On the 12th of January 2010, an earthquake rivalling some of the most intense in the world, erupted 25 km from Port-au-Prince, capital of Haiti. Devastating both the land and the people, it reached an intensity of IX on the modified Mercalli scale (intensity I is hardly felt, intensity XII is disastrous) and a magnitude of 7.0 on the Moment Magnitude scale (ranging from 1 to 9). On the 28th of January 2010, the Haiti government confirmed a death toll of 170,000 people with thousands more trapped under buildings and rubble. But what created this disaster?
Some basic geology: We know that the Earth is formed of different layers compiled of various minerals. The outer layer is called the lithosphere and is composed of the crust and the highest portion of the mantle. This layer is separated into tectonic plates which cover the surface of the Earth. These plates move over the top of the upper mantle, the asthenosphere, and also move against each other. The tectonic plates are fundamental to maintain atmospheric composition amongst other things. There are currently around 7-8 major plates and many smaller ones. It is at the boundaries between these plates that stress occurs, through movements of the plates against each other.
Three types of boundaries exist. Divergent boundaries, predictably, are found where two plates slide apart creating ridges and rifts. Convergent boundaries are the exact opposite. They originate where two plates are moving towards each other, creating trenches. Transform boundaries are found when plates slide along side each other. These kinds of boundaries cause what is known as transform faults which are at the origin of many earthquakes. As the plates are not perfectly formed, there is friction when they slide against each other, making the movement difficult and causing a certain amount of tectonic stress. This stress is cumulative over a range of the boundary and can, when reaching a peak that is greater than the resistance the surrounding rocks can provide, create a seizure, resulting in an earthquake. The original point of rupture of the earthquake is called a hypocenter; a term you will have heard more often is the epicentre of an earthquake, which is the same location but at ground level. Interestingly, these geological happenings are also responsible for other natural disasters. If the epicentre of seismic activity is offshore, it will undoubtedly lead to a tsunami.
Why is the intensity or magnitude of the earthquake measured by different scales? Although the Richter scale is most commonly known, it lacks specificity when it comes to very large earthquake, marking them all 7. To replace it, the Moment Magnitude scale was implemented in 1979 which makes up for this whilst following the main Richter scale formula. Another scale called Modified Mercalli scale denotes the effects of an earthquake on the people, buildings, etc.
So what happened in Haiti? Scientists have identified that the Caribbean tectonic plate has been shifting eastwards along the North American plate by around 20 mm a year. The stress incurred by this shift gathered stress and ultimately led to the rupture of the Enriquillo-Plantain Garden fault zone which has been locked now for 250 years. This zone has been responsible for many other earthquakes in Haiti and Jamaica, leading to questions about whether the disaster could’ve been avoided. Knowing the zone is a hazard, should the government, having been warned by 7 different groups of scientists predicting major risks of seismic activity in the area, have taken more steps to prepare?