Revisiting Seismic Design Post Türkiye/Syria Quake Part 1 - Earthquake Causes

1. Introduction

On February 6, 2023, at a time when the COVID-19 pandemic was nearing its end, a magnitude 7.8 earthquake struck the border between Syria and Turkey, specifically in Türkiye. The incident has been reported to have caused an unprecedented loss of human lives, with 50,000 reported fatalities and several hundred thousand injuries.

Turkey earthquake: Death toll could increase eight-fold, WHO says - BBC News

Kahramanmaras earthquake - Google Maps

While the media reports on earthquake damages often criticize the inadequacies of seismic design, there has been no specific indication of which aspect of the seismic design was flawed. It is indeed challenging to pinpoint exactly what went wrong in the seismic design.

Moreover, when facing extensive earthquake damage, it is acknowledged that human errors may have played a role. However, similar to the inability to perfectly prepare for natural disasters such as floods and droughts, achieving flawless seismic design is nearly impossible as long as humanity continues to utilize gravel and sand— the most cost-effective natural materials— in the construction of structures.

Kahramanmaraş'taki depremde İMO binası ayakta kalırken çevresindeki yapılar yerle bir oldu (haberturk.com)

The reason I accepted the manuscript request from MOTIVE was not to point out the shortcomings of seismic design without having access to the observed seismic waveforms and spectra. Instead, I hope that civil and architectural engineers will recognize seismic damage as the result of the interaction between seismic waves inputting into structures and the resonance of the structures. My intention is to encourage insightful perspectives on this matter.

Furthermore, it is essential to utilize artificial intelligence algorithms, such as deep learning, in the assessment of seismic performance and seismic reinforcement methods. The reason for advocating the use of this technology lies in its appropriateness, especially in enhancing the vulnerable seismic performance of widely adopted piloting structures, often used for parking space optimization.

All architectural and civil structures have evolved over time in response to the natural environment and contemporary circumstances, and every object subject to Earth's gravity has been constructed to withstand its influence.

Natural materials, such as stone, possess strength against compression but are weak in tension. The invention of the arch structure during ancient Roman times allowed for the construction of grand structures and bridges, overcoming such material weaknesses.

However, arch structures using stone as a material can adequately withstand vertical loads, such as gravity, but they exhibit structural weaknesses when subjected to horizontal seismic loads. Thus, structures that may be structurally robust against vertical forces can become powerless against horizontal forces. Seismic design is the engineering practice of designing structures to withstand horizontal forces, particularly seismic loads.

However, in the face of tremendous seismic events such as the 1995 Great Hanshin Earthquake in Japan, which led to the destruction of structures amounting to 20 million tons of debris, the 2011 earthquake and tsunami causing damage to the Fukushima nuclear power plant, and the recent earthquake on the Syria-Turkey border, the invention of reinforced concrete that flourished in the 20th century construction culture is now seen through the lens of a quaint slogan—minimizing human casualties due to collapse, rather than ensuring absolute safety.

If we intend to term the engineering practice of safely designing structures against major earthquakes as seismic design, it should be capable of usage without the need for repairs immediately after an earthquake and should preserve not only human lives but also the national assets.

However, ensuring the safe design of structures against powerful seismic events without accompanying a shift in new conceptual approaches is by no means an easy task.

We intuitively understand that objects with weak stiffness are easily shaken, while those with strong stiffness are not easily shaken. How then is it possible to construct tall buildings with structures prone to shaking in earthquake-prone regions like Tokyo in Japan or Los Angeles in the United States?

All issues related to vibrations, including earthquakes, can be explained by resonance phenomena, and seismic design aims to prevent or overcome resonance phenomena. It is through effective seismic design that the construction of tall buildings with structures susceptible to shaking is made possible in earthquake-prone areas like Tokyo and Los Angeles.

Resonance refers to the phenomenon of reciprocal vibrations between two objects and necessarily involves the concept of frequency.

In other words, if identical tuning forks are placed apart, and an impact is applied to one of them, causing energy to be transmitted through the air, the untouched tuning fork on the other side will start vibrating. This phenomenon is referred to as resonance.

Here, the expression "identical tuning forks" signifies that the vibrational frequencies of the two objects are the same.

The relationship between earthquakes and structures is similar to this. When an earthquake occurs, causing significant shaking at the Earth's surface, all objects on the surface experience forces. Objects with vibrational frequencies close to that of the earthquake may experience greater forces and are prone to collapse.

Therefore, understanding the vibrational characteristics of earthquakes and designing structures to withstand them is the essence of seismic design.

