Today, Haiti experienced a rather large 6.1 aftershock that was located 56 km from Port-Au-Prince.  Aftershocks are earthquakes.  In Haiti, there have been more than 40 aftershocks ranging from 3.0 to 5.9 as seen in the map here.  Today’s aftershock was the largest one since the 7.0 earthquake last Tuesday.  The relief efforts are underway, but the enormity of the task appears daunting, yet when we listen to those who are there, and who are working directly with victims through aid organizations, we see some progress, and the determination on the part of Haitian people and the workers who are there to help.

Map of the aftershocks of the January 12th 7.0 earthquake. Click on the map to go to the USGS active map and data.

As President Obama has said, the immediate task is search and rescue, providing medical treatment and aid, and bringing food and water to the Haitian people.  But in the long run, he said that we’ll need to work closely with the government and people of Haiti to help them preserve and reclaim their nation.  This will happen through a massive humanitarian effort, and that  effort is underway.

According to a GDACS report, an earthquake of magnitude 6.1 occurred in the very highly populated region of Ouest in Haiti.  The center of the map locates the epicenter of this earthquake.  As with the 7.0 earthquake, the aftershock had a considerable effect on the population in this part of Haiti.  As you can see in the map to the left, aftershocks have been considerable in the area of the original 7.0 earthquake (the largest red circle).  The USGS has made a preliminary assessment of the earthquake, the aftershocks, and the future seismic activity in the region, and you can read their report here.

The 7.0 earthquake of a week ago was one of the most powerful earthquakes to happen in Haiti in more than 200 years.  That earthquake, which occurred within miles from the capital, Port-Au-Prince, was the result of movement along the Enriquillo-Plantain Garden fault zone.  The movement in this area is caused by the relative movement of the North American and Caribbean tectonic plates.  Geologists describe this motion as a strike-slip or transform fault movement where one side of a fault slides horizontally past the other.

Diagram of a strike-slip fault, similar to the fault system in Haiti

As in the case of the 7.0 earthquake, the 6.1 aftershock today, and the other 40 or so aftershocks since last Tuesday, the epicenters of these quakes are fairly shallow, e.g. about 5 – 7 miles beneath the surface.  The energy of shallow quakes such as these is concentrated near the surface of the earth resulting in enormous ground movement and damage, especially in Haiti, which buildings have been constructed without regards to earthquake preparedness.

The nature of the earthquakes that occur along the boundary between the North American and the Caribbean tectonic plates has been documented by geologists.  In a Scientific American article, it was noted that:

the two plates are “shearing the island, crushing it, grinding it.” So although such a large earthquake has not shaken Haiti since the 18th century, “this is quite an earthquake-prone region,” Blanpied said. And it is not the biggest quake to hit the Island of Hispaniola (which Haiti shares with the Dominican Republic) in recent history. A magnitude 8.0 earthquake rocked the Dominican Republic in 1946.

Someone asked me today how the earthquake in Haiti differed from the earthquake that caused the 2004 devastating Indian Ocean tsunami.  The earthquake produced the most destructive tsunami in recorded history, killing more than 230,000 people.  The earthquake had a magnitude between 9.1 and 9.3, and was the second largest earthquake recorded, and resulted in faulting of rocks that lasted about 8 minutes.  Again a record.  But this earthquake happened not as the result of two plates sliding past one another, but when one plates sinks beneath another plate.

Subduction zone showing on crustal plate sliding under another. Diagram, from NASA

Geologists call this subduction.  In the case of the earthquake that occurred in the Indian Ocean, the India plate slides beneath or under the Burma Plate.  According to geologists, a rupture of about 250 miles long and about 60 miles wide and 19 miles beneath the seabed caused the earthquake.  This was a huge earthquake, and geologists, after studying seismic records have determined that aftershocks from this quake continued for months after the December quake.

The tsunami that was produced by this massive quake affected fourteen countries, and produced waves as high as 100 feet.  Indonesia, Sri Lanka, India, and Thailand were the hardest hit.