We know that when waves (light or seismic waves) penetrate inhomogeneous media or media with structures, they will diverge. This is because waves are composed of components with different frequencies, but how does the structure and structure affect the frequency?
For example: when light passes through a prism, components of different frequencies are refracted at different speeds. Why do different frequencies have different speeds?
How are different frequencies in seismic surface waves separated by different media?
我们知道波（光或者地震波）在穿透不均匀介质或者存在构造的介质时，会发生频散。这是因为波是由不同频率的成分组成的，但结构和构造是如何影响到频率的呢？
例如：光线透过三棱镜时，不同频率的成分具有不同的速度而发生折射，为何不同频率会具有不同的速度？
地震面波中不同频率是如何被不同的介质分开的？
It is similar maths that describes dispersion in light and sound waves, but without maths it is perhaps simpler to use sound waves to give a simple and perhaps not very accurate description of what happens.
When a sound wave goes through a solid, all the atoms or molecules move backward and forward as the wave goes past, once a cycle.
The atoms or molecules are in a structure that behaves similarly to if they were held in place by springs. Each atom in the structure has frequencies it likes to vibrate and frequencies it doesn't like to vibrate. These frequencies are affected by the weight of the atoms or molecules, the stiffness of the structure, and the shape of the structure. When a sound wave goes past, if the frequency is close to a frequency that the atom or molecule likes to vibrate at, it interacts more strongly with the wave than at other frequencies. This can cause loss, and it can cause a change in speed, at least partly because the atom may vibrate a bit behind the original wave and add its own contribution back into the wave with a small delay.
For this reason different frequencies can travel at different speeds through materials.
It is similar maths that describes dispersion in light and sound waves, but without maths it is perhaps simpler to use sound waves to give a simple and perhaps not very accurate description of what happens.
When a sound wave goes through a solid, all the atoms or molecules move backward and forward as the wave goes past, once a cycle.
The atoms or molecules are in a structure that behaves similarly to if they were held in place by springs. Each atom in the structure has frequencies it likes to vibrate and frequencies it doesn't like to vibrate. These frequencies are affected by the weight of the atoms or molecules, the stiffness of the structure, and the shape of the structure. When a sound wave goes past, if the frequency is close to a frequency that the atom or molecule likes to vibrate at, it interacts more strongly with the wave than at other frequencies. This can cause loss, and it can cause a change in speed, at least partly because the atom may vibrate a bit behind the original wave and add its own contribution back into the wave with a small delay.
For this reason different frequencies can travel at different speeds through materials.
In general, even monochromatic wave can diverge or not when pass through inhomogeneous media. It depend on the relation between wavelength and the size of inhomogenities. For better understanding you can study separately 2 questions: what is scattering (or diffraction) of monochromatic wave in nonhomogeneous media (a) and what is dispersion of polychromatic wave in homogeneous media (b). For optical waves the both questions are described in any textbook in Physical Optics (for example, F. Pedrotti and L. Pedrotti, "Introduction to Optics"). Then you can apply your knowledges to more complicated example - propagation of polychromatic wave through nonhomogeneous media.
As the most simple explanation of dispersion the answer of M. White is sufficient.
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