In this blog post, we explore the scientific principles behind earthquake occurrence and the response measures we can take to minimize the resulting damage.
The powerful magnitude 9.0 earthquake that struck Japan in 2011 caused immense devastation across the entire country. It also triggered a tsunami, resulting in massive loss of life and property damage. This earthquake was also linked to the Fukushima nuclear power plant accident, causing significant international repercussions. A similar major earthquake struck Chile in 2010. That earthquake measured magnitude 9.5, ranking among the strongest recorded in history. Thus, earthquakes transcend being mere natural disasters; they are among the most challenging catastrophes for humanity to prepare for. So, what exactly is an earthquake, and why do they occur? To understand this, we need to examine the structure of the Earth’s interior and its operating principles.
An earthquake is literally a natural phenomenon where “the earth shakes.” More precisely, it occurs when underground rock suddenly fractures due to forces within the Earth, and the resulting shockwaves cause the ground to shake. The energy released is transmitted as seismic waves. As these waves propagate to the Earth’s surface, people feel the earthquake. Two primary scales are used to measure an earthquake’s strength: magnitude and intensity. While many people frequently encounter the terms magnitude and intensity in news or earthquake-related articles, these two concepts are often confused.
Magnitude is a scale representing the absolute strength of an earthquake. Commonly used scales include the Richter scale or the moment magnitude scale. The earthquake magnitude we often encounter in news reports is typically based on magnitude. In contrast, intensity is the relative strength of an earthquake actually felt at a specific location. Even for the same earthquake, it feels stronger closer to the epicenter and weaker farther away. This is similar to how ripples spread out when a stone is thrown into water.
Why do earthquakes occur? Answering this question requires an understanding of the Earth’s plate tectonics theory. The ground we stand on is not fixed; instead, massive plates forming the Earth’s surface are constantly moving. These plates float atop the upper mantle and move very slowly but steadily with the mantle’s motion. When plates collide or push against each other, the resulting impact triggers earthquakes. Because plate movement occurs so gradually, people don’t feel it. However, when this accumulated movement is suddenly released over a long period, it creates the powerful natural phenomenon known as an earthquake.
The shock generated when plates move is transmitted through seismic waves, which are classified into four types: P-waves, S-waves, L-waves, and R-waves. Among these, P-waves are the first to arrive. While they generally cause less damage, systems that detect them can quickly alert authorities to an earthquake’s occurrence. S-waves arrive next, and it is from this point that major damage begins. Particularly in the case of powerful major earthquakes, liquefaction can occur, where the ground behaves like a liquid. This weakens building foundations, significantly increasing the risk of collapse.
An earthquake does not occur in a single event but unfolds in several stages. It can generally be divided into three stages: the foreshocks that occur before the main shock, the main shock delivering the most powerful impact, and the aftershocks that follow the main shock. After the main shock, hundreds of aftershocks may follow, which can also cause additional damage and require caution.
Many people say that precursory phenomena may appear before an earthquake occurs, but there are no scientifically verified precursors. For instance, sudden mass animal movements or unusual cloud formations are sometimes cited as earthquake precursors, but these claims lack scientific evidence. To date, there is no reliable method for predicting earthquakes. Even in countries like Japan, where earthquakes are frequent, the current capability is limited to issuing warnings as quickly as possible after detecting P-waves through emergency earthquake early warning systems.
So, how should we respond when an earthquake strikes? The most crucial action is protecting yourself from falling debris. If you are inside a building, prioritizing taking cover under a table and protecting your head is essential. Especially in countries like ours, where most buildings are constructed of concrete, the risk of collapse is high, making staying inside potentially more dangerous. However, attempting to exit the building during an earthquake carries a significant risk of falling objects striking your head. Therefore, remaining in a safe location until the shaking subsides is safer. Outside buildings, it is crucial to move to the widest, most open space possible. Avoid moving rashly during the earthquake; instead, protect your head and wait until the shaking stops.
Earthquakes are one of the natural disasters humanity cannot control, and especially powerful major earthquakes have greater destructive power the larger their magnitude. No matter how thorough the seismic design, it is difficult to prevent building collapse during earthquakes of magnitude 8.5 or higher. While it is impossible to be completely safe from such disasters, advance preparation and education to minimize damage are essential. Living in seismically designed buildings and knowing how to respond during an earthquake are the most important ways to protect your life from earthquakes.
