Seismic shaking and its impacts

Overview

Seismic shaking and socio-economic impacts of a major 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 is seismic shaking, and why is it the principal cause of earthquake deaths?

   Model answer

   Seismic shaking is the violent movement of the ground surface as seismic waves pass through it, moving buildings horizontally and vertically many times in a matter of seconds. It is the principal cause of death because almost all earthquake fatalities result from the collapse of structures onto the people inside them, rather than from the ground itself opening or from any direct effect of the waves on the body. The saying that earthquakes do not kill people but buildings do is broadly accurate outside tsunami-affected coasts.

2. 2

   Why do buildings fail under horizontal shaking when they stand up perfectly well to their own weight?

   Model answer

   Because they are designed for vertical loads. Walls, columns and foundations carry the weight of the structure downwards, and masonry in particular is strong in compression. Horizontal shaking applies forces sideways, and the building's own mass makes those forces large. Walls are pushed out of plane and topple, joints between walls and floors pull apart, columns snap where they meet floor slabs, and each floor may slide relative to the one below, so the building racks into a parallelogram and then collapses. Unreinforced masonry and heavy concrete floors are especially lethal because they are heavy and have almost no tensile strength.

3. 3

   What is resonance, and why does building height matter?

   Model answer

   Every structure has a natural period at which it sways most readily, and taller, more flexible buildings have longer natural periods than short, stiff ones. If the dominant period of the ground shaking matches the natural period of the building, each successive wave adds to the sway rather than cancelling it, so the movement builds up dramatically. This is resonance, and it explains why in a single earthquake mid-rise buildings may collapse while both the low houses and the tallest towers around them survive: the ground shaking happened to match the natural period of that height of building.

4. 4

   Why does the duration of shaking matter as much as its peak intensity?

   Model answer

   Because damage accumulates. A structure can usually survive one or two strong cycles, but each cycle of sideways movement cracks masonry a little further, loosens joints, and weakens columns, so it takes progressively less force to move it. A great subduction earthquake may shake the ground for two or three minutes rather than a few seconds, and buildings that withstood the first ten seconds fail in the second minute. Long duration also gives more time for soft ground to lose strength and for landslides to be set off.

5. 5

   How does seismic shaking cause landslides, and why are they so destructive?

   Model answer

   Shaking adds repeated horizontal forces to a slope and momentarily reduces the friction holding material in place, so slopes that were stable under gravity alone fail. Steep terrain, weathered rock, loose slope deposits and wet ground all increase the likelihood. Landslides are destructive because they can bury whole settlements without warning, they block the roads that rescue teams need, they cut water, power and communication lines, and where they dam a river they create a lake that may burst later and flood the valley below. In mountainous regions landslides can kill more people than the collapse of buildings.

6. 6

   Why is fire a major secondary hazard of earthquakes, particularly in cities?

   Model answer

   Shaking ruptures gas mains and fuel tanks and knocks over heaters and cookers while shorting electrical wiring, providing both fuel and ignition at the same moment across a wide area. Fires therefore start in many places simultaneously. They spread rapidly where buildings are close together and contain timber, and firefighting fails because water mains have also been broken so there is no pressure, roads are blocked by rubble and collapsed buildings, and the emergency services are overwhelmed by rescue demands. Historically, fire has caused a large share of the total destruction in some urban earthquakes.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.