Various: Updated 1/15/10: A number of publications have written reports about the 7.0-magnitude earthquake (on the Richter scale) that rocked Haiti on Tuesday. It was the most powerful in the region for more than 100 years.According to Jian Lin, a geophysicist at the Woods Hole Oceanographic Institution (WHOI), even though the quake was "large but not huge," there were three factors that made it particularly devastating: First, it was centered just 10 miles southwest of the capital city, Port-au-Prince; second, the quake was shallow--only about 10–15 kilometers below the land's surface; third, and more importantly, many homes and buildings in the economically poor country were not built to withstand such a force and collapsed or crumbled.All of these circumstances made the earthquake a "worst-case scenario," Lin said.There is a highly complex tangle of tectonic faults near the intersection of the Caribbean and North American crustal plates.
The quake struck on a 50 to 60-km stretch of the more than 500-km-long Enriquillo-Plantain Garden Fault, which runs generally east–west through Haiti, to the Dominican Republic to the east and Jamaica to the west.It is a "strike-slip" fault, according to the US Geological Survey, meaning the plates on either side of the fault line were sliding in opposite directions. In this case, the Caribbean Plate south of the fault line was sliding east and the smaller Gonvave Platelet north of the fault was sliding west.But most of the time, Earth's plates do not slide smoothly past one another. They stick in one spot for perhaps years or hundreds of years, until enough pressure builds along the fault and the landmasses suddenly jerk forward to relieve the pressure, releasing massive amounts of energy throughout the surrounding area.Such seismic areas "accumulate stresses all the time," says Lin, who has extensively studied a nearby, major fault, the Septentrional Fault, which runs east–west at the northern side of the Hispaniola island that makes up Haiti and the Dominican Republic. In 1946, an 8.1-magnitude quake, more than 30 times more powerful than this week's quake, struck near the northeastern corner of the Hispaniola.Compounding the problem, he says, is that in addition to the Caribbean and North American plates, there is a wide zone between the two plates made up of a patchwork of smaller "block" plates, or "platelets"—such as the Gonvave Platelet—that make it difficult to assess the forces in the region and how they interact with one another. "If you live in adjacent areas, such as the Dominican Republic, Jamaica and Puerto Rico, you are surrounded by faults."Residents of such areas, Lin says, should learn to recognize basic earthquake warning signs such as small rapid tremors that could indicate a bigger quake is coming.The geologist says that aftershocks, some of them significant, can be expected in the coming days, weeks, months, years, "even tens of years." But now that the stress has been relieved along that 50- to 60-km portion of the Enriquillo-Plantain Garden Fault, Lin says this particular fault patch should not experience another quake of equal or greater magnitude for perhaps 100 years.However, the other nine-tenths of that fault and the myriad networks of faults throughout the Caribbean are definitely "active," he adds. Related coverage Slate asks how do we know what the magnitude was of earthquakes before modern seismographs came along, especially as the Richter scale is just 75 years old? The answer lies in reviewing historical accounts of damage to buildings, the distance at which people felt tremors, and reports on changes in the soil. In a related piece Slate looks at whether humans can affect the frequency of earthquakes through mining, development, or dam building. ScienceNOW's Richard A. Kerr talks to seismologist William McCann, who suggests that it could have been a lot worse. The quake ruptured only a part of the fault segment that caused a magnitude-7.5 quake over 200 years ago, he says, and that quake was about 20 times more powerful than the one that struck near Port-au-Prince on Tuesday.The Associated Press reports that scientists said they warned officials in Haiti two years ago that their country was ripe for a major earthquake.Sensors on underground magnets that steer particles around the four-mile Fermilab's Tevatron ring apparently picked up seismic ripples from the earthquake, says Symmetry magazine. Related Links How do you measure an earthquake from 250 years ago? Are there more big earthquakes than there used to be? Haiti quake could have been even worse How satellites help analyse the Haiti earthquakeThe Haiti earthquake epicenter is marked by the star along the displaced portion (shown in red) of the Enriquillo-Plantain Garden Fault. The fault lines with small arrows denote a different kind of fault called thrust faults, where one plate dives under another. Strike-slip faults grind past one another. The dotted lines at bottom denote complex seafloor formations. (Source: P. Jansma, and G. Mattioli, Geol. Soc. Amer. Spec. Paper 385, 13, 2005. )