Istanbul: Acute earthquake hazard confirmed

Tension under the Sea of ​​Marmara suffices for a quake of magnitude 7.1 to 7.4

Istanbul is seismically in an ejection seat. How high the tension on the North Anatolian Fault right in front of the city, researchers have now determined. © gece33 / iStock
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Pent-up voltage: A new measuring system confirms the acute earthquake danger for the metropolis Istanbul - and quantifies it for the first time. The measurements show that the North Anatolian Fault under the Marmara Sea is completely blocked. As a result, enough tension has accumulated to trigger an earthquake with a magnitude of 7.1 to 7.4, as the researchers report in the journal Nature Communications. For the residents of Istanbul, such an earthquake would be fatal.

Istanbul is sitting on a seismic time bomb. Because immediately south of the metropolis under the Sea of ​​Marmara is the North Anatolian Fault - an active plate boundary. Here, the Eurasian and the Anatolian terrestrial plates slowly pass each other. Because the rocks are becoming entangled again and again, the tension in earthquakes, which have hitherto wandered ever further to the west, is being discharged to Istanbul. But the 150-kilometer section under the Sea of ​​Marmara has not been broken since 1766 - it forms a seismic gap.

Creep or blockade?

So far, however, it was unclear how high the tension is under the Sea of ​​Marmara - and if not a slow creep has reduced some of it. The reason for this: the signals from GPS and radar satellites are largely swallowed up by the covering seawater. Therefore, researchers have so far been able to estimate only on the basis of the GPS measurements on the banks, whether and how much the fault moves at this point.

Location of the gauges (triangles) and previously registered seismic activity (red) at the Marmara Sea fault in the center remained calm for a long time Lange et al./ Nature Communications, CC-by-sa 4.0

Now, however, Dietrich Lange from the GEOMAR Helmholtz Center for Ocean Research Kiel and his team have used a novel measuring system based on acoustic signals to directly measure the underground movements at the bottom of the Sea of ​​Marmara. For two and a half years, ten measuring devices in 800 meters of water depth carried out more than 650, 000 distance measurements on both sides of the fault.

Only through these data, the researchers could now determine whether the North Anatolian Fault at this point is completely entangled or creeping moves without seismic activity. display

Fault is completely blocked

The result: "Our measurements show that the fault zone is entangled in the Sea of ​​Marmara and therefore tectonic tensions build up, " says Lange. The superficial offset rate along the measurement network was almost zero, as the measurements showed. This speaks against a creeping offset and a complete blockade of the fault in this 150-kilometer-long section, the researchers said.

According to their findings, the rock of the plate boundary is completely entangled up to a depth of at least three kilometers, probably even down to even deeper layers of rock. "Assuming that the last rupture of the North Anatolian Fault in the Marmara Sea took place in 1766, it has since accumulated a four-meter offset deficit, " the researchers report.

Potential for a quake of magnitude 7.1 to 7.4

"This is the first direct proof of the tension build up on the seafloor south of Istanbul, " says Lange. If this voltage discharges abruptly in an earthquake, this would have devastating consequences. According to researchers, this would cause an earthquake with a magnitude of 7.1 to 7.4, depending on the length of the fracture. For the nearby metropolitan area of ​​Istanbul and its 15 million inhabitants that would be fatal.

The last strong quake in the Marmara Sea on May 22, 1766 had a magnitude of 7.5 and caused severe damage. Houses were destroyed, port facilities were damaged and thousands of people died from drought and tidal waves. (Nature Communications, 2019; doi: 10.1038 / s41467-019-11016-z)

Source: GEOMAR Helmholtz Center for Ocean Research Kiel

- Nadja Podbregar