The 15-million city is situated very close to the so-called North Anatolian Fault Zone which runs just outside the city, below the Marmara Sea. Scientists appreciate that an earthquake of magnitude M7 or greater will hit region in the coming years.
A team led by Marco Bohnhoff from the GFZ German Research Centre for Geoscience presents a study indicating that the next major earthquake is more likely to originate in Istanbul's eastern Marmara Sea. "This is both good news and bad news for the city with over 15 million inhabitants". The good news: "The rupture propagation will then run eastwards i.e. away from the city", explains the researcher. "The bad news is that there will only be a very short early warning phase of a few seconds."
The constant build-up of energy from the interlocking of the tectonic plates causes plate movement to come to a halt until a great tremor releases this energy. The extent of such seismic threat to the city of Istanbul actually depends on how strongly the plates are entangled and on the exact nucleation point of the earthquake. The estimations presented by Marco Bohnhoff and his team are based on the analysis of numerous small quakes along the Marmara fault. Results have shown that the degree of locking in the western part of the fracture zone is lower and that the two tectonic plates are creeping past one another at a very slow rate. During this process small tremors of the same signature, so-called "repeaters", occur at distinct recurrence times. Further east, close to Istanbul, however, repeaters have not been observed and the tectonic plates appear to be completely locked here. This leads to a build-up of tectonic energy and increases the probability of a large earthquake there.
Such observations were possible due to a new high-precision seismicity catalog for the region. For this purpose, the researchers have thoroughly evaluated the earthquake activity by combining the two major Turkish Earthquake Measurement Networks with measurement data from the GFZ Plate Boarder Observatory within the framework of a German-Turkish cooperation project. "In this way we have found recurring earthquakes below the western Marmara Sea" says Bohnhoff. "From this we deduce that below the western Marmara Sea the two tectonic plates (for the most part -- 25-75%) are moving slowly past each other thus accumulating less energy than if they were completely locked."
And what will happen if it actually comes to the feared strong earthquake below the western Marmara Sea? "In such a case there would likewise be good news and bad news," says Bohnhoff. Good would be a somewhat longer early warning period, bad would be the fact that the rupture propagation would then take place in the direction of Istanbul resulting in more severe ground shaking than if the origin was further east. However, the current data obtained suggests the opposite: an earthquake with an epicenter at the gates of the city, which would allow the people only very little time to find protection, but which would trigger less powerful ground movements.
The results of the research study have been published in the Geophysical Journal International.
Source: GFZ Potsdam
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