A magnitude 7.0 earthquake occurred today on Tuesday, January 12 at 21:53:09 UTC (4:53 pm EST). Location was 18.45° N, 72.45° W - about 15 km (10 miles) southwest of Port-Au-Prince, Haiti. The depth was about 10 km (6.3 mi), a relatively shallow earthquake.
The earthquake basically occurred on a fault (the Enriquillo-Plaintain Garden fault system) associated with the boundary between the North American plate and the Caribbean Plate. The Caribbean plate is moving eastward by about 20 mm/yr (0.8 in/yr) relative to the North American plate (geologists would refer to this as a left-lateral strike-slip fault). It sticks, stress builds, then it snaps and moves.
This is a bad one. Whatever minimal infrastructure Port-Au-Prince had is now likely gone. The death toll from the quake and its afteraffects will be horrendous.