A magnitude-6.3 earthquake shook Christchurch, New Zealand yesterday, collapsing buildings, triggering landslides and flooding, and killing dozens of people. A more powerful magnitude-7.1 quake rattled the city last September but didn't cause nearly as much damage, with no fatalities. Why do some earthquakes kill hundreds or thousands of people while others do little damage? There are several factors that determine just how destructive an earthquake can be:
Location: This one is kind of obvious—an earthquake that hits in a populated area is more likely to do damage than one that hits an unpopulated area or the middle of the ocean.
Magnitude: Scientists assign a number to represent the amount of seismic energy released by an earthquake. The Richter magnitude scale, as it is known, is logarithmic, so each step up represents an increase in energy of a factor of 10. The more energy in an earthquake, the more destructive it can be.
Depth: Earthquakes can happen anywhere from at the surface to 700 kilometers below. In general, deeper earthquakes are less damaging because their energy dissipates before it reaches the surface. The recent New Zealand earthquake is thought to have occurred at a more shallow depth than the one last year.
Distance from the epicenter: The epicenter is the point at the surface right above where the earthquake originates and is usually the place where the earthquake's intensity is the greatest.
Local geologic conditions: The nature of the ground at the surface of an earthquake can have a profound influence on the level of damage. Loose, sandy, soggy soil, like in Mexico City, can liquefy if the shaking is strong and long enough, for example. That doesn't bode well for any structures on the surface.
Secondary effects: Earthquakes can trigger landslides, fires, floods or tsunamis. It was not the 2004 Sumatran-Andaman earthquake that caused so much damage in 2004 but the Indian Ocean tsunami it triggered. Nearly a quarter of a million people in 14 countries were killed when coastal communities were inundated by the water.
Architecture: Even the strongest buildings may not survive a bad earthquake, but architecture plays a huge role in what and who survives a quake. The January 2010 Haiti earthquake, for example, was made far worse by poor construction, weak cement and unenforced building codes.