Basic Geophysics: Body Waves

Summary

This video script delves into seismic body waves, using the 2011 Great Tohoku Earthquake as a case study. It explains the nature of P-waves and S-waves, their different properties, and how they travel through the Earth. The script highlights how seismic waves help scientists determine earthquake origins, map Earth's internal structure, and measure ground motion. Through clear explanations of wave frequencies, velocities, and the use of seismographs, viewers gain insight into how seismic data aids in understanding earthquakes and the Earth's composition. The video emphasizes the importance of seismic waves in both earthquake localization and geological studies.

Takeaways

• 😀 Seismic waves from earthquakes spread out in all directions, causing the earth to oscillate and can be measured with seismometers.
• 😀 Body waves, such as P-waves and S-waves, are seismic waves that travel through the earth's interior and oscillate the earth's surface.
• 😀 P-waves (primary waves) are the fastest and first to arrive. They move longitudinally, causing oscillation parallel to the direction of propagation.
• 😀 S-waves (secondary waves) arrive after P-waves and oscillate transversely, moving perpendicular to the direction of propagation.
• 😀 Surface waves are the last to arrive and have larger amplitudes than P-waves and S-waves, traveling along the earth's surface.
• 😀 The frequency and wavelength of seismic waves are linked, with higher frequencies corresponding to faster oscillations and smaller wavelengths.
• 😀 P-waves travel faster than S-waves, with P-wave velocities approximately 1.7 times faster than S-waves in typical rock types.
• 😀 S-waves cannot travel through liquids (such as water or the earth's liquid outer core) because they require a shearing medium to propagate.
• 😀 By measuring the arrival times of seismic waves at different seismometer locations, the earthquake's epicenter and hypocenter can be determined.
• 😀 Seismic body waves are used to investigate the earth's internal structures, allowing geologists to create tomographic maps of the earth's interior.

Q & A

What are seismic body waves and how do they relate to acoustic waves?

Seismic body waves are waves that travel through the interior of the Earth, causing the ground to oscillate. These waves are similar to acoustic waves, which require a medium to propagate. In seismic body waves, the Earth acts as the propagation medium, causing longitudinal oscillations (P-waves) and transverse oscillations (S-waves).

What is the significance of the P-wave and S-wave arrival times in seismology?

P-waves are the first to arrive because they are faster than S-waves. By analyzing the arrival times of these waves at different seismometer stations, scientists can calculate the distance to the earthquake’s epicenter and determine other properties, such as the hypocenter and the earthquake's depth.

What distinguishes P-waves from S-waves?

P-waves (Primary waves) are faster and move in the same direction as the wave propagation, meaning they are longitudinal waves. S-waves (Secondary waves) are slower, oscillating perpendicular to the propagation direction, and are transverse waves. P-waves can travel through solids, liquids, and gases, while S-waves cannot travel through liquids.

Why do S-waves not travel through the Earth's outer core?

S-waves are transverse waves that require a medium capable of shearing, such as solid materials. The Earth's outer core is liquid, which means it cannot support shear stress, thus preventing S-waves from passing through it.

How are seismic waves used to determine the depth of an earthquake's source?

The difference in travel times between P-waves and S-waves can be used to determine the earthquake's depth. By analyzing these time differences across various seismic stations, seismologists can calculate the vertical distance to the earthquake's hypocenter.

What is the relationship between seismic wave frequency, wavelength, and propagation velocity?

Seismic wave frequency is inversely related to wavelength. Higher frequencies correspond to shorter wavelengths. The relationship is governed by the equation λ = v / f, where λ is the wavelength, v is the propagation velocity, and f is the frequency. In seismic waves, P-waves typically have higher frequencies and shorter wavelengths than S-waves.

How does the speed of seismic waves vary with rock type?

The speed of seismic waves depends on the type of rock. For instance, P-wave velocities are 2-4 km/s in sandstone and 4-7 km/s in granite. The speed increases with depth, reaching up to 14 km/s in the Earth's lower mantle, influenced by the rock's crystal structure.

What role do seismic body waves play in earthquake localization?

Seismic body waves are essential for earthquake localization. By measuring the arrival times of P-waves at different seismometer stations around the world, scientists can triangulate the earthquake's epicenter and determine its origin or hypocenter.

What is Earth tomography, and how does it use seismic waves?

Earth tomography is a technique used to study the Earth's internal structure. By analyzing how seismic waves travel through different layers of the Earth, scientists can infer the properties and compositions of the Earth's interior, especially in regions the size of seismic wavelengths.

Why do P-waves arrive before S-waves on a seismogram?

P-waves travel faster than S-waves because they are longitudinal waves, causing particles to oscillate in the same direction as the wave's motion. S-waves, being transverse, have slower propagation speeds, leading to a delayed arrival compared to P-waves.