Anatolian Earthquake is the Result of Compressive Plate Movement

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Location of the Van earthquake of October 2011 (image from USGS)

Location of the Van earthquake of October 2011 – Image courtesy of USGS

A major earthquake which struck today in eastern Turkey was the result of complex faulting as the block of the earth’s crust broadly corresponding to the country is squeezed between two others. Early reports indicate that there have been many casualties and significant damage to buildings.

The tremor occurred in the extreme east of the country on 23 October 2011, just ten miles from the city of Van. Reported by the USGS as having a magnitude of 7.2, the quake, though releasing substantially less energy than the catastrophic event to strike in Japan in March 2011, is nevertheless one of the largest, and potentially most devastating, of the quakes to have struck so far in 2011.

The Tectonic Setting of Eastern Turkey

Turkey is highly seismically active and the large-scale faulting which generates the many earthquakes to strike there is complicated. At its simplest, the situation can be described as one in which the northwards movement of the Arabian and African tectonic plates against the Eurasian plate (a by-product of which is the mighty Caucasus mountain range) causes extreme stresses.

Caught between these plates is a large block of the earth’s crust known as the Anatolian block. Bounded by many faults (the most significant being the North and East Anatolian Faults) this block of crust is being squeezed and moved westwards.

Istanbul lies in an earthquake zone (image by Tambako the Jaguar)

Istanbul lies in an earthquake zone -Image by Tambako the Jaguar

Although it is too early to say in detail on which fault the most recent earthquake occurred, the earthquake was close to the eastern end of the block, close to the plate boundaries. According to the USGS, it is associated with fault thrusting (reverse faulting) on the fault complex within the convergence zone.

Historic Seismicity of the Anatolian Block

The Turkey Quake of October 2011 measured 7.2 – Image courtesy of USGS

Major – and damaging – earthquakes are by no means rare in Anatolia. Within the past twenty years there have been seven earthquakes with magnitude of greater than 6.0, between them claiming almost 20,000 lives, while the period 1939-44 saw five earthquakes with a magnitude greater than 7.0. Turkey’s largest recorded earthquake, an M8.0, occurred in central Anatolia in 1668, killing thousands.

Given its tectonic history, a major earthquake in Anatolia is by no means an unusual event and other major earthquake events are to be expected in the region as the central block continues to be squeezed westwards and lateral movement occurs along the fault complexes of both North and east Anatolian Faults.

A significant concern is the city of Istanbul: Erdik et al note that ‘Earthquake records spanning two millennia indicate that, on average, at least one medium intensity…earthquake has affected Istanbul in every 50 years’ while Dr Roger Musson, of the British Geological Survey, told Decoded Science that he believes that the city may be due for a significant earthquake event in the near future (interview October 10 2011).

Sources
BBC website. “Casualties as eastern Turkey hit by 7.2 magnitude quake.” bbc.co.uk, accessed 23 October 2011.

Colorado University. “Geological, Tectonic and Seismological Aspects of the 17 August 1999 Izmit Earthquake.” Accessed 23 October 2011.

Erdik, M. Demircioglu, M., Sesetyan K., Durukal, E and Siyahi, B. “Earthquake Hazard in Marmara Region, Turkey.” (2004). Accessed 23 October 2011.

United States Geological Survey. “Historic World Earthquakes.” Accessed 23 October 2011.

United States Geological Survey. “Magnitude 7.2 – Eastern Turkey.” Accessed 23 October 2011.

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