Expert explains when aftershocks will end

The 5.9-magnitude earthquake that occurred yesterday (October 16) in the Kale district of Malatya caused panic.

Following the earthquake, which was also felt in surrounding provinces, aftershocks were recorded.

Geological Engineer Prof. Dr. Okan Tüysüz wrote an article explaining the nature of aftershocks following a main earthquake and how long they may continue to occur.

Tüysüz's assessments are as follows:

"An earthquake occurs as a result of the rocks forming the Earth's crust breaking along planes called 'faults' due to the forces (stress) applied by moving pieces that form the outer part of the Earth, known as 'plates'. There is a direct proportion between the size (area) of faults, which are tens or sometimes hundreds of kilometers long and usually 10-15 kilometers deep, and the magnitude of the earthquake that will occur if they break. In other words, the larger the length and depth of the fault, the greater the magnitude of the earthquake that will occur.

THE NATURE OF AFTERSHOCKS

During an earthquake, the rocks on the two sides of the fault plane (fault blocks) move meters relative to each other, especially in large earthquakes. This movement of the blocks continues for a while after the earthquake, and this ongoing movement causes smaller earthquakes to occur following the main earthquake. These smaller earthquakes following the main shock are known as 'aftershocks' and, according to general acceptance, can reach a magnitude up to one degree lower than the main shock. For example, the aftershocks of a magnitude 7 earthquake can reach up to 6. The number of aftershocks can be highly variable; generally, as the earthquake magnitude increases, more aftershocks occur and aftershocks persist for a longer period.

The region where aftershocks occur in the period following the main earthquake shock is known as the 'aftershock zone'. The aftershock zone that occurs in the approximately 24 hours following the main shock is quite consistent with the epicenter area where the main shock is most effective. However, subsequent aftershocks spread over a wider area around the fault that created the earthquake. Generally, the spread area of aftershocks is accepted as twice the length of the fault broken in the earthquake.

During the main shock of the earthquake, that is, while the fault is breaking, the rocks along the fault, especially in areas close to the earthquake focus, are broken, fragmented, and become dynamically unstable. As an example of this, we can give a wooden slat that is squeezed and broken with two hands. As the slat is squeezed and bent, it begins to crackle, that is, to crack. These crackles can be compared to foreshocks. The breaking of the slat and the sound waves resulting from it can be an example of the main shock. The broken pieces of the slat vibrate for a while and then stop. This stage is also an example of aftershocks. When the fault moves and the rocks break, this movement continues for a while longer. Some of the irregularities on the broken fault plane cannot withstand the movements created by the main shock, and these also break, creating aftershocks smaller than the main shock. Over time, the movement of the fault blocks dampens, the rocks adapt to the new situation, and the aftershocks disappear.

WHAT DOES THE FREQUENCY OF AFTERSHOCKS DEPEND ON?

The number of aftershocks is highest immediately after the main shock occurs, but they decrease over time in terms of both number and magnitude. Although it is not the same for every earthquake, the number of aftershocks drops to about one-tenth in the first 10 days after the main shock, and to one percent in 100 days. The larger the main shock, the longer it takes for the aftershocks to dampen. Relatively large aftershocks occur less frequently, while small aftershocks occur more frequently. The frequency of aftershocks also varies according to their magnitude. For example, magnitude 4 aftershocks occur about 10 times more frequently than magnitude 5 aftershocks. In most cases, the largest aftershock occurs within 3 days following the main shock, but aftershocks that appear after many years are also possible. Aftershocks of some earthquakes can occur years, decades, or even longer periods later. Relatively large aftershocks occur far from the epicenter of the main earthquake and usually at the edge or around the 'aftershock zone'.

Although losses and damages have been experienced in smaller earthquakes in our country, it can generally be accepted that earthquakes of magnitude 6 and larger can have destructive effects. In this case, while the aftershocks of main earthquakes of magnitude 7 and larger are destructive, the aftershocks of smaller earthquakes can increase the damage caused by the main shock. Some of the structures damaged in the major earthquakes that occurred on February 6, 2023, were destroyed due to subsequent aftershocks. Taking this into account, it should be considered that aftershocks will follow after an earthquake occurs, and damaged structures should not be entered.

In developed countries, although there are regulations defining the procedures to be followed in cases such as the need to enter damaged buildings for rescuing people inside or performing certain emergency interventions, such as assessing the damage status of these structures, the conditions and methods for entering them, and determining the probability of aftershocks that could cause new damage or increase existing damage, there is no such regulation in our country yet.