“How prepared are we for a major earthquake?”
The earthquake story you need to know now, from Korea's top earthquake expert.

The story of earthquakes told by Professor Hong Tae-kyung, Korea's leading earthquake expert and the first person the media looks for when an earthquake occurs.
This book contains essential knowledge about earthquakes, from the principles of their occurrence, observation and analysis methods, to earthquake disasters and response, earthquakes around the Korean Peninsula, and the applications of seismology.
This book provides an easy-to-understand explanation of plate tectonics and the mechanisms of faults, as well as methods for observing and analyzing earthquakes, and the types and patterns of occurrence of various earthquakes.
It also presents lessons we can learn from major earthquakes that occurred on the Korean Peninsula and in Japan, and addresses the importance of countermeasures to reduce earthquake disasters, earthquake-resistant design, and early warning systems.
Furthermore, we explore how seismology is used in diverse fields, such as nuclear power plant safety inspections, volcanic activity monitoring, and nuclear test detection, and we broadly examine earthquakes occurring not only on Earth but also on other celestial bodies.
Through this book, readers will be able to obtain accurate and useful information about earthquakes and raise awareness.

An earthquake is a natural phenomenon that occurs when a huge amount of energy accumulated underground is suddenly released along a fault.
Where does this immense power come from? Sometimes, earthquakes are triggered by human activities, such as large-scale explosions like nuclear tests. Some research suggests that tidal effects caused by the gravitational pull between the Earth and the Moon are linked to earthquakes in specific areas, such as the Nankai Trough in Japan. However, most earthquakes are closely linked to the movements of the Earth's interior.

--- p.20

Most earthquakes in Korea, including those that occur at depths of about 5 to 15 km, occur within several dozen kilometers underground.
This is because stress is easily relieved near the shallower surface, but at that depth, compressive force is continuously applied and stress gradually accumulates.
When this accumulated stress reaches its limit, an earthquake occurs at the weakest point, causing tectonic displacement.
So what about deeper depths? In fact, at very deep depths, the high temperatures and pressures cause ductile deformation of rocks, releasing stress, making earthquakes less likely to occur.

--- p.28

The saying “softness overcomes strength” also applies to the role of water in earthquakes, as water acts as a catalyst to promote earthquakes by reducing compressive stress in fault zones.
Water flowing into the underground space slowly fills the space, causing significant changes in the stress environment of the fault.
Water creates tension in a radial direction, which reduces the compressive stress on the fault, and consequently reduces friction, which promotes the occurrence of earthquakes.
The greater the amount of water, the lower the compressive stress, so the frequency of earthquakes may increase when groundwater inflow or water is injected into a fault zone.

--- p.39

An induced earthquake is an earthquake that occurs when human activities directly and dominantly affect a fault that previously had little chance of occurring, while a triggered earthquake is an earthquake that occurs when an external factor acts as a small trigger on a fault where stress has already reached a critical point.
This can be metaphorically understood as a cup with little water and a cup full of water.
The amount of water needed to overflow each cup is different.
An induced earthquake is like pouring a lot of water into an empty cup at once and causing it to overflow, while a triggered earthquake is like adding a few drops of water to an already full cup and causing it to overflow.

--- p.43

Super earthquakes don't just shake the Earth's crust.
Surface waves that travel around the Earth cause strong vibrations, which can trigger a chain of earthquakes in various parts of the Earth.
Additionally, megaquakes affect the Earth's gravitational field because they cause displacement of large masses of material.
As a result, gravitational waves are generated and propagate at the speed of light.
During the 2011 Great East Japan Earthquake, these gravitational waves were detected by seismometers and radio telescopes, and the effect of gravitational waves on the medium was observed, causing vertical movement of the medium that rose and fell by about 20 cm for 500 seconds, even on the Korean Peninsula, 1,500 km away.
Every person and every facility on the Korean Peninsula has undergone this transformation, but it has happened gradually over a long period of time, so we have not been aware of it.
The long-term impact of these gradual, large-scale changes is not yet known.

