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Seismic Velocity Structure
Velocity structure is a spatial distribution and variation of seismic wave velocities that travel underneath the Earth's surface or other planetary bodies. These velocities can be indicative of the subsurface conditions such as the material composition, density, temperature, (porosity and state of stress, among other factors). The ability to analyse and interpret the velocity structure enables geophysicists to construct more accurate models of subsurface geology, which is instrumental in resource exploration, earthquake seismology, and understanding Earth's geological evolution.

History
The development of velocity structure evolved alongside the advancement of seismology. The model of Earth's velocity structure rapidly developed since the invention of the seismogram in the 19th Century which allows seismic waves to be recorded and studied.

20th Century:
Andrija Mohorovičić's identified the Mohorovičić discontinuity in 1909 which marked the boundary between crust and mantle. At the Mohorovičić discontinuity, the seismic velocity change abruptly from 6.7–7.2 km/s to 7.6–8.6 km/s. (firsrt 1-D model in 1910??)

Beno Gutenberg with his continuous work in early to mid 20th Century identified a seismic velocity discontinuity at the core-mantle boundary (CMB). With the advent of the World Wide Standardized Seismograph Network (WWSSN) in the 1960s, data collection on global seismic activity and velocities improved significantly. The development of seismology supported the plate tectonics theory to gain acceptance in the 1960s. Meanwhile, plate tectonics offered a framework for understanding the lateral and vertical variation in seismic velocities across different geological settings.

In the 70s to the 80s, researchers like Aki, Dziewonski, and others contributed significantly to the development of algorithms and methodologies crucial for seismic tomography which help studying the velocity structure of the Earth.

A global velocity structure model of the Earth was built upon the Preliminary Reference Earth Model (PREM) developed by Adam M. Dziewonsk and Don L. Anderson in 1981. [show figures, try to make a new figure with PREM data?]

In 1984, the Global Seismic Network (GSN) was launched by the Incorporated Research Institutions for Seismology (IRIS) to replace the WWSSN and serve as a permanent network to monitor Earthquake.

21st Century:
With the advancement in seismic tomography and the expanding GSN the velocity structure of the Earth had been refined. Furthermore, increased computational capabilities facilitated complex modeling and inversion techniques to deduce the Earth’s velocity structure with unprecedented precision.

Current advancement included the refinement of velocity structure in the inner core and the use of different technique such as ambient noise tomography to image the velocity structure.

Principle (in studying seismic velocity structure)
The study of Earth's velocity structure is rooted in principles of seismic waves. The fact that seismic wave travel at different velocity allow researchers to study the internal structure of the Earth ?. The change in seismic velocity will change the direction of seismic wave crossing boundary between materials with different elastic properties which lead to seismic refraction governed by Snell's Law. On the other hand, the seismic reflection also serves as a principle to study velocity structure. Seismic wave reflects when encountering a layer with different material properties. By analysing the travel time of a reflected seismic wave, it is able to reconstruct the seismic velocity structure.

By using different types of seismic waves, researchers are able to reveal the seismic velocity structure by analysing the propagation velocity and properties of Earth's internal structure. The speed at which P-waves travel varies with the type, temperature, and pressure of the material. For instance, they travel faster through denser rocks than through lighter ones and even faster through solid rock than through molten material or water. The property that P-waves can travel through solids, liquids, and gases, allowing them to traverse the entire Earth enables researchers to study both the crust and deeper structures, including the core[?]. The S-waves, unlike P-waves only travel through solids. This behavior has been crucial in determining that the outer core of the Earth is liquid, as S-waves are not observed to pass through this region. The speed of S-waves provides information about the rigidity (shear modulus) of Earth materials. Variations in S-wave velocities help identify differences in rock composition, temperature, and other properties.

(If the Earth is made of isotropic material and equal density, there won't be any difference in the velocity structure, thus, the variation of velocity structure indicate the anisotropic or aspericity properties of the Earth .)

Limitation/Uncertainty
The velocity structure of the inner core has proven challenging to study. The most direct way to study the inner core velocity structure would be through direction observation. However, the seismic phase PKJKP is difficult to directly observe due to the weak compressional-shear wave conversion in the inner-outer core boundary (ICB) and the strong attenuation of the shear wave in the inner core. While recent advancement…