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Tectonic Hazards

Earthquake characteristics and seismic waves

  • 2 things to know
  • Component 3 Section A

Overview

Focus of an earthquake, epicentre of an earthquake and P waves.

What you need to know

Answer each one before you reveal it.

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Key term

Define the focus of an earthquake.

Key term

Define the epicentre of an earthquake.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define the focus of an earthquake.

    Model answer

    The focus, or hypocentre, is the point underground where the fault first ruptures. Seismic waves radiate outwards in all directions from that point.

  2. 2

    Define the epicentre of an earthquake.

    Model answer

    The epicentre is the point on the surface directly above the focus. Shaking is usually strongest there and weakens with distance.

  3. 3

    Classify earthquakes by focal depth.

    Model answer

    Shallow earthquakes have a focus less than 70 km deep, and intermediate ones between 70 and 300 km. Deep earthquakes have a focus between 300 and 700 km.

  4. 4

    Why are shallow earthquakes more damaging than deep ones of the same magnitude?

    Model answer

    Energy from a shallow focus travels only a short distance to the surface and is less attenuated. Deep events spread the same energy through a far greater volume of rock.

  5. 5

    Describe P waves.

    Model answer

    P waves are primary body waves that compress and stretch rock along the direction of travel. Speeds reach about 6 to 8 km s⁻¹ in the crust, and they pass through solids and liquids.

  6. 6

    Describe S waves.

    Model answer

    S waves are secondary body waves that shake rock at right angles to the direction of travel. Speeds are around 3 to 4 km s⁻¹, and they cannot pass through liquids.

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