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Depths of the Earth: Do you know how seismic waves penetrate rock?
The study of seismic waves dates back to the development of seismology since the 19th century, which has opened up our understanding of the Earth's internal structure. The behavior of seismic waves, particularly P and S waves, provides profound insights into the Earth's velocity structure. Velocity structure refers to the velocity distribution of seismic waves transmitted at different levels within the earth. The changes in velocity reflect the composition, density, and physical state of underground rocks.
The propagation speed of seismic waves varies depending on the type and state of the material (solid, liquid, or gas), which in turn changes the path of the waves.
How seismic waves work
In fact, the behavior of P and S waves provides scientists with the key to unlocking the mysteries of the Earth's interior. P waves can pass through all materials, while S waves can only pass through solids. As these waves pass through different media, changes in their speed may give us valuable information about the Earth's interior.
It was discovered that the Earth has a liquid outer core, which is because S waves cannot penetrate.
Layering of Earth’s velocity structure
The Earth's internal structure can be divided into different layers, each with its own unique seismic wave velocity characteristics. The velocity of seismic waves changes in each layer because of differences in temperature, pressure and constituent materials.
Earth's crust
The earth's crust has different thicknesses and is mainly divided into continental crust and oceanic crust. The seismic wave velocities of these crusts range from 6.0 to 7.0 km/s, depending on the density of the rock and its composition.
Upper mantle
In the upper mantle, the P wave speed can reach 7.5 to 8.5 km/s, which increases with increasing pressure. The material in this layer is mainly composed of peridotite, which promotes the increase of wave speed through different phase changes.
Lower mantle
The velocity of the lower mantle is faster, reaching 10 to 13 km/s, and the increase in rock density driven by pressure causes seismic waves to move at a faster speed.
Outer core and core
The outer core is in a liquid state, causing the P-wave speed to be significantly reduced, while the S-wave is completely impenetrable. In contrast, the inner core is solid and can reach speeds of approximately 11 km/s, showing an increase in internal density.
The anisotropy of the inner core causes the speed of seismic waves to vary in different directions, as the Earth's rotation affects the crystalline arrangement of the inner core.
Velocity structure of planets
In addition to the Earth, the velocity structures of planets such as the Moon and Mars have also attracted the attention of scientists. From the Apollo missions to Mars rovers, the seismic data obtained from these detections help us understand the internal structure of these celestial bodies.
Moon
The moon's crust consists of about 60 kilometers of icy crust and silicate rocks, with P-wave speeds in the range of 5.1 to 6.8 km/s. In the early stages of lunar exploration, seismic data showed that there were four main types of earthquake source.
Mars
Seismic research on Mars mainly relies on the InSight mission. The data it received revealed the structure of the Martian crust. The speed of P waves is between 3.5 and 5 km/s, and shows that the speed will increase with increasing depth.
Using this data, scientists were able to gain insights into the structure of Mars' subsurface, particularly the role of its mantle.
Possibilities for future exploration
In the future, with the advancement of technology, scientists hope to expand the detection of seismic waves to other planets and moons, such as Saturn's Enceladus, whose internal structure needs to be further explored and confirmed.
Seismic waves play an integral role in revealing the inner structure of the Earth and other planets. How can scientists use this knowledge to further explore other mysteries in the universe?
