AS THE appalling death toll of last Tuesday鈥檚 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鈥檚 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鈥檚 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
鈥渢he port city of Izmit is most vulnerable鈥� (Geophysical Journal
International, vol 128, p 594). And so it proved last week when a quake
struck almost beneath the city.
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鈥淥ur results suggest that earthquakes interact,鈥� 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鈥檚 quake, next in line, says
Turkish-born seismologist Nafi Toks枚z, now at the Massachusetts Institute
of Technology. 鈥淭here 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.鈥�
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鈥檚 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鈥檚 Neotectonics
Research Centre in London believes an Istanbul quake is inevitable, he thinks
last week鈥檚 quake may have increased stress in the fault to the east, away from
Istanbul, rather than to the west. 鈥淭he evidence for that is that the
aftershocks have been mainly to the east,鈥� he says. But Toks枚z dismisses
this: 鈥淭he location of aftershocks does not have any relation to where the next
quake will occur.鈥�
So Istanbul holds its breath. 鈥淭here has not been a direct seismic hit on a
major world city since Tokyo in 1923,鈥� says Stewart. That killed 140 000 in a
city with half the population of Istanbul today.
