Strong earthquake strikes near the Marmara Fault, damaging 77 buildings and frightening Istanbul residents

Haluk Eyidoğan, Ph.D., Istanbul Technical University, and Volkan Sevilgen, M.S.c., Temblor, Inc.
 
Some 77 public buildings were assessed as heavily damaged. Of the public buildings, 1 hospital, 4 schools, and 9 administrative buildings are severely damaged. Some 29 schools of various degrees of damage were forced to close, at least temporarily.
 
Citation: Eyidogan, H., Sevilgen, V., (2019), Strong earthquake strikes near the Marmara Fault, damaging 77 buildings and frightening Istanbul residents, Temblor, http://doi.org/10.32858/temblor.047
 

In the rush to call relatives after the shaking, cell phone and internet services were temporarily disabled in Istanbul
 

On 26 September 2019, there was a strong earthquake in the Marmara Sea (Northwestern Turkey) with a magnitude of M 5.7 on the Main Marmara Fault (MMF). The public took to the streets without knowing what to do. The press and social media have been shaken by the news with a wide range of information. Some of the mobile phone operators were disabled.

After every strong earthquake in Marmara, questions and discussions continued for hours on maps and graphics, dominated by the question of ‘what is the danger of a still larger earthquake in Marmara’ and the question of whether a larger earthquake would be triggered after this earthquake. In contrast, there was no debate on how to reduce earthquake risks in the megacity of Istanbul.

 

 
The M 5.7 quake heavily damaged 77 public buildings

The closest settlement to the earthquake episode was Silivri, 22 km to the north, but the earthquake was felt in many places around the Sea of ​​Marmara, including Istanbul. Strong enough to topple some vases and kitchenware, the largest recorded horizontal accelerations were 0.081 g in Silivri and 0.085 g in Büyükçekmece, 34 km away. Istanbul suffered widespread damage to varying degrees. Although the damage assessment is still continuing, according to the latest information, an average of 3,600 buildings have been examined, and 77 public buildings were assessed as heavily damaged. Of the public buildings, 1 hospital, 4 schools, and 9 administrative buildings are severely damaged. Some 29 schools of various degrees of damage were forced to close, at least temporarily.

 

 

 

 
Seismicity started to ramp up four days before the M 5.7

Four days before the earthquake of magnitude 5.7 on 26 September 2019, four micro-earthquakes of magnitude 2.1-2.2 were recorded in the same area on 22 September 2019. On September 24, 2019, there was an earthquake of 3.0 magnitude in the same place, and 30 minutes after this earthquake, a magnitude 4.6 magnitude was felt near the northern shores of the Marmara Sea. Finally, on September 26, 2019, there was a strong earthquake of 5.7, which was widely felt in the Marmara region, and many people tried to shelter outside their homes for a few nights. Numerous aftershocks followed the magnitude 5.7.

 

 
The M 5.7 was probably not on the main Marmara Fault

The fault rupture orientation as inferred from the recorded seismic waves (‘focal mechanism’) of the 5.7 magnitude earthquake shows that a fault struck on a 45-degree dipping plane with predominant thrust slip. In contrast, the Marmara fault is believed to be near-vertically inclined with ‘right-lateral’ slip (whichever side you are on, the other moves to the right). Therefore, the earthquake most likely occurred on the secondary faults in the Marmara Sea, as also suggested by the alignment of its aftershocks.

 

What’s next?

Earthquake risk for Istanbul still remains high even though the recent seismicity didn’t seem to occur on the Main Marmara Fault, so the continuing effort is needed to prepare the city against future earthquake disasters.

 

References

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Aurélia Hubert-Ferrari, Aykut Barka, Eric Jacques, Süleyman S. Nalbant, Bertrand Meyer, Rolando Armijo, Paul Tapponnier, and Geoffrey C. P. King (2000), Seismic hazard in the Marmara Sea region following the 17 August 1999 Izmit earthquake, Nature, 404, 269–273, doi.org/10.1038/35005054

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General Directorate of Mineral Research, Turkey

Kandilli Observatory and Earthquake Research Institute

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U.S. Geological Survey, Focal Mechanism