After this morning's magnitude 6.1 earthquake struck outside the already ravaged Haitian capital of Port-au-Prince, survivors and aid workers must be wondering "when will it end?" On this point, unfortunately, scientists are not very encouraging.
When I spoke last week with Uri ten Brink of the United States Geological Survey after the initial 7.0 quake hit, he warned that the biggest thing working against seismologists monitoring the region's faults was that they're poorly understood, but that additional quakes west of the original were well within the range of possibility.
(in a recent study he wrote that assessing seismic hazard in the region "is as if we would try to assess earthquake hazards in California without knowing of the existence of the San Andreas Fault system and its rate of motion.")
He listed a series of possibilities for future quakes in the area, two of which are relevant to today's quake:
1. The Enriquillo fault (which hosted the original quake on January 12. see map) could rupture further to the west, out along Haiti's southern Tiburon peninsula. The fault extends through the more sparsely populated region of Haiti, offshore and runs near to Kingston, Jamaica. Only a small portion of it -- ten Brink estimates 50-60 kilometers (31-37 miles) -- ruptured yesterday.
The quake could set in motion a string of powerful tremors, each further west than the last, as the rest of the fault unloads its pent up stress. The trouble is knowing when. In 2004, a 1200-kilometer (746-mile-)-long stretch of the Sumatra fault broke, unleashing a magnitude 9.2 earthquake and a horrifying tsunami that pummeled coastlines throughout the Indian Ocean. But only three months later the fault popped again along a previously locked section further south, letting loose a magnitude 8.6 tremor.
However, it may be years, even decades, before the next big quake hits the Enriquillo fault. "We can't go around saying, 'oh look out, there's going to be another one in three months,'" ten Brink said. "Ultimately it will reach Jamaica, but it could be 10 or 20 years. We have no way of knowing."
Given that this morning's quake was indeed further west than last week's, it could be the beginning of a series of ruptures. But we can't say for sure -- we only have one data point for now. More likely, given how close the two quakes were in space and time to one another, this morning's quake is indeed an 'aftershock,' a product of the second scenario ten Brink outlined:
2. Within 50 kilometers (31 miles) of a quake, faults are prone to what scientists call "near field" effects -- large changes in tectonics stresses that can greatly increase the chance of a fault rupturing. This is different than the long term fault behavior noted above, and there is a small probability that the area immediately west of yesterday's quake could rupture any day.
Either way, the danger for people in Haiti certainly has not passed.
Image: USGS
Tags: Earthquakes, Geology, Geophysics, Natural Disasters
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