Shocking aftershocks in Haiti
On Jan. 20, eight days after Port-au-Prince, the capital of Haiti, was destroyed by an earthquake, the area was rocked by a strong aftershock, rated magnitude 5.9. To people who were still living in the streets and parks, the shock was a reminder of the danger of living in a geologic fault zone. But as wags once said about the tenth nuclear bomb to devastate a city, the shock mainly bounced the rubble, according to the reports we heard.
By Jan. 24, 52 aftershocks registering 4.5 or above had been recorded in Haiti.
The Jan. 12 quake (magnitude 7.0) struck about 25 kilometers west of Haiti's capital, Port-au-Prince, along the Enriquillo fault, near the boundary between the North American and Caribbean plates. The astonishing devastation may have killed upwards of 150,000 people, according to reports from the government.
The giant plates that comprise Earth's crust float on the semi-molten mantle. As the plates move, earthquakes help relieve stress that builds up along their edges.
But aftershocks prove that the first big quake, called the "main shock," does not end the story. And so The Why Files got to wondering: Why do earthquakes follow earthquakes? Do some follow-on quakes take years to appear? And can a better understanding of earthquake sequences help seismologists make better forecasts of future shaking?
Aftershock vs. triggered earthquake
To understand the shaking that follows an earthquake, we must distinguish two types of follow-on quakes: aftershocks and triggered earthquakes. Technically, aftershocks occur in the same section of fault (or in nearby faults) that broke in the main shock, usually within days or weeks. Triggered earthquakes occur in an adjacent or nearby section of the fault, and may happen years afterwards.
To people in Haiti who are trying to find and bury the dead and put their lives back together, this may seem a distinction without a difference: both aftershocks and triggered earthquakes, by definition, follow a main shock, and both can be dangerous.
Geologically, both types of follow-on quakes help readjust the fault after pieces of crust have slid against each other during a main shock. And in terms of earthquake safety, both categories seem to follow general patterns that has made them hot topics in seismology, where "prediction" has been a dirty word but where there is a growing interest in making probability forecasts for active faults.
Earthquake activity in the Caribbean
Taking stock of aftershock
As mentioned, an aftershock is an earthquake that follows a main shock and occurs along the same section of fault, or on a nearby fault. A magnitude 7 quake, like the one that just struck Haiti, usually breaks tens of miles of a fault, says Susan Hough, a seismologist who studies California earthquakes for the U.S. Geological Survey.
And that movement creates conditions for aftershocks. The main shock may leave an unbroken section that still needs relief, or it can put added stress on nearby faults. The earthquake "is shoving the real estate around, and that affects the surrounding crust, where there are lots of little faults," says Hough. "An aftershock is not necessarily on the same fault, typically they are on smaller faults nearby."
The aftershocks of a big earthquake are "relatively predictable ... and follow fairly well-established empirical rules," says Hough, (see #1 in the bibliography). "The largest aftershock is typically one unit smaller than the main shock. So if we see one magnitude 6, we see 10 magnitude 5's and 100 magnitude 4's. (The Richter scale for gauging earth movement has been supplanted by a magnitude scale that measures energy release. A magnitude 6 quake releases 30 times as much energy as a magnitude 5.)
Indeed, the Jan. 20 aftershock, measured at 5.9, was just more than 1 unit of magnitude smaller than the Jan. 12 main shock.
The rate of aftershock will fall with time, and aftershocks are finished when earthquakes fall to the background rate, says Hough.
Speaking eight days after the Jan. 12 earthquake, Hough said "As a general probability, over the next month, there are even odds of an earthquake at least as large as 5.6 in Haiti, and there is a small chance, 2 to 3 percent, that they could have another 7, or even greater."
But despite the typical aftershock curve, she warns, "Aftershocks are not as well-behaved as people think. They are overwhelmingly smaller than the main shock, but there is a chance that something bigger could happen."
Main shocks do not just produce aftershocks, as Hough stresses. In Haiti, "You worry on this type of long fault, like on the San Andreas, you have a segment at either end that hasn't broken yet."
Terry Devitt, editor; Steve Furay, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive