In 1999, an eastern portion of a 150-kilometer-long segment of the North Anatolian Fault ruptured beneath Turkey’s Marmara Sea. The result was the Izmit earthquake, a 7.6 magnitude quake that struck Turkey, leveling several cities and towns and taking thousands of lives. Since then, scientists have kept a close eye on the area, known as the Main Marmara Fault, to find out if the segment is a locked or creeping fault.
Along locked segments, energy builds up slowly over time, before it is released rapidly in massive earthquakes. Creeping faults, on the other hand, continuously release energy without significant seismic commotion. The Main Marmara Fault’s manner of stress relief matters because a major earthquake along the North Anatolian Fault could threaten several big cities around the Marmara Sea, including Istanbul.
The main tool scientists use to determine whether faults are locked or creeping is the coupling coefficient—a number that describes the amount that seismic events deform faults. However, researchers don’t have enough information about the seismic history of the Marmara Sea region to make the calculation, and the Marmara Sea makes measuring the fault strain with GPS profiles difficult.
Here Schmittbuhl et al. sought to fill in the gap with a high-resolution look at the seismicity profile of the Main Marmara Fault between 2007 and 2012. The team used seismic wave data collected by 132 seismic stations installed near the fault to create a catalog of earthquake events over the study period. Altogether, the stations picked up on 1936 earthquake events around the fault.
A closer look at those events revealed four distinct segments along the Main Marmara Fault. On the western side of the fault, in the Tekirdag basin and the Central basin, seismic events occurred fairly often and at a variety of depths—from Earth’s surface down to 17 kilometers below. In the eastern Cinarcik basin, seismic events were also abundant, but in the centrally located Kumburgaz basin, seismic events were scarce. The study also found that in both the Cinarcik basin to the east and the Tekirdag basin to the west, the most seismic activity took place around 8 kilometers below the surface. In the Central basin, seismic activity was recorded deeper down, around 13 kilometers beneath the surface.
The researchers calculated the seismic slip rate in the Central basin and found it was at least 2 orders of magnitude larger than that of the Kumburgaz segment, indicating that the fault may be releasing energy gradually, without major earthquake events. The Kumburgaz segment, however, appears to be locked and could eventually produce a major earthquake on the scale of the 1999 Izmit event. (Geochemistry, Geophysics, Geosystems, doi:10.1002/2015GC006120, 2015)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2016), Characterizing the fault beneath the Marmara Sea, Eos, 97, doi:10.1029/2016EO047027. Published on 29 February 2016.