--- p.53~54

Earthquakes, like people, can record birth information.
Just as when a person is born, basic information such as parents, place and time of birth, and the physical condition of the newborn are recorded on a birth certificate, earthquake information such as the time of occurrence, location, and magnitude are quickly calculated and announced. This information is called epicenter information.

--- p.65

Digitizing analog records like this required meticulous care and patience, and even converting a single seismic waveform took a long time.
Analyzing the massive amount of data required more time and effort.
During the digitization process, research results can vary depending on how accurately the researcher converts the data.
Based on these analog records, the internal structure of the Earth, consisting of the crust, mantle, and core, was already discovered in the 1930s.
These achievements, achieved in an era before digital technology, offer a glimpse into the passion and dedication of seismologists at the time.

--- p.71

Understanding the Earth through seismic analysis, experiments, and exploration is like putting together a puzzle.
It is difficult to understand the whole picture with just one puzzle piece, but by fitting the different puzzle pieces together, you can eventually grasp the whole picture and reality.
Sometimes it may seem like you're repeatedly fitting similar pieces together, but this is part of a process of gradually understanding reality by gathering shadows projected from different angles.
Understanding the Earth is also a gradual process that involves synthesizing various observations and research results.

--- p.87

One of the most frequently asked questions about earthquake disasters is whether frequent small earthquakes will relieve stress on fault planes and reduce the risk of earthquakes.
It is true that when an earthquake occurs, the stress on the fault plane decreases, but the amount of stress relieved by small earthquakes is very small.
For example, the energy of 1,000 magnitude 2.0 earthquakes is equivalent to the energy of one magnitude 4.0 earthquake.
That means it would take 1,000 magnitude 2.0 earthquakes to release enough stress to cause a magnitude 4.0 earthquake.
Although the amount of stress released by a small earthquake is so small, it plays a significant role in weakening the fault plane.
Small earthquakes cause small amounts of damage along the fault plane, creating a weak fault plane that is more susceptible to large earthquakes.

--- p.100

If you are inside a building when an earthquake occurs, keep the front door open to secure an exit and turn off the gas valve to prevent a fire.
Afterwards, you should hide under a desk or table to protect yourself from falling objects.
If you are using an elevator, you should get off at the nearest floor immediately.
After an earthquake, there is a high possibility of aftershocks.
Damaged buildings can collapse even with a small impact, so you should immediately exit and move to a safe place.
After the building stops shaking, quickly move to a designated outdoor shelter or safe open space.
Earthquake shelters are mostly designated as open spaces that are safe from falling objects, such as school playgrounds or parks.
Tsunami shelters are provided in coastal areas, and public buildings and open spaces on high ground are used.
It is important to check the shelter in advance, and always use the stairs when evacuating.
Because an earthquake can cause the elevator to break down.

--- p.109~109

However, this does not mean that we can give up on efforts to detect earthquakes in advance.
The scientific community is currently focusing on methods for predicting earthquakes through direct exploration of fault zones.
For example, methods for detecting micro-earthquakes or measuring strain in fault zones are representative examples.
It is important to monitor these microearthquakes that occur along fault lines because they can weaken the fault plane and lead to a larger earthquake.
In addition, research is actively being conducted to estimate the state of the fault zone by measuring the electrical resistivity of the fault plane, and to determine the possibility of an earthquake by calculating the accumulated stress.
As seismologists continue to synthesize diverse information to predict earthquakes, earthquake prediction technology is expected to further advance in the future.

