • Tectonic earthquakes.
  • Ground deformation due to magma ascent.
  • Changes in electrical potential and local magnetic fields.
  • Gas emissions.
  • Water temperature fluctuations in crater lakes.

Volcano Preventative Measures

  • Population evacuation.
  • Diverting lava flows with ditches.
  • Solidifying lava with cold water.
  • Distributing masks to the population.
  • Draining crater lakes.
  • Constructing hemispherical, non-combustible shelters.

The Wilson Cycle

The Wilson Cycle describes a cyclical pattern of lithospheric plate movement:

  1. Initial Volcanism: Magma emerges along an elongated zone, creating a fracture in the lithospheric plate.
  2. Rift Formation: The fracture widens, and magma continues to rise, forming new oceanic lithosphere upon cooling. This creates a rift valley surrounded by elevated terrain.
  3. Seafloor Spreading: New magma pushes existing lithosphere apart, causing divergent plate movement. This expands the ocean floor.
  4. Subduction: Oceanic lithosphere is consumed as it subducts beneath another plate, often involving continental lithosphere and accumulated sediments.
  5. Ocean Closure: The separating ridge may also subduct, leading to the eventual collision of continents.
  6. Continental Collision: The collision forms mountain ranges with suture lines, like the Himalayas. Erosion eventually reduces the mountains, restarting the cycle.