The nature and origin of earthquakes

Earthquakes are sudden vibrations or shock waves in the Earth's crust caused by the release of built-up tension or compression at a specific point known as the focus. This is the location where the stress between tectonic plates is discharged. Directly above the focus, on the Earth's surface, lies the epicentre, marking the area of most intense surface shaking.

Classification by depth of focus

• Shallow-focus earthquakes - These occur relatively close to the Earth's surface.
• Deep-focus earthquakes - These take place at considerable depth below the surface.

Types of seismic waves and their effects

Earthquakes generate different types of waves that travel through the Earth, each with distinct characteristics and impacts on the ground and structures.

Categories of seismic waves

• Primary (P) waves - Known as pressure waves, these are the fastest seismic waves and can travel through both solids and liquids. They cause the ground to shake backwards and forwards.
• Secondary (S) waves - Also called shear waves, these move with a sideways motion and are unable to move through liquids. They make the ground move horizontally, often leading to considerable structural damage.
• Surface waves - When P and S waves reach the Earth's surface, some transform into surface waves, which are slower but cause the most damage:
  • Love waves - Cause the ground to move sideways.
  • Rayleigh waves - Cause the ground to move up and down.

Earthquakes in relation to plate boundaries

The occurrence of earthquakes is closely linked to the movement and interaction of tectonic plates, with specific patterns observed at different types of plate boundaries.

Earthquake activity at plate boundaries

• Subduction zones - At these boundaries, an oceanic plate is forced beneath a continental plate, descending into the mantle. This process can create some of the deepest earthquakes, reaching depths of up to 700 km, as the cold, rigid slab distorts and cracks.
• Mid-ocean ridges - Here, brittle faults occur as magma cools, solidifies and then cracks due to upwelling magma from below. Earthquakes in these areas are generally small because the brittle faults cannot extend more than a few kilometres.
• Away from plate boundaries - Not all earthquakes occur at tectonic margins. Some are triggered by human activities such as the construction of large dams, mining operations, hydraulic fracturing (fracking), and nuclear weapon testing.

Primary and secondary hazards of earthquakes

Earthquakes result in a range of immediate and subsequent hazards that pose significant risks to life, property, and infrastructure.

Primary hazards of earthquakes

• Ground shaking - The direct result of seismic waves, causing loss of life.

Secondary hazards of earthquakes

• Ground failure and soil liquefaction - This can lead to total or partial destruction of buildings.
• Landslides and rockfalls - These can cause loss of livelihood.
• Debris flow and mudflow - These contribute to destruction.
• Tsunamis - These can cause widespread destruction.
• Additional impacts - Secondary effects also include interruption of water supplies, breakage of sewage disposal systems, loss of public utilities such as electricity and gas, floods due to collapsed dams, release of hazardous materials, fires, and the spread of chronic illness due to lack of sanitary conditions.

Characteristics of earthquake hazard-event profiles

Understanding the nature of earthquake events through hazard-event profiles helps in assessing their potential impact and planning effective responses.

Key features of earthquake hazard profiles

These profiles illustrate the sudden, intense, and often unpredictable nature of earthquakes, highlighting the importance of preparedness and rapid response strategies in affected areas.