Prof. Dr. Haluk Eyidoğan
27.12.2020 – The Sivrice Earthquake of 27 December 2020
I decided to write this article after examining the magnitude 5.5 earthquake that occurred on 27 December 2020 just to the northwest of Lake Hazar (Sivrice). I sense you wondering what the connection is. In our profession, we describe this lake as a ‘fault lake’ (sag pond). That is why there is a connection. Below, I will try to explain this connection without going into too much technical detail. Young faults and earthquakes The displacement movement along young (active, live) fault zones creates various tectonic* phenomena that differ from one another. Depending on the physical differences in the structure of the earth’s crust in the region where the young fault is located, and on the type of fault (strike-slip, normal and reverse fault), it produces stresses and movements of various types and levels. For example, around the main young faults, many geological phenomena emerge, such as scattered fractures, deflected stream beds, ridges, fault valleys, alluvial cones, landslides, fault lakes, water springs and interrupted sedimentary masses. When you install seismic recorders around young fault zones and record data, we can observe numerous small earthquakes. In earthquake archives, we may come across records of damaging earthquakes related to this fault from past years. Along young fault zones, we can see traces of damage from old earthquakes in existing settlements and in archaeological findings. As a result of research carried out over many years in Anatolia, which is still ongoing, close to 500 young, live faults have been identified and mapped in detail. Faults and fault lakes In Anatolia, numerous fault lakes have formed along the major fault zones. Lakes Manyas, İznik, Sapanca and Abant on the North Anatolian Fault Zone, part of Lake Burdur and the other lakes in the lake district of western Anatolia, and Lakes Gölbaşı, Azaplı and Hazar on the East Anatolian Fault are examples that emerged when the depression basins formed during the movement of the faults turned into lakes. Depending on the arrangement of the faults, the width of the fault zone and the type of movement, the shapes of the resulting depression basins and lakes can also differ. Depending on the right- and left-stepping, branching or bending (flexure) patterns of the fault zones in the field, the resulting tectonic structure—and therefore the geographical structure—also differs. In places where tensile, extensional stress is dominant, pits, basins and lakes form, while in places under compressional stress, ridges, folds and elevations form (Figure 1). There are three models proposed and discussed for the formation mechanism of basins and lakes caused by strike-slip faults. These are: a) the releasing bend model, b) the pull-apart basin model and c) the negative flower structure model. If more detailed geophysical, geological and seismic studies are carried out within such lakes, it will become clear which of the discussed models is the more valid.
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