In the summer of 2016, a large earthquake took place beneath the Sea of Marmara, a body of water enclosed by Turkey. But it was so slow that nobody without specialist monitors detected it. Occurring south of Istanbul, in the zone of the Earth known as the mid-crustal depth, the event went on for more than 50 days, emitting energy that, had it been released in one go, would have resulted in a magnitude 5.8 earthquake.
The phenomenon was recorded by researchers from the GFZ German Research Centre for Geosciences using super-sensitive borehole monitors, specially developed in the US by UNAVCO. Dr Patricia Martínez-Garzón from GFZ explains that over the past 20 years these periods of ‘slow energy release’ (there is debate as to whether they can truly be called earthquakes) have been recorded in other parts of the world prone to seismic activity, including the San Andreas Fault in California, the Cascadia Subduction Zone which spans the US and Canada, and in parts of Japan. She adds that they can vary enormously in length, from just half-an-hour to many months, with some geological evidence even suggesting releases of energy that lasted many years. The depth at which they can occur is also uncertain. Though most have been recorded several kilometres underground in the brittle-ductile transition zone (around 15km in depth), they can also occur within the first four kilometres beneath the surface.
Scientists cannot say exactly how these slow-motion events relate to earthquakes felt on the surface, though Martínez-Garzón points out that during the same period in 2016 the shallower parts of the Earth’s crust near Istanbul saw the highest number of moderate earthquakes in years. ‘We cannot say for certain that these two events are related, we can just say that they definitely occurred at the same time,’ she says.
Monitoring these slow deformation events is particularly important for Istanbul. The city sits just above the North Anatolian Fault, the tectonic boundary that separates Eurasia from the Anatolian plate. The fault has produced a number of ruptures during the 20th century, with the last major earthquake taking place in 1999 near Izmit causing almost 20,000 fatalities. A portion of the fault, running just south of Istanbul is currently identified as a ‘seismic gap’ and considered overdue to produce a large earthquake.
In light of this, Martínez-Garzón emphasises the importance of taking into account all types of energy release to better monitor earthquakes. ‘This slower seismic deformation may be important when calculating recurrence periods. One also needs to account for the amount of energy released in this type of event and how the adjacent fault segments are affected.’
In the July issue of Geographical, our Dossier series will be taking an in-depth look at the future of earthquake detection and how technological advances in the field can help better prepare us for future disasters. Subscribe today and make sure you don’t miss a thing: geog.gr/subscribe.
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