In the Turkish-Syrian border area near the city of Gaziantep, which was hit by the catastrophic earthquake on Monday, the earth does not come to rest. In the first 24 hours after the main tremor with a magnitude of 7.8, at least 24 aftershocks with magnitudes of more than five occurred. The first of these aftershocks took place eleven minutes after the original earthquake and had a magnitude of 6.7. Nine hours later, about 100 kilometers further north, the strongest aftershock so far with a magnitude of 7.5 hit. All of these aftershocks are strong enough to completely collapse already damaged houses and severely hamper and endanger the work of the rescue teams.
Aftershocks are a normal and natural consequence of earthquakes. There are several causes for their occurrence, two of which are particularly important: An earthquake is a fracture in a rock along a weak zone, the so-called fault. But because no rock in the earth’s crust is homogeneous and uniform, such a fracture does not occur continuously and evenly. Rather, the rock breaks abruptly because the break is slowed down by resistant zones in the rock or by layer boundaries in the rock, only to then spread further at great speed a little later.
Aftershocks ensure that stresses are released even on rough surfaces
Such a fracture process does not leave a uniformly smooth fracture surface, but rather a very rough surface that is not completely fractured everywhere. Aftershocks later ensure that the mechanical stresses in the rock are balanced out at these points as well. Any compensation of this kind is an earthquake. The greater the initial roughness, the greater this aftershock.
The second reason for aftershocks is related to the tectonic tension that has been reduced over a large area by the main tremor. The cause of Monday’s severe main tremor was the slow but steady southwesterly movement of the Anatolian Plate relative to the Arabian Plate. This movement is tearing at the rocks in the zone of weakness, the East Anatolian Fault. If these tearing forces exceed the strength of the rock, it breaks; there is an earthquake. As a consequence, the tensile forces are dissipated along the area of the earthquake. In the case of Monday’s earthquake, this focal area was about 100 kilometers long and in some places reached a depth of almost 50 kilometers.
Edges are also under tension
However, the reduction in stresses along the focus area means that the edges are under more stress after an earthquake than before. On one side of the rim, the quake has reduced tensions, but on the other, unbroken side, they are as high as before. A large voltage difference develops, which can trigger further earthquakes. These movements in the earth’s crust are also called aftershocks.
The most severe aftershock nine hours after the main tremor had such a cause. It occurred at the northeastern edge of the original focus area, about 100 kilometers from the hypocentre. The tension contrast that developed there ensured that a secondary fault broke and the tension that had accumulated over decades was also reduced there.
How many strong aftershocks there are depends on the strength of the main tremor. After very severe earthquakes, such as the earthquake with a magnitude of 9.2 off the Indonesian island of Sumatra at Christmas 2004, the earth can still tremble years later. However, geophysicists have not yet been able to predict the exact number of aftershocks. They also find it difficult to predict the distribution and strength of aftershocks.
Researchers at the United States Geological Survey’s Earthquake Center in Golden, Colorado, recently released an experimental aftershock forecast, which is updated daily, following each major earthquake in North America. However, such experimental predictions for regions outside of North America have not yet been made. What is certain, however, is that further aftershocks can be expected in the southeast of Turkey in the coming weeks, although their number and strength will continue to decrease over time.