Environment

The earthquake occurred at a relatively shallow depth, which allowed it to retain more of its' energy as it hit the land, thus making it more catastrophic. Since the earthquake was located so close to the capital, with 3.5 million living there, there were more casualties.  
The earthquake was born out of 250 years of stress. A 2007 study concluded that the worst-case scenario would be a 7.2 magnitude earthquake.