Read more about the Haiti earthquake in Slate.
Tens of thousands of people are thought to have died in the 7.0-magnitude earthquake that hit Haiti Tuesday, destroying much of the capital city of Port-au-Prince. The disaster raises a number of questions.
Are Haitian buildings designed to resist earthquakes?
No, for the most part. Haiti has no national building codes. Some construction companies (especially those working with relatively generous budgets) do voluntarily follow codes like the French or Canadian standards or the International Building Code.
Even when developing countries have codes in place to make sure buildings can withstand strong winds or earthquakes, they're rarely enforced, and construction companies end up cutting corners to save money. (The added cost of earthquake-proofing a building is usually around 4 percent.) After a quake hit Mexico City in 1985, for example, investigators discovered that some of the buildings that were destroyed had too much sand mixed in with the concrete, making them weaker. *
What's safer, big buildings in the city or ramshackle houses outside?
That depends entirely on the construction. Big-city buildings with steel-reinforced columns and strong joints are sturdier than slapdash country houses made of stacked concrete blocks. (Adobe huts, more common in earthquake-prone areas of Southeast Asia, are especially dangerous, since they not only crush the inhabitants but suffocate them as well.) But a rural house made of a wood frame with sheet metal nailed to the outside is going to be a lot safer than a giant office building without proper reinforcement. In other words, the height and location of a building matter less than how well it's built.
The Dominican Republic seems to have emerged relatively unscathed. How come?
Because its population centers are so far east. Santo Domingo, the Dominican Republic's capital and its largest city, is about 150 miles east of Port-au-Prince, while the earthquake was centered about 10 miles west of the Haitian capital. Naturally, the effects of a quake get weaker the further one gets from the center.
This map shows how strong the vibrations were in different areas of the island during the initial quake. Shaking intensity is measured both by the speed of the Earth's movement, or "peak ground velocity," and by a subjective measurement called the Modified Mercalli Intensity Scale, based on how the tremor "feels." In Port-au-Prince, the quake rated an "X," meaning the shaking was "violent" or "extreme." In Santo Domingo, by contrast, the quake rated a "III," meaning the vibrations felt "similar to the passing of a truck."
Why do some island earthquakes create tsunamis while others do not?
Because of where and how they happen. If an earthquake occurs underneath the ocean floor, such that the earth's plates shift vertically, the movement from one plate sliding up and another sliding down causes a rush of water that can turn into a tidal wave. The earthquake in Haiti, by contrast, was both underneath the land—about 10 miles west of Port-au-Prince—and horizontally shifting. So there was a lot of shaking ground but no tsunami.
There are two principles that describe the typical behavior of aftershocks. The first, called Omori's Law, predicts that most shocks will occur immediately following the earthquake and become less and less frequent over time. The other, known as Båth's Law, states that the largest aftershock is, on average, about 1.2 magnitudes smaller than the main quake. (The Richter scale is logarithmic, meaning that an earthquake measuring magnitude 6.0 is 10 times larger than one measuring 5.0.)
While a stronger earthquake produces more aftershocks than a weaker one, big aftershocks are rarer than small ones—again, by a factor of 10 per order of magnitude. For example, a 4.0 magnitude aftershock in Haiti on any given day is 10 times less likely than an aftershock of 3.0 magnitude. *