Factors affecting severity of earthquakes: – focus and epicentre

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Earthquake Hazards: Focus and Epicentre

Earthquake and Volcanic Hazards and Impacts

Factors Affecting Severity of Earthquakes: Focus and Epicentre

Earthquakes are powerful natural hazards that can cause significant damage and loss of life. The severity of an earthquake is influenced by several factors, including the magnitude of the earthquake, the distance from the epicentre, the depth of the focus, and the local geology. Understanding the relationship between the focus and the epicentre is crucial for assessing earthquake hazards.

Focus and Epicentre: Definitions

The focus (also known as the hypocentre) is the point within the Earth where an earthquake originates. It is the source of the seismic waves.

The epicentre is the point on the Earth's surface that is directly above the focus.

Suggested diagram: A diagram showing a vertical cut through the Earth with the focus located at a depth and the epicentre directly above it on the surface. Seismic waves radiating outwards from the focus are also depicted.

Relationship between Focus and Epicentre

The distance between the focus and the epicentre is a key factor in determining the intensity of ground shaking at a particular location. The depth of the focus also plays a significant role.

Impact of Focus Depth

The depth at which an earthquake originates significantly affects the intensity of shaking felt at the surface.

  • Shallow-focus earthquakes (0-70 km): These are the most common and generally the most destructive. The seismic waves have less distance to travel to the surface, resulting in stronger ground shaking.
  • Intermediate-focus earthquakes (70-300 km): These earthquakes cause moderate ground shaking.
  • Deep-focus earthquakes (300-700 km): These earthquakes cause the least amount of ground shaking at the surface. The seismic waves are significantly attenuated (weakened) as they travel through the Earth.

Impact of Distance from the Epicentre

The intensity of ground shaking decreases with increasing distance from the epicentre. This is because the energy of the seismic waves spreads out in all directions.

Distance from Epicentre Intensity of Shaking
Close to the Epicentre Very Strong
Moderate Distance Strong
Further Away Weak

Conclusion

The focus and epicentre are fundamental concepts in understanding earthquake hazards. The depth of the focus and the distance from the epicentre are critical factors influencing the severity of ground shaking and the potential for damage. A shallow focus combined with proximity to the epicentre will result in the most severe impacts.