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15 OPINION maltatoday | WEDNESDAY • 8 FEBRUARY 2023 AN extremely large earthquake has oc- curred in the southeast of Turkey, near the border with Syria. Data from seis- mometers which measure shaking of the ground caused by earthquake waves sug- gest this this event, in the early morning of 6 February, was a magnitude 7.8 out of 10 on the moment magnitude scale. Seismic waves were picked up by sensors around the world (you can watch them ripple through Europe) including places as far away as the UK. This was a really big one. The shaking caused by energy travel- ling outwards from the source or epi- centre has already had terrible conse- quences for people living nearby. Many buildings have collapsed, at least 5,000 people are thought to have died across the two countries, and there are reports of damage to gas pipelines leading to fires. A second very large earthquake of 7.5 magnitude also occurred in central Turkey around lunchtime. Why this happened here This area of Turkey is prone to earth- quakes as it lies at the intersection of three of the tectonic plates that make up the Earth's crust: the Anatolian, Ara- bian and African plates. Arabia is mov- ing northwards into Europe, causing the Anatolian plate (which Turkey sits on) to be pushed out westwards. The movement of the tectonic plates builds up pressure on fault zones at their boundaries. It is the sudden re- lease of this pressure that causes earth- quakes and ground shaking. This latest earthquake is likely to have happened on one of the major faults that marks the boundaries between the Anatolian and Arabian plates: either the East Anatolian fault or the Dead Sea Transform fault. These are both "strike- slip faults", which means they accom- modate some motion of plates moving past each other. 'Significantly bigger' than previous earthquakes While this area has many earthquakes every year caused by the ongoing mo- tion of the tectonic plates, today's earth- quake is particularly large and devastat- ing as so much energy was released. The United States Geological Survey (USGS) states that only three earth- quakes bigger than magnitude 6 have happened within 250km of this location since 1970. At magnitude 7.8, the Feb- ruary 6 event is significantly bigger than ones the area has experienced before, releasing more than twice as much en- ergy as the largest previously recorded earthquake in the region (magnitude 7.4). Modern seismologists use the moment magnitude scale, which represents the amount of energy released by an earth- quake (the Richter scale is outdated, though is sometimes wrongly quoted in the news). This scale is non-linear: each step up represents 32 times more energy released. That means a magni- tude 7.8 actually releases around 16,000 times more energy than the more mod- erate magnitude 5 earthquakes that might usually happen in the region. We tend to think of earthquake ener- gy as coming from a single location, or epicentre, but they are actually caused by movement along an area of a fault. The bigger the earthquake the larger the fault area that will have moved. For something as large as this magnitude 7.8 there is likely to have been move- ment over an area roughly 190km long and 25km wide. This means the shaking will be felt over a very large area. Severe to violent shaking (enough to cause significant property damage) is estimated to have been felt by 610,000 people in the surrounding area up to around 80km away north-eastwards along the tectonic plate boundary. Light shaking was felt as far away as Turkey's largest city Istanbul (around 815km away), as well as Baghdad in Iraq (800km) and Cairo in Egypt (950km). What about aftershocks? After major earthquakes there will be many smaller earthquakes known as af- tershocks as the crust readjusts to the changes in stress. These can continue for days to years after the initial event. In the first 12 hours after the initial tremor in southeast Turkey there were already three other earthquakes above magnitude 6.0. The first was a 6.7 which happened only 11 minutes after the first shock, and there have been hundreds of smaller magnitude aftershocks. The second, magnitude 7.5 earth- quake occurred further to the north on a different but adjacent fault system: the Sürgü Fault. Technically this one was powerful enough to count as a separate earth- quake in its own right, though it is like- ly to have been triggered by the first earthquake, and it will generate its own series of aftershocks. While aftershocks are usually signif- icantly smaller than the main shock, they can have equally devastating con- sequences, further damaging infra- structure that was damaged by the first earthquake and hampering rescue ef- forts. As the aftermath of these major earth- quakes continues to be felt by the peo- ple living in this region, we can only hope that international aid gets to Tur- key and Syria as soon as possible to help in ongoing rescue efforts, amid the on- going aftershocks. Jenny Jenkins is Assistant Professor, Department of Earth Sciences, Durham University Turkey-Syria earthquakes: a seismologist explains what has happened