A seismic swarm in progress in Western Turkey, near Izmir

Temblor

7 years ago

The city of Izmir in Western Turkey is close to the location of the recent seismic swarm. Additionally, faults associated with the Gediz Graben run near the city, posing a significant seismic hazard. (Photo from: istanbulinvestments.com)

Over the last 5 days, there has been a seismic swarm in progress in Western Turkey near the city of Izmir, on the Aegean Sea, which has a population of close to 3 million people. This swarm began on Saturday, and the largest quake was a M=5.0 at a depth of 13 km. The next day, there was another M=5.0 about 5 km to the northwest. In total, there have been approximately 250 earthquakes in this region since the swarm began. All of these appear to be extensional faulting events. Fortunately, none of these quakes have been large enough to cause damage. However, they are close to a major fault system, which runs through two populated areas, that is capable of generating large magnitude earthquakes.

This Temblor map shows the location of the active earthquake swarms in Western Turkey. The swarm to the northeast appears to be occurring on the Halitpasa fault, while the smaller one to the southwest is likely on a fault associated with the Manisa Fault. The Gediz Graben is also labeled, and is a major source of seismic hazard for this region of Turkey since it passes through the city of Izmir, which is home to nearly 3 million people.

Western Turkey is an area of active continental extension, and contains many east-west trending grabens. According to the USGS, a graben is a down-dropped block of the earth’s crust resulting from extension. What this means is that for the last 25 million years, Western Turkey has been experiencing north-south extension. Couple this with influence of the African plate subducting beneath the Anatolian plate, and you have a recipe for significant seismic activity.

This figure from Xacier Le Pichon and Corné Kreemer (2010) shows GPS velocities in Turkey. The ‘Anatolian plate’ is spinning counter-clockwise relative to a pole of rotation offshore Egypt (the ‘AT-EU star). As it does so, it is also spreading in western Turkey, visible by the subtle splaying or spreading of the vectors. This leads to the extensional faults that have been activated in the current swarm. The Aegean Sea lithosphere is subducting under Crete in the central Mediterranean Sea.

In the area around the ongoing swarm, normal faults associated with the Gediz Graben dominate. Based on the locations of the earthquakes in the map above, it is possible that the smaller swarm to the southwest occurred on the Manisa Fault, a 50 km-long normal fault that is subsidiary to the Gediz Graben. However, for this to be the case, the Manisa Fault would have to be dipping at an extremely low angle. While there are papers suggesting the Manisa Fault is low angle, based on the depth of these shocks, we believe that this smaller swarm occurred on a secondary, unmapped structure associated with this major fault.

In contrast, the larger swarm to the northeast appears to have occurred on the Halitpasa Fault, which is just to the west of Lake Marmara. This swarm is not only larger, but continues to be active, as there have been around 10 small quakes in the last 24 hours.

One of the reasons why swarms are important to highlight is because on occasion, they can be a precursor to larger events. While this may not happen here, pointing out an increase in seismic activity is essential, as even the smaller normal faults are capable of rupturing in M=6.0 earthquakes. Furthermore, the Gediz Graben in Western Turkey is capable of rupturing in M=7.0+ earthquakes, which could be devastating to the cities of Izmir or Manisa. Based on the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor, a M=6.25 earthquake is likely in your lifetime for this area. What this means is that a large-scale rupture along the Gediz Graben would be a rare event, while a smaller shock on an associated structure is likely.

This Temblor map show the Global Earthquake Activity Rate (GEAR) model for Turkey, with the recent swarm circled. What this map shows, using global strain rates, and seismicity since 1977, is that a M=6.25 earthquake is likely in your lifetime in the location of the active swarm.

References
USGS
European Mediterranean Seismological Centre
Erdin Bozkurt & Hasan Sözbilir (2006) Evolution of the Large-scale Active Manisa Fault, Southwest Turkey: Implications on Fault Development and Regional Tectonics, Geodinamica Acta, 19:6, 427-453, DOI: 10.3166/ga.19.427-453

Xavier Le Pichon and Corne Kreemer, The Miocene-to-Present Kinematic Evolution of the Eastern Mediterranean and Middle East and Its Implications for Dynamics, Annu. Rev. Earth Planet. Sci. 2010. 38:323–51