By Charles Carrigan
By now we've all seen the news: last Tuesday, January 12, at 4:53:10 p.m. local Haiti time, a massive earthquake of magnitude 7.0 devastated thousands in Haiti.
Just 7.5 minutes later, the first aftershock struck, itself a sizeable earthquake with a magnitude of 5.9.
As of Sunday, January 17, 49 aftershocks greater than magnitude 4.0 have occurred, two-thirds of which occurred in the first 10 hours following the initial earthquake.
It is a distressing tragedy as we witness the intolerable suffering that was caused by the event. Unfortunately, it is an event the world has seen before, and one the world will likely see again at some time in the future.
Some may remember similar events in recent years, such the magnitude 7.9 magnitude earthquake that struck Eastern Sichuan, China, on May 12, 2008, and left over 87,000 people dead. Or the 7.6 magnitude earthquake in Pakistan on October 8, 2005, which killed over 80,000. The list goes on: 31,000 killed in a 6.6 magnitude earthquake in southeastern Iran in 2003; 20,000 killed in India by a 7.7 magnitude earthquake in 2001; and over 17,000 killed in Turkey by a 7.6 magnitude earthquake in 1999. And I don't mean to minimize the suffering of these just listed, but the number of deaths in each of those pale in comparison to the more than 227,000 lives lost in Indonesia the day after Christmas 2004 in a whopping 9.1 magnitude earthquake.
The earthquake that struck Haiti just a few days ago occurred on a strike-slip fault, the kind of fault where the two blocks of crust slide side-by-side. The San Andreas Fault of California is a similar kind of fault, as is the Anatolia Fault in Turkey that was associated with the 1999 earthquake there. Given the number of active faults in the Caribbean, the earthquake in Haiti was not terribly surprising to geologists. It occurred in an active tectonic area, like the vast majority of earthquakes, especially very large ones, do. This area, in fact, was predicted by seismologists to be one where a large earthquake was likely to happen. Unfortunately, simply knowing which areas are at increased risk of large earthquakes is not sufficient to spare thousands from the devastation they can cause.
Since the late 1960s, geologists have recognized that the Earth's surface is broken up into tectonic plates, rigid blocks of crust and underlying mantle separated by faults and moving laterally across the Earth's surface. There are seven major plates, such as the North and South American plates, and eight minor ones. Haiti lies on the northern edge of the Caribbean Plate, one of Earth's minor tectonic plates (Fig. 1, below). Roughly rectangular in shape, it sits sandwiched between the North and South American plates. The Caribbean Plate includes most of the nations of Central America (e.g., Honduras, Nicaragua, Costa Rica, etc.) as well as the islands of Jamaica, Hispaniola (shared by Haiti and the Dominican Republic), Puerto Rico, and the many smaller islands of the Lesser Antilles (e.g., the Virgin Islands, Antigua, Guadeloupe, Martinique, St. Vincent, Grenada, etc.).
The North and South American plates are moving to the west relative to the Caribbean plate, meaning that the northern and southern plate boundaries of the Caribbean plate are strike-slip faults. The northern boundary runs to the north of Hispaniola and continues west to the south of Cuba. On the eastern side of the plate, the ocean crust in the Atlantic (part of the North American plate) is subducting underneath the Caribbean plate, a process that has produced the volcanic islands of the Lesser Antilles. On the western side of the Caribbean plate lies another subduction zone, where the Cocos plate in the Pacific Ocean is moving to the east and diving back into the mantle under the Caribbean plate. As such, the Caribbean Plate does not include a mid-ocean ridge spreading center, nor is any part of the Caribbean Plate subducting back into the mantle under another plate. This set of conditions makes the Caribbean plate a unique plate on Earth's surface, in that it is neither growing nor shrinking in size.
The main boundary fault on the northern side of the Caribbean Plate is about one hundred miles to the north of Port Au Prince, but a second fault of similar nature passes along the southern border of Haiti, going right past the capital city. This fault extends east into the Dominican Republic and also west to Kingston, Jamaica. The epicenter for the recent earth quake was along this second, more southerly fault, only about 12 miles from Port Au Prince and at a relatively shallow depth of about 8 miles (Fig. 2, below). The large size of the earthquake, its shallow depth, and close proximity to a large population center are all reasons that it caused so much destruction.
Large magnitude earthquakes like this one have also occurred in the U.S. In the last decade, 63 earthquakes of magnitude 6.0 or higher have occurred in the U.S., six of which were above magnitude 7.0. These powerful events, however, have not resulted in the deaths of tens of thousands, or even thousands. Some of them did not occur near large population centers, but nonetheless only two deaths in central California in 2003 were attributed to earthquakes in the U.S. over the past 10 years.
The key difference is building codes; buildings in the U.S. and other more developed nations are built to withstand earthquakes, and in many poorer countries of the world, they are not. It is by and large not the earthquake itself that kills people, but rather the falling buildings, built in ways that were not capable of handling the intense shaking. Although these earthquakes are unpredictable, they are not unforeseen; large earthquakes like this one will eventually occur near cities such as Los Angeles or San Francisco, but the damage is not expected to be at the scale seen in Haiti primarily due to building codes.
There are some things about the Earth that we cannot change. There will always be earthquakes in seismically active zones, and some of them will be very large. Geology can tell us where they are likely to occur, but of course cannot predict them perfectly. We can only make ourselves aware of the risks that nature at times presents to us, and work to protect ourselves and our neighbors accordingly.
Unfortunately, many of our neighbors in lesser developed nations do not have this information, and may not have the means to do what is necessary to protect themselves even if they did. Beyond earthquakes, many other natural disasters represent risk to ever-increasing human populations, and many also lack access to basic necessities such as clean water and other natural resources. There remains incredible opportunity for professionals in geology and related scientific fields to use their skills to impact the world for the betterment of all people.
Fig. 1
Fig. 1: Tectonics of the Caribbean Region. Arrows indicate directions of plate motions.
Fig. 2
Fig. 2: Location of the main Earthquake (white target symbol) and aftershocks (orange symbols) in Haiti.
Dr. Charles Carrigan is an Associate Professor of Geology at ONU. He received his B.S. from ONU in 1996, his M.S. from Vanderbilt in 2000, and his Ph.D. from the University of Michigan in 2005. He and his wife Dana and their two girls reside in Bourbonnais, Ill.
All earthquake data from USGS Web site
Earthquake image from The Church of the Nazarene
Figure images from Google Earth.
