Is Istanbul next?
By Fred Pearce AS THE appalling death toll of last Tuesday’s earthquake in Turkey continues to rise, geologists are warning that things could be even worse next time. They say the quake leaves Istanbul vulnerable to a direct hit. As the world’s first quake beneath a city of more than 10 million people, it could kill several times more people than the 40 000 estimated to have died in last week’s disaster. In 1997, Turkish and American geologists charted a series of 10 earlier quakes, each with a magnitude greater than 6.7, that occurred in a progression from east to west along the 1500-kilometre North Anatolian fault. They explained the progression by showing that each quake increased stress in the zone where the next quake occurred. The team, headed by Ross Stein of the US Geological Survey in Menlo Park, California, and Aykut Barka of Istanbul Technical University, concluded that “the port city of Izmit is most vulnerable” (Geophysical Journal International, vol 128, p 594). And so it proved last week when a quake struck almost beneath the city. “Our results suggest that earthquakes interact,” says Stein. His study showed that following a major shock, the earthquake probability in the next decade for areas stressed by the fault jumps threefold. The march of quakes along the North Anatolian fault line makes Istanbul, 100 kilometres from the epicentre of last week’s quake, next in line, says Turkish-born seismologist Nafi Toksöz, now at the Massachusetts Institute of Technology. “There is a well-defined seismic gap west for 150 kilometres west of Izmit that includes Istanbul. It has all the right ingredients for a quake with a magnitude of more than 7.” Not all experts think the Izmit quake will trigger one under Istanbul. Dan McKenzie, a geophysicist at the University of Cambridge, says no other fault line in the world shows such a simple progression as that claimed for the North Anatolian fault. It could simply be chance, he says. But Stein argues that the simple path of the earthquakes is due to the fault’s unusual geometry. The fault is almost in a straight line and largely isolated from other faults, which makes the transfer of stress along its length relatively straightforward. Another theory, however, is that the Izmit quake might even give Istanbul a stay of execution. Although Iain Stewart of Brunel University’s Neotectonics Research Centre in London believes an Istanbul quake is inevitable, he thinks last week’s quake may have increased stress in the fault to the east, away from Istanbul, rather than to the west. “The evidence for that is that the aftershocks have been mainly to the east,” he says. But Toksöz dismisses this: “The location of aftershocks does not have any relation to where the next quake will occur.” So Istanbul holds its breath. “There has not been a direct seismic hit on a major world city since Tokyo in 1923,