The nature of a seismic event

Overview

Focus and the epicentre of an earthquake and magnitude and the intensity of an earthquake.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What actually happens when an earthquake occurs?

   Model answer

   Two blocks of rock that have been locked together along a fault suddenly slip past each other. Plate movement has been steadily loading the rock with stress, and because friction prevents the fault from sliding freely the rock deforms elastically, storing the strain like a bent spring. When the accumulated stress exceeds the frictional resistance the fault ruptures, the rock snaps back towards its original shape, and the stored energy is released as seismic waves that radiate outwards through the Earth. This build-up and sudden release is called elastic rebound.

2. 2

   Distinguish between the focus and the epicentre of an earthquake.

   Model answer

   The focus, or hypocentre, is the point underground where the rupture begins and from which the seismic waves are released. The epicentre is the point on the Earth's surface directly above the focus. The distinction matters because damage depends on distance from the focus: a shallow focus puts the surface close to the source and produces intense local shaking, whereas a deep focus of the same magnitude produces weaker shaking spread over a wider area.

3. 3

   How are earthquakes classified by focal depth, and where does each type occur?

   Model answer

   Shallow-focus earthquakes have foci within about 70 km of the surface and occur at all margin types, including constructive, conservative and collision margins where there is no descending slab. Intermediate-focus earthquakes have foci from about 70 to 300 km deep and deep-focus earthquakes from about 300 to 700 km, and both are confined to destructive margins because only there is cold, brittle lithosphere carried down into the mantle. Shallow-focus events are much the most common and cause the great majority of damage.

4. 4

   Why does a shallow-focus earthquake usually cause more damage than a deeper one of the same magnitude?

   Model answer

   Because seismic waves lose energy as they travel, spreading out and being absorbed by the rock they pass through. A focus 10 km below a city delivers waves that have travelled only a short distance, so they arrive with high amplitude and shake the ground violently over a small area. A focus 300 km deep delivers waves that have already dissipated much of their energy before reaching the surface, so the shaking is weaker, though it is felt across a much wider region.

5. 5

   What is an aftershock, and why do aftershocks matter for management?

   Model answer

   An aftershock is a smaller earthquake following the main shock on the same fault or nearby, caused by the readjustment of rock to the new stress distribution. Aftershocks can continue for weeks or months, becoming less frequent over time, and occasionally one is nearly as large as the main event. They matter because they collapse buildings already weakened by the main shock, killing rescuers and survivors, they trigger further landslides, and they force repeated evacuation of shelters, so search and rescue and damage assessment must be carried out under continuing risk.

6. 6

   Distinguish between the magnitude and the intensity of an earthquake.

   Model answer

   Magnitude measures the energy released at the source, so a given earthquake has only one magnitude, calculated from the amplitude recorded on seismographs. It is measured on a logarithmic scale, so each whole step upwards represents about a thirty-fold increase in energy released. Intensity measures the strength of shaking and the damage experienced at a particular place, so a single earthquake has many intensity values, high near the epicentre and decreasing outwards. Intensity is what determines the human impact, and it depends on ground conditions and building quality as well as on magnitude.

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