Haiti aftershock: Why a 6.1 quake isn't that powerful

A magnitude 6.1 aftershock struck near Port-au-Prince on Wednesday morning. With so many buildings damaged by the major Haiti earthquake last week, there are concerns about danger. But there's a world of difference between today's quake and the 7.0 earthquake that did so much damage last week.

The aftershock that hit Haiti shortly after 6 a.m. on Wednesday frightened Haitians traumatized by last week's much stronger earthquake and raised concerns that it could bring down buildings weakened by the original earthquake and cause more loss of life.

The US Geological Survey (USGS) said the latest earthquake has a magnitude of 6.1 and that it struck about 35 miles southwest of the capital. Like the earlier 7.0 earthquake that killed tens of thousands in the Port-au-Prince area, this one was relatively shallow, occurring about 6.2 miles below the surface.

Though there was fear and concern in Haiti over the latest earthquake ("aftershock" simply means an earthquake associated with an earlier, generally more powerful one in the same general location) early indications from Port-au-Prince were that limited additional damage was caused.

While a 6.1 magnitude earthquake sounds almost as strong as a 7.0 earthquake, the difference of the destructive power between the two is greater than an order of magnitude. Unlike temperature scales, in which units of increase are constant, the method used to measure earthquake magnitudes is logarithmic. What this generally means is that the amount of shaking at ground level caused by a 5.0 earthquake is 10 times less than that caused by a 6.0 earthquake and 100 times less of that caused by a 7.0 earthquake.

Earthquakes of the magnitude felt in Haiti today are also far more common. The USGS says there are roughly 132 earthquakes a year with a magnitude between 6 an 6.9 and just 17 between 7 and 7.9 and one, on average, of 8.0 or greater. On Monday, a 6.0 quake shook Guatemala. On Tuesday and Wednesday, there have been four other earthquakes under 6.0 -- in Santiago del Estero, Argentina, Tucuman, Argentina, in the Cayman Islands, and in the Solomon Islands.