Is magnitude all that matters for determining earthquake damage?
The type and quality of construction has a tremendous effect on what happens during an earthquake. Damage and fatalities are directly affected by the construction in an earthquake. For example, many more people died in the 1988 Armenia earthquake, where people live in mud houses, than in the 1989 earthquake in Loma Prieta. Most buildings in California’s earthquake country are designed to be earthquake-safe.
Damage from Earthquakes
We know that earthquakes kill lots of people. However, the ground shaking almost never kills people, and the ground does not swallow someone up. Fatalities depend somewhat on an earthquake's size and the type of ground people inhabit. But much of what determines the number of fatalities depends on the quality of 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 than the earthquake itself.
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.
- Not size. Only about 2,000 people died in the 1960 Great Chilean earthquake, the largest earthquake ever recorded. The Indian Ocean earthquake of 2004 was one of the largest ever, but most of the 230,000 fatalities were caused by the tsunami, not the earthquake itself.
- Ground type. Solid bedrock vibrates less than soft sediments, so there is less damage on bedrock. Sediments that are saturated with water 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.
Earthquake effects on buildings are seen in this animation: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/6/SeismicBuilding-Narrated480.mov.
In earthquake-prone areas, city planners try to reduce hazards. 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.
liquefaction: Clay, silt, and sand saturated with water become like quicksand, lose their strength, and behave more like a liquid than a solid.
- 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.
Use this resource to answer the questions that follow.
Chile Assesses Earthquake Damage
1. How many people were killed?
2. How many people have been displaced?
3. How large was the earthquake?
4. What damage could be seen?
5. How capable is Chile in dealing with an earthquake?
6. Why is Chile better prepared for an earthquake than Haiti?
1. In the map of expected Modified Mercalli Intensity for the Bay Area of a hypothetical earthquake on the Hayward Fault, why do you think there is red and black north of the bay and up the Sacramento River? Why do you think there are much safer areas in rings around the bay?
2. What causes liquefaction and why is it damaging?
3. If a 9.2 earthquake struck near Anchorage, Alaska today, what do you think the fatalities would be compared with the quake in 1964?