--- p.118~119

During the 2011 Great East Japan Earthquake, Japan used its early warning system to prevent accidents and reduce damage by halting high-speed trains and immediately cutting off electricity to hospitals.
In Korea, the rapid delivery of earthquake information contributed to reducing public anxiety during the 2016 Gyeongju earthquake and the 2017 Pohang earthquake.
In particular, in the Gyeongju earthquake, the time from observation to announcement was recorded as 26 seconds, and if it had been faster, the damage could have been reduced further.
Since then, the system has been continuously improved, and in the case of the Ganghwa earthquake in 2023, earthquake warnings were announced nationwide and disaster text messages were sent out 9 seconds after the earthquake occurred, and in the case of the Gyeongju earthquake in November of the same year, seismic waves were detected 5 seconds after the earthquake occurred, so it can be said that the system is approaching the level of advanced countries in earthquake prevention.

--- p.124

The 2011 Great East Japan Earthquake clearly demonstrated the destructive power of tsunamis.
The sight of a tsunami suddenly crossing the coastline and engulfing the land was deeply shocking to many people.
Tsunamis can travel at speeds of up to hundreds of kilometers per hour, making escaping in slower vehicles or on foot potentially dangerous.
If you notice a tsunami, move to higher ground without hesitation or quickly evacuate to a high place such as a nearby high-rise building.

--- p.131

The metropolitan area, home to over 33 million people, suffered significant damage from the 7.9 magnitude Great Kanto Earthquake in 1923, and a 2005 survey confirmed the existence of a major fault line between 4 and 26 kilometers beneath Tokyo.
If an earthquake occurs on this fault, the seismic waves from the direct earthquake occurring directly under the city can be transmitted to the surface, causing serious damage.
Additionally, sedimentary layers in eastern Tokyo could further amplify seismic activity.
In addition, the possibility of a large earthquake occurring in the Chubu and Kansai regions is being raised.
In particular, the probability that a large earthquake of magnitude 8 or higher will occur in the Tokai region, including the sea off Tokyo, and the Nankai Trough region within 30 years is 80 percent.

--- p.176

Our film "Baekdu Mountain," released in 2019, rekindled public interest in the Baekdu Mountain volcanic eruption.
The film's plot involves Mount Baekdu erupting two or three times, causing massive earthquakes, and using a nuclear bomb to reduce the pressure in the magma chamber to prevent the final explosion.
Since the film's release, there has been a lot of interest in the truth of its content. The structure of the magma chamber at the bottom of Mt. Baekdu depicted in the film was far from factual, and the possibility of a magnitude 7 earthquake occurring due to a volcanic eruption is also low.
Earthquakes caused by volcanic eruptions are generally small earthquakes of magnitude 3 or less.
Additionally, detonating a nuclear bomb to relieve the pressure in the magma chamber is extremely dangerous.
This is because the strong seismic waves generated by a nuclear explosion can cause large stress changes in the magma chamber, which can create bubbles and accelerate volcanic eruptions.
--- p.192~193

There are also cases in Korea where seismology has been used as an important event analysis tool.
The sinking of the Cheonan warship on March 26, 2010, shocked the entire nation, and there was controversy surrounding the exact time of the incident and the cause of the sinking.
The main hypotheses suggested included a North Korean torpedo attack, grounding on a reef, and fatigue failure of the ship.
Afterwards, a reporter from a newspaper received a tip and obtained seismic data from Baengnyeong Island and the surrounding areas through the Korea Meteorological Administration and asked me to analyze it. The analysis results confirmed a clear signal of an explosion.
At first, no clear signal was visible, but after separating the high-frequency signals, it was revealed that the seismograph recorded not only the P-wave, S-wave, and multiple reflections from the underwater explosion, but also the sound of the explosion propagating through the air.
The results of identifying the location of the explosion using data from three observatories were consistent with the sinking point announced by the government's joint investigation team.
Additionally, the exact time of the explosion could be determined using seismic waveform records.
This study was also selected as a notable study by the Seismological Society of America in 2011.

--- p.203

Mars exploration has now moved beyond the government's domain and into an era where private companies such as SpaceX are participating.
Space exploration is no longer a distant dream, but a tangible goal, and seismology is playing a role in its advancement.
Observing earthquakes on Mars will be more than just a scientific discovery; it will be an important first step toward exploring the possibility of human life on Mars.

--- p.213