Do buildings in Haiti have earthquake codes? Why do some quakes cause tsunamis and others don't? And what's a "hospital ship," anyway?

Experts predict that the aftershocks from the Haiti quake could last another week or so. You can monitor the aftershocks and their magnitudes here.

A Navy hospital ship, the USNS Comfort, is standing by in Baltimore for a possible trip to Haiti. What's a hospital ship?
A ship that's converted into a hospital, of course. The Comfort has all the facilities of a top-of-the-line hospital on land: 1,000 beds, 12 operating rooms, radiological services, a medical lab, a pharmacy, an intensive care ward, dental services, and a morgue. It's also got a staff of more than 1,400.

Most hospital ships are maintained by the military for use in war zones. But their secondary mission is to support disaster relief in the United States and worldwide. It's a war crime to fire on a hospital ship, according to the Hague Convention of 1907. The same treaty says that hospital ships must be clearly marked, must avoid interfering with war ships, and must help wounded people no matter what country they're from.

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Explainer thanks John Ebel of Boston College and Brian Tucker of GeoHazards International.

Correction, Jan. 15, 2010: This article originally implied that any amount of sand weakens concrete. Sand is an ingredient of concrete. Too much sand, however, weakens it. (Return to the corrected sentence.)

Correction, Jan. 19, 2010: This article stated incorrectly that larger earthquakes produce fewer aftershocks than smaller ones. ( Return to the corrected paragraph.)