In other words, actively preparing for earthquakes, a natural disaster, involves a seismic approach that understands the characteristics of seismic waves. In the context of reinforced concrete structures, practical measures to prepare for earthquakes include improving the connection of main reinforcing bars, adjusting the spacing of stirrups, and enhancing the detailing of reinforcement, all aimed at mitigating the impact of earthquakes.

2. The principle of earthquake occurrence

In ancient times, the causes of earthquakes were often speculated from a religious perspective, such as the wrath of gods or the anger of monsters living beneath the Earth. However, starting from the late 19th century, scientific interpretations of earthquakes began to emerge through the observation of seismic activities.

To generate seismic waves in the rigid lithosphere composed of solid rock, there are two direct scenarios to consider: either an impactful force acts on a certain part of the lithosphere, leading to the formation of faults as a seismic result, or the lithosphere becomes incapable of withstanding the forces acting upon it, resulting in abrupt destruction. The former hypothesis suggests that faults are formed as a result of seismic forces, while the latter proposes that the breakdown of rocks forming faults is the cause of earthquakes.

In other words, the former attributes the creation of faults to an impactful force, namely earthquakes, and the latter claims that earthquakes arise from fault movements.

The debate about which comes first, the chicken or the egg, found resolution in the theory of plate tectonics based on continental drift. According to this theory, widely accepted as the correct interpretation, earthquakes occur as a result of faults created through plate movements.

2.1 Theory of plate tectonics

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To understand seismic movements as a result of fault activities, it becomes crucial to determine the cause of forces leading to fault formation. The plate tectonics theory emerged in the late 1960s as an explanatory framework for this phenomenon.

When aligning the western coast of Europe and Africa, which border the Atlantic Ocean, with the eastern coast of North and South America, the junction points fit remarkably well. Moreover, various aspects, such as the geological features, rock strata, and the distribution of organisms and fossils, align significantly in the connecting regions of these continents.

From these observations, it was once believed that a single continent had split around 100 million years ago and had drifted east and west, leading to the creation of the Atlantic Ocean in the process. The proponent of this plate tectonics theory was the German geologist Alfred Wegener, who first formulated this concept in 1912.

As another new piece of evidence supporting the theory of continental drift, there was a development in the 1950s related to paleomagnetism.

When igneous rocks cool below a certain temperature, the minerals they contain acquire a magnetic alignment corresponding to the Earth's magnetic field (with the north being the N pole and the south being the S pole). Therefore, if continents had not moved, rocks from the same era on any continent should exhibit the same magnetic alignment. However, in reality, this is not the case. To resolve this inconsistency, it became necessary to accept the idea that continents had indeed moved.

Additionally, estimating the thickness of marine sediment and the age of underwater rocks reveals that the closer one is to the mid-ocean ridge, known as the mid-oceanic ridge, the thinner the sediment and the younger the age of the rocks. Even in the oldest ocean floor, it is known that the age does not exceed 200 million years. To explain this fact, a hypothesis is required that involves the eruption of lava near the mid-ocean ridge, leading to the creation of new oceanic crust.

Furthermore, since the total area of the ocean floor cannot decrease, there must be somewhere where the ocean floor disappears, and these locations where the ocean floor is consumed are known as subduction zones. Assuming a movement rate of 5 to 10 cm per year for the creation and consumption of oceanic crust, calculations suggest that the entire Earth's surface could be completely transformed into new terrain within the next 200 million years.

Another hypothesis regarding the mass extinction of dinosaurs millions of years ago attributes their demise to a colossal meteorite impact, causing drastic climatic changes. This impact-induced extinction theory suggests that dinosaurs, being reptiles with limited temperature regulation capabilities, perished due to the environmental consequences of a massive meteorite collision. However, despite extensive research, scientists have not found direct evidence on Earth of a large-scale impact corresponding to the time of the dinosaur extinction. Yet, the subduction of oceanic crust, supported by plate tectonics, raises the possibility that the traces of such an impact may have been erased by the subduction process.

Connecting the mid-ocean ridges and subduction zones on the Earth's surface, the outer layer of the Earth is divided into 13 large and small plates, including the Pacific Plate, Eurasian Plate, and North American Plate. These plates, which consist of rock layers with a thickness of about 70 km beneath the oceans and even more under continents, are confirmed to be moving in various directions at speeds of approximately 5-7 cm per year through measurements using artificial satellites (Figure 1.1).

At the boundaries of each plate,

• Two plates may overlap, with one plate sliding beneath the other and disappearing into the Earth's mantle.

• Plates can collide, causing one to be forced upward.

• Two plates might collide and slide past each other horizontally.

• Additionally, when two plates move apart, magma can rise through the gap, leading to the creation of new crust.

The discovery of fossils of marine organisms at the summit of the Himalayan mountain range is further evidence supporting the fact that plates collide and force material upward. Examining the epicenters of earthquakes that occurred from the late 1960s to the present to investigate the correlation between these plates and earthquakes reveals that most earthquakes originate at or near the boundaries of these plates or within the interior of the plates near their boundaries (Figure 1.2).

2.2 the driving force behind the plate movement

What force drives the movement of these plates?
The plates are positioned on a relatively weak layer of magma called the asthenosphere within the Earth. Since there is a convection phenomenon in all liquids where hot substances rise and cold substances sink, the convective movement of magma within the Earth acts as a force pulling down the lower part of the plates.
Additionally, considering the weight of the rock masses forming the plates, which is heavier and denser than the magma beneath the plates, it is believed that gravitational forces cause the plates to be drawn into the magma and move (Figure 1.3).

Thus, plate tectonics are driven by the temperature differences between the Earth's surface, which is cooler, and the hotter, thermally unstable interior. Ultimately, the ultimate cause of earthquakes can be attributed to the heat within the Earth, primarily supplied by the decay of radioactive substances contained within the Earth.

2.3 Elastic rebound theory 

So, what is the relationship between plate tectonics and earthquakes? The theory that connects these two facts is the theory of elastic rebound.

In other words, when one plate slides beneath another due to the convection currents in the magma, it doesn't smoothly glide without any friction. As the Earth's surface has irregularities, the upper plate is pulled and sinks due to friction. During this process, the upper plate experiences a force trying to pull it down into the subduction zone formed by the surface of the lower plate.

Due to this force, the upper plate undergoes stress, and when this stress accumulates and exceeds the elastic limit of rocks, energy is released through a phenomenon known as faulting, a type of rock destruction, causing earthquakes (Figure 1.4).

This phenomenon is referred to as the theory of elastic rebound.

When observing the phenomenon of seismic elastic rebound on a broader scale, considering the entire crust, the location where energy is released moves along the plate boundaries. From the perspective of a specific region, the accumulation and release of deformation energy occur repeatedly, resulting in seismic events with periodicity.

Therefore, by measuring the deformation of the crust at a particular location, it becomes possible to predict specific earthquakes.

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Looking at the distribution of earthquake-prone areas known so far, earthquakes are not evenly distributed worldwide. Instead, they tend to occur more frequently in specific regions.

Particularly earthquake-prone areas include the region from Southeast Asia through the Middle East to Southern Europe, referred to as the Eurasian Seismic Belt, and the region from the Pacific region to the west of South America, known as the Circum-Pacific Seismic Belt.

Since the late 19th century, seismographs have been installed worldwide, and the seismic observation network has gradually improved. By examining the distribution of earthquake epicenters obtained through such networks, the existence of the mentioned seismic belts becomes more evident (Figure-1.2).

The alignment of these seismic belts with the boundaries of tectonic plates provides clear evidence supporting the theory of elastic rebound.

Earthquakes primarily caused by the elastic rebound of the crust tend to have deep hypocenters and involve a significant release of energy. Moreover, they often occur under the sea, contributing to the frequent occurrence of tsunamis.

However, earthquakes are not limited to occurrences only at plate boundaries; they frequently happen within the continental regions of China, where there are no plate boundaries. The elastic rebound theory alone cannot explain the causes of these earthquakes. Instead, within the plates, compressive or tensile forces are at play, leading to the rupture of weak rock masses within the plates and causing earthquakes. These earthquakes are referred to as intraplate earthquakes (Figure-1.4).

Intraplate earthquakes, characteristic of having a relatively low energy release, can still cause significant damage due to their proximity to densely populated areas and shallow depth beneath the Earth's surface, resulting in intense shaking. For instance, the Tangshan earthquake in Northern Hebei Province, China, on July 28, 1976, claimed the lives of 242,000 people, with 164,000 reported injuries.

Concerning the Korean Peninsula, we can consider intraplate earthquakes. Historically, there are records of numerous earthquakes during the Three Kingdoms period. In recent times, notable events include the 1936 Jirisan earthquake and the 1978 Hongseong earthquake. While the occurrence of earthquakes in recent years may be relatively low, considering the cyclical nature of seismic activity, it is crucial to recognize that the Korean Peninsula is not exempt from seismic hazards.

#Turkey-Syria Earthquake 2023
#Seismic Design Challenges
#Plate Tectonics

About the Editor

GyusikJeon

Doctor of Architectural Engineering Over 20 years of experience

Dr. Gyusik Jeon, with over 20 years of experience in Architectural Engineering, has a diverse career including roles at KEPCO's Power Research Institute (1980-1997), Technical Director at Unison Construction (1997-2000), and currently works with a seismic base isolators production company. He graduated from Busan National University in Civil Engineering and holds a Master's and Ph.D. from the University of Tokyo's Seismic Research Institute.

 

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