For earthquakes, what will determine the frequency of the earthquake wave? e.g. Magnitude or soil?

Earth’s outