6.20: Earthquake Damage
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Is earthquake magnitude the most important factor in determining damage?
The type of construction has a tremendous effect on what happens during an earthquake. Damage and deaths are directly affected by the construction in an earthquake. For example, enormous damage was done in the 2011 Christchurch, New Zealand earthquake. However, far less was damaged in an earthquake of the same magnitude near the area in Iceland seen above.
Damage from Earthquakes
We know that earthquakes can kill lots of people. However, the ground shaking alone almost never kills people. Unlike in cartoons, the ground does not swallow people up! Deaths depend somewhat on an earthquake's size and the type of ground people inhabit. But a very important factor is the quality of the structures. People are killed when structures fall on them. More damage is done and more people are killed by the fires that follow an earthquake. Of course, the number of deaths is different from the property damage that a quake does.
What Makes Earthquakes Deadly?
What makes an earthquake deadly?
- Population density. The magnitude 9.2 Great Alaska Earthquake, near Anchorage, of 1964 resulted in only 131 deaths. At the time few people lived in the area (Figure below).
A landslide in a neighborhood in Anchorage, Alaska, after the 1964 Great Alaska earthquake.
- Size, but not size. No one dies in a magnitude 1.0 earthquake. But large quakes do not always kill a lot of people. Only about 2,000 people died in the 1960 Great Chilean earthquake. This was the largest earthquake ever recorded. The Indian Ocean earthquake of 2004 was one of the largest ever. Still, most of the 230,000 fatalities were caused by the tsunami, not the earthquake.
- Ground type. Solid bedrock vibrates less than soft sediments, so there is less damage on bedrock. Sometimes sediments become saturated with water. They then undergo liquefaction and become like quicksand (Figure below). Soil on a hillside may become a landslide.
Liquefaction of sediments in Mexico City caused the collapse of many buildings in the 1985 earthquake.
In earthquake-prone areas, city planners try to reduce hazards. They look at both the likelihood of a quake and the damage one would do. For example, in the San Francisco Bay Area, maps show how much shaking is expected for different ground types (Figure below). This allows planners to locate new hospitals and schools more safely.
The expected Modified Mercalli Intensity Scale for an earthquake of magnitude 7.1 on the northern portion of the Hayward Fault.
- Seismic waves rarely kill anyone. Structures falling on people and fires or tsunamis after the earthquake cause many more fatalities.
- City planning can lessen the damage done by earthquakes.
- Population density and ground type affect the number of fatalities.
- What causes liquefaction? Why is it damaging?
- A 9.2 earthquake struck near Anchorage, Alaska in 1964, but few people died. If a similar quake struck today, how many fatalities would there be compared with the quake in 1964? Explain your reasoning.
- Why do city planners look at Mercalli maps rather than predictions of earthquake magnitude?