More science from the Haiti earthquake

Various : As more aftershocks hit Haiti this morning after last week’s earthquake, scientists have been trying to assess what further damage the country may suffer.David Kerridge, who is head of Earth hazards at the British Geological Survey (BGS), says that “with an earthquake of this size and the mountainous terrain there is a strong possibility of landslides which may have caused many causalities in more remote parts of the island. Due to disruptions in communications the full extent of the disaster might not be clear for a few days.” Kristina Bartlett Brody of ScienceNews has talked to David Applegate, senior science adviser for natural hazards with the US Geological Survey. “Our folks and others are acquiring all the imagery they can,” he says, “in order to examine possible landslide-dammed drainages that could create subsequent flash flood hazard, identify surface rupture and look for the extent of ... ground failure.” Haiti’s political situation had made it a difficult place to do science, says Eric Calais , a geophysicist at Purdue University in West Lafayette, Indiana, who has used GPS stations to monitor the area since 2003 (see left). “A lot of researchers who otherwise would have liked to work in Haiti decided not to.... There is very little science infrastructure.” AftershocksBut while scientists, particularly those who work in the region, try and collect their instruments , Haiti has suffered more than 50 aftershocks measuring more than magnitude 4.5 which have hampered both relief work and in the ability of researchers to get to their instruments. The largest aftershock, a 6.1-magnitude tremor, struck southwest of Port-au-Prince at 6:03 EST this morning.Brian Baptie, also from BGS says that “earthquakes of this size always have aftershocks that can last for many weeks” which may hamper rescue efforts and cause buildings to collapse that were affect by the original seismic event.Nevertheless, some conclusions can be drawn from the existing data—based on GPS readings Calais has created a simulation showing how the ground has deformed since the earthquake (see below). Was the quake predictable? Paul Mann (see right image) from the University of Texas at Austin’s Jackson School of Geosciences has explained in more detail how he and his colleagues back in 2008 forecasted that Haiti was going to suffer a severe earthquake in the near term."Earthquake prediction to a seismologist means that the epicentral location, date and size of the event can be determined before the earthquake occurs on faults that may have been quiescent for years or even centuries,” he says."In our Haiti studies, we estimated the size of the future event (7.2 magnitude on the Ritchter scale) based on the time we inferred since the fault was last active [the last major earthquake was in 1751], the rate of slip along the fault as determined from GPS measurements in Haiti published by D. M. Manaker et al. (2008) (7 mm/yr), and the location of the rupture (the Enriquillo-Plantain Garden fault zone (EPGFZ), a strike-slip fault we have studied from geologic mapping).""All attempts to precisely predict the exact date of a major earthquakes on the many plate boundaries and seismically active intraplate areas on the planet have ended in failure,” he says. But attempts “to forecast the likely sites of future earthquakes based on GPS studies and fault mapping have been more successful since they are less specific (ie, no certain date or epicentral locations)."Mann and his group calculated that if the ground is moving at 7 mm per year, and the last major earthquake happened 250 years ago, then there is an accumulated strain of 1.7 meters (7 mm X 250 yrs = 1.7 meters). Other regions that have had ground displacement of a similar amount have had earthquakes of 7.0-magnitude, which led to their prediction of a 7.0-magnitude earthquake for Haiti. Last week’s earthquake was exactly a 7.0 magnitude earthquake .Mann and his colleagues were particularly worried about the large population situated on the fault line —the city of Port-au-Prince has a population of 2 million people. The city is well known for poor construction, and the surrounding area has many steep hills of loose soil that have been built on. In an earthquake this soil could buckle and quickly undermine the buildings’ foundations.It was for these reasons, says Mann, that they presented their warning to geologists and policy makers at the 2008 Caribbean conference . It wasn’t a prediction, he adds, but a forecast, “similar to the way this term is used by meteorologists,” he says, “who forecast future weather trends but cannot predict exact weather conditions on specific dates weeks or months into the future.” The next big one?The research that Mann and his colleagues have done in the region suggests that the next big earthquake could be along the Septentrional fault zone of the northern Dominican Republic ."This second, northern strand of the Carib-Noam plate boundary has a faster rate than the Enriquillo-Plantain Garden fault zone and therefore accumulates strain at a faster rate,” says Mann. According to their calculations, although there hasn’t been a major quake for 800 years along the fault, they forecast that a 7.5 magnitude earthquake is likely sometime in the future. “We have no idea when this fault might rupture: tomorrow or 100 yrs from now,” Mann says, “but recent events show the importance of this type of research, and retrofitting older buildings in order to avoid a repetition of the Haiti disaster along that part of the plate boundary.” Related Links After Haiti, worries about other big quakes InsideScienceNews Geologists to evaluate future Haiti risks Naturenews Interseismic Plate coupling and strain partitioning in the Northeastern Caribbean Actively evolving microplate formation by oblique collision and sideways motion along strike-slip faults: An example from the northeastern Caribbean plate margin (1995) Powerpoint presentation delivered by Mann et al. at 18th Caribbean Geological Conference (2008) (138MB PPT) Paleoseismicity of the North American-Caribbean plate boundary (Septentrional fault), Dominican Repulbic (1993)