The seismic gap south of Istanbul

Geoscientists expect an earthquake along the North Anatolian Fault
Geoscientists expect an earthquake along the North Anatolian Fault

The chain of earthquakes along the North Anatolian fault shows a gap south of Istanbul. The expected earthquakes in this region represent an extreme danger for the Turkish megacity. A new computer study now shows that the tensions in this part of the fault zone could trigger several earthquakes instead of one individual large quake event. In the latest issue of Nature Geosciences (Vol 3, doi:10.1038/NGEO739) Tobias Hergert of the Karlsruhe Institute for Technology and Oliver Heidbach of the GFZ German Research Centre for Geosciences present the results of the computer simulation, which was developed within the framework of the CEDIM (Centre for Disaster Management and Risk Reduction Technology).project “Megacity Istanbul”.

The Izmit-Earthquake of August 1999 resulted in 18,000 death victims and was, with a magnitude of 7.4, the most recent quake of a series, which began in 1939 to the east of Turkey and gradually ran along the plate border between the Anatolian and the Eurasian Plate from east to west. Therefore, the next quake in this series is expected to take place west of Izmit, i.e. south of Istanbul. The city has, thus, a threatening earthquake risk.

An important factor in judging seismic hazard is the movement rates of the tectonic fault. For their study Hergert and Heidbach divided the area into 640,000 elements, in order to determine, three-dimensionally, the kinetics of the fault system. “The model results show that the movement rates at the main fault are between 10 and 45% smaller than accepted to-date” explains Oliver Heidbach of the GFZ. “In addition the movement rates vary by 40% along the main fault” The authors interpret this variability as an indication that the built-up tension in the Earth’s crust can also unload in two or three earthquakes with a smaller magnitude rather than in one enormous quake. This, however, by no means implies an all-clear for Istanbul. The authors explicitly point out in their article that the short distance of the main fault to Istanbul still represents an extreme earthquake risk for the megacity. The fault zone is less than 20 kilometres from the city boundary, disaster precaution before the occurrence of a quake is essential.

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