In this blog post, we’ll explore how seismic waves—which cause significant damage during earthquakes—are used to investigate the Earth’s internal structure and determine the location of earthquakes and the thickness of the crust.
In the past, South Korea was classified as an earthquake-safe zone. However, as the frequency of earthquakes in the country has recently increased sharply, people who had lived their lives without giving much thought to earthquakes have become flustered and fearful when faced with sudden tremors. As a result, when people hear the term “seismic waves,” they may think of them only as a terrifying phenomenon that causes great damage. However, seismic waves are actually put to very useful applications in academic research and the field of earth sciences.
Seismic waves are waves that carry energy accumulated within rocks when force is applied to them, causing them to deform rapidly beyond their elastic limit and fracture. In other words, seismic waves are a major cause of the damage resulting from earthquakes. Seismic waves can be broadly divided into body waves—such as P-waves and S-waves—and surface waves, which propagate along the Earth’s surface. P-waves, short for “Primary Waves,” are the first seismic waves to arrive when an earthquake occurs. S-waves, short for “Secondary Waves,” are seismic waves that arrive after the P-waves. Surface waves include Love waves and Rayleigh waves; as they propagate along the Earth’s surface, they cause significant vibrations and have a major impact on earthquake damage. Among these, P-waves and S-waves are particularly widely used in seismic observation and research on the Earth’s internal structure.
P-waves are longitudinal waves in which the direction of vibration in the medium is parallel to the direction of wave propagation; with a speed of approximately 5–8 km/s in the Earth’s crust, they are the fastest of all seismic waves. They can travel not only through solids but also through liquids and gases. S-waves are transverse waves in which the direction of vibration in the medium is perpendicular to the direction of wave propagation; their speed in the Earth’s crust is approximately 3–4 km/s. Unlike P-waves, they cannot travel through liquids or gases and propagate only through solids. Because S-waves cause shear motion that shakes the ground side-to-side or up-and-down, they can cause significant damage to buildings and people; surface waves also have a major impact on earthquake damage.
However, these seismic waves are already being utilized in various research applications. A prime example is the exploration of the Earth’s internal structure. Seismic waves are reflected or refracted at interfaces where the properties of the medium differ, and even the same type of seismic wave travels at different speeds depending on the density and state of the medium. By leveraging these characteristics, scientists have been able to confirm the existence of the Earth’s outer and inner cores. As seismic waves pass through the boundary between the crust and the mantle, their speed changes significantly; at the boundary between the mantle and the outer core, the speed of P-waves suddenly decreases, and S-waves cease to pass through. This led to the discovery of the outer core, and the fact that S-waves cannot pass through the outer core also revealed that the outer core is in a liquid state. Furthermore, at the boundary between the outer and inner cores, the speed of P-waves changes significantly once again; these characteristics of seismic waves allowed scientists to confirm the existence of the inner core as well.
In addition to exploring the Earth’s internal structure, seismic waves are used to determine the location of the hypocenter—the point where an earthquake originates—and the epicenter—the point on the Earth’s surface directly above the hypocenter. First, the distance between the focus and an observation station can be calculated using the PS time—the difference between the arrival times of the P-wave and the S-wave. By calculating the distance from each observation station to the focus, circles can be drawn with each station at the center and the distance to the focus as the radius. The intersection of these circles allows us to estimate the location of the focus, where the earthquake originated, and by projecting this onto the Earth’s surface, we can determine the location of the epicenter.
Seismic waves can also be used to determine the thickness of the Earth’s crust. Seismic waves travel faster in the mantle than in the crust. When an earthquake occurs, seismic waves spread out in all directions; there are direct waves that arrive at a given point directly through the crust, and refracted waves that arrive after passing through the mantle. At nearby locations, direct waves arrive first because it takes time for the seismic waves to travel down into the mantle and back up again. Conversely, at distant locations, refracted waves arrive before direct waves because seismic waves travel faster in the mantle.
The distance to the point where direct and refracted waves arrive simultaneously is called the intersection distance. By using this intersection distance, along with the seismic wave velocities in the crust and the mantle, we can determine the thickness of the crust. Therefore, seismic waves are not merely a phenomenon that transmits vibrations caused by earthquakes; they are also utilized as an important research tool for exploring the Earth’s interior and understanding the structure of the crust.
As such, seismic waves have two sides: while they can cause significant damage to our lives and paralyze urban functions, they also provide great academic value. Rather than viewing seismic waves—which can be both a poison and a cure in our lives—as merely a negative force, it is important to understand their characteristics and conduct more diverse research into ways to utilize them scientifically.
