A class "major" earthquake of magnitude 7.0 struck 10 miles Southwest of Port-au-Prince, Haiti Tuesday night at 4:53 p.m. local time, shortly before dark fell over the Caribbean island of Hispaniola. This earthquake was pretty much a worst-case-scenario because of several factors. The initial 7.0 was shallow, only 6.2 miles deep. Also, it was devastatingly close to the highest population density in the affected region, the capital of Haiti. Finally, this was a "strike-slip" type of earthquake, where two plates slide past each other in a horizontal fashion instead of one being uplifted by the other.
When this happens, buildings are shaken side-to-side instead of up and down. This type of horizontal shaking of a structure usually leads to building failure, as the weight of the roof causes an implosion and walls collapse. In Haiti, most buildings are not reinforced concrete, and not earthquake-resistant. This likely means many structures failed in the initial earthquake. If they did not do so in that first major quake, they probably later collapsed during one of the 35 aftershocks (as of 10 a.m.) that occurred overnight, 14 of which were magnitude 5.0 or greater. You can see a full list of all the earthquakes and aftershocks here. The initial 7.0 was felt in the surrounding countries in the Caribbean, as displayed here on the Shake Map:
For your reference, here are the classes of earthquakes on the Richter Scale, with the strength of the earthquake felt increasing logarithmically:
For your reference, here are the classes of earthquakes on the Richter Scale, with the strength of the earthquake felt increasing logarithmically:
This part of the Caribbean has several fault lines running through it, which makes it particularly prone to seismic activity:
