Haiti's 7.0 Quake: A Complex, Active Seismic Region

Graphic by Woods Hole Oceanographic Institution. - h/t: spacefuture

WHOI Expert: Haiti quake occurred in complex, active seismic region

By Jason McManus via The Woods Hole Oceanographic Institution

The magnitude 7.0 earthquake that triggered disastrous destruction and mounting death tolls in Haiti this week occurred in a highly complex tangle of tectonic faults near the intersection of the Caribbean and North American crustal plates, according to Ian Lin, a quake expert at the Woods Hole Oceanographic Institution (WHOI) who has studied faults in the region and throughout the world. 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 Island that makes up Haiti and Dominican Republic. It triggered a tsunami and left 20,000 people homeless, according to the U.S. Geological Survey.

Lin said that 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 Jan. 12 earthquake a "worst-case scenario," Lin said. Preliminary estimates of the death toll ranged from thousands to hundreds of thousands. "It should be a wake-up call for the entire Caribbean," Lin said. [It should be a wake-up call for us all: Gangs Armed With Machetes Loot Port-Au-Prince]

The quake struck on a 50-60-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 U.S. 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.

The Haiti earthquake epicenter is marked by the star along the displaced portion (shown in red) of the Enriquillo-Plantain Garden Fault. The 7.0 magnitude quake struck along about one-tenth of the 500-km-long strike-slip fault. The region sits on a complex seismic area made up of numerous faults and plates. 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: Jansma, P. and Mattioli, G., 2005, GPS results from Puerto Rico and the Virgin Islands: constraints on tectonic setting and rates of active faulting, Geol. Soc. Amer. Spec. Paper 385 (ed. Paul Mann), 13-30.)

But most of the time, the 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. A similar, more familiar, scenario exists along California's San Andreas Fault.

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. Compounding the problem, he says, is that in addition to the Caribbean and North American plates, a wide zone between the two plates is 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."

Some residents of earthquake zones know that after the quake's faster, but smaller, primary, or "p" wave hits, there is usually a few-second-to-one-minute wait until a larger, more powerful surface, or "s" wave strikes, Lin says. P waves come first but have smaller amplitudes and are less destructive; S waves, though slower, are larger in amplitude and, hence, more destructive.

Lin said the Haiti quake did not trigger an extreme ocean wave such as a tsunami, partly because it was large but not huge and was centered under land rather than the sea.

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 50-60-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.

"A lot of people," Lin says, "forget [earthquakes] quickly and do not take the words of geologists seriously. But if your house is close to an active fault, it is best that you do not forget where you live."

The Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the oceans’ role in the changing global environment.