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What is a fault line and how do earthquakes happen?

The answer to what a fault line is lies in the movement of the blocks that make up the Earth’s crust. According to the theory of plate tectonics, stress builds up in rocks as the plates shift relative to one another. When that stress exceeds the strength of the rocks, sudden rupture and slip occur along the fault; the energy released spreads as seismic waves and creates an earthquake.

Types of faults and the elastic rebound theory

Faults are classified by the direction in which blocks move relative to one another. In strike-slip faults, horizontal movement is dominant; in normal faults, downward movement caused by extension is seen; and in reverse faults, upward movement caused by compression stands out. The elastic rebound theory, one of the main explanations for earthquakes, says rocks are strained over a long period and then suddenly release the energy that has built up at the moment of rupture.

  • Strike-slip fault: Blocks slide past each other horizontally.
  • Normal fault: The crust is stretched, and one block moves downward.
  • Reverse fault: The crust is compressed, and one block is pushed upward.

Major fault zones in Turkey’s earthquake reality

The North Anatolian Fault Zone is a right-lateral strike-slip fault zone about 1,200 kilometers long. Its average annual slip rate is accepted at around 2 centimeters, although this can vary between 17 and 25 mm depending on the segment. The 17 August 1999 Gölcük earthquake is listed in sources at Mw 7.4-7.6 and produced a surface rupture of about 120 km; the 12 November 1999 Düzce earthquake had a magnitude of Mw 7.1 and created about 40 km of surface rupture.

The East Anatolian Fault Zone is a left-lateral strike-slip zone extending from Karlıova in Bingöl toward Hatay. According to AFAD, on 6 February 2023, two earthquakes with magnitudes of Mw 7.7 and Mw 7.6 struck the same day with epicenters in Pazarcık and Elbistan in Kahramanmaraş; both were associated with this fault zone and directly affected 11 provinces.

What does seismic gap mean?

A seismic gap is an assessment term used for segments that have not produced a major rupture for a long time. However, this does not mean an earthquake can be predicted for a specific day or hour. For that reason, experts focus less on giving a time frame and more on reducing risk through stress accumulation, historical ruptures and the condition of the building stock.

Why are magnitude, intensity and building safety different issues?

The difference between moment magnitude and the Richter scale

Moment magnitude (Mw) is based on the fault area that ruptures and the amount of slip; the Richter scale defines local magnitude. Today, Mw is considered more useful for comparing the energy of large earthquakes. Intensity, by contrast, describes the impact of an earthquake on people, buildings and the ground; for this reason, two earthquakes with the same magnitude can produce different outcomes in different places.

Why can’t earthquakes be predicted?

Under today’s scientific framework, it is not possible to determine the exact time, location or magnitude of an earthquake in advance with certainty. Damage, however, can be reduced, and here the Turkey Earthquake Building Code is one of the key tools. The regulation entered into force on 1 January 2019; the establishment of AFAD (Turkey’s Disaster and Emergency Management Authority) in 2009 and the updated Turkey Active Fault Map published by MTA, Turkey’s General Directorate of Mineral Research and Exploration, in 2013 are also among the main reference sources for understanding the hazard and strengthening preparedness.

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