“Earthquakes do not announce their arrival”

9 Feb 2023

The earthquake that struck the Turkish-Syrian border region has wrought devastation. In our interview, LMU geophysicist Heiner Igel explains how such a powerful earthquake could occur and what makes assessing future risks so difficult.

Syrians try to salvage some belongings from the rubble in the city of Atarib, in the western Aleppo countryside, in the aftermath of a major earthquake in the region.

© IMAGO / ZUMA Wire / Juma Mohammad

What happened in this region on Monday?

Heiner Igel: The region in which the earthquake occurred is located on the boundaries of tectonic plates. Huge landmasses – the Arabian and the Anatolian plates – meet there, while the African plate additionally pushes up from the south. This gives rise to faults, along which earthquakes take place. Generally speaking, earthquakes are nothing unusual there. If anything, it’s unusual that no earthquake on this scale happened in that place or region over a period of up to 1,000 years or so.

To what extent is this region considered to be at risk?

It’s a region in which earthquakes are expected, no doubt about it. In recent decades, however, there have only been earthquakes of magnitudes between 6 and 7 and none of almost magnitude 8 like the current one. When we jump from magnitude 6 to 8, the energy increases by a factor of 1,000. This means that the current earthquakes radiated many times more energy in the form of seismic waves than the other earthquakes that occurred over the past decades. We see this in the terrible damage it caused, with whole neighborhoods reduced to rubble.

According to the US Geological Survey, the tremor had a magnitude of 7.8. What does this number mean?

7.8 is a number on the Richter scale which describes the energy of an earthquake at its source. The stronger the quake, the larger the fault plane where the Earth gets displaced. In Turkey, the plates moved against each other in a horizontal direction – the technical term for this is a strike slip. This fault plane covered an area of around 100-200 kilometers by 20-30 kilometers. And at the fault plane, the two sides moved past each other most likely by about 3-5 meters. This means that the seismic waves emitted have a very large amplitude. The deflection in the region of the fault plane can be in the range of dozens of centimeters. Although the movements are relatively slow, they are devastating in their effects. These movements happen to be close to the resonant frequencies of buildings, causing them to collapse. It’s simply catastrophic for buildings when movement amplitudes and oscillation periods of this kind act upon them.

The danger for the region has not passed of course, even after the devastating quakes. Can earthquakes be predicted?

For earthquakes, we have only probabilistic forecasting to work with. We’re familiar with this concept from rainfall forecasting, for example. Rain forecasts indicate the probability that it will rain in a specific place say in 48 hours for example. But such predictions are fraught with uncertainties, and it’s a similar story with earthquakes. We actually know pretty well where earthquakes occur. And we also know pretty well what the greatest magnitudes are likely to be in a specific region. What we categorically cannot do is predict the exact time, the magnitude, and the precise location of a quake.

One thing we can do is use information from GPS observation satellites to carry out long-term monitoring of the deformation that leads to the build-up of stress in faults. From this data, scientists can calculate the deficit arising from tectonic movements. There are various regions – and the border region between Turkey and Syria is one – where you more or less know when there is a deficit of, say, five to eight meters. And if you also know that no earthquake has occurred for a long time, then the possibility of quakes occurring that can reach magnitudes of between 7 and 8 should be no surprise.

However, observation data on the movement of plates in the Turkish-Syrian border region goes back only a few decades of course. And while we know that the Syrian city of Aleppo, for example, was destroyed in the 12th century and that a large earthquake must have occurred in this region; and we know that earthquakes are to be expected in this region; nevertheless, earthquakes do not announce their arrival. Moreover, the very high magnitude and, even more so, the occurrence of two quakes in such quick succession, are very unusual. That genuinely could not have been foreseen.

How do you rate the danger of future earthquakes in the region or in the wider area?

In the epicentral region, there will be aftershocks over the coming weeks to months which will reach magnitudes that can lead to further damage. Unfortunately, the population there must prepare for this and carefully assess which buildings are still safe.

For a long time, people have been warning about a large earthquake that could hit Istanbul, for example, in the near future. Is it conceivable that the tensions which the current earthquake has released could have been transferred to another place, increasing the risk there?

Yes, such transfer of stress does exist. It can make earthquakes that are overdue to happen in areas some distance away more likely, or they can delay their occurrence. This depends most of all on the direction of future seismic centers from the previous epicenter. In fact, experts do expect a larger quake to strike the Istanbul region over the coming decades, due to a fault that runs through the Sea of Marmara. When it will happen, nobody can predict.

Are there precautions that can be taken in areas threatened by earthquakes?

The best precaution is earthquake-resistant building design, as practiced in places like California and Japan. In California, for example, the housing in Silicon Valley consists of prefabricated wooden buildings with wooden frames. The structures are no more than one or two stories high. If they collapse, the people inside them have a good chance of surviving. Large buildings, including skyscrapers, are designed so that they can absorb vibrations. Therefore, earthquake-resistant building design is very much achievable. In the region affected by the recent quake – we saw this in the pictures of the disaster area – sadly there were very many multi-story houses built in a way that meant they could not withstand such vibrations. This is the tragic aspect of this earthquake, which led to there being such a large number of victims and such extensive damage.

Heiner Igel is Professor of Geophysics/Seismology at the Department of Earth and Environmental Sciences. His main research interests include studying the seismic waves caused by earthquakes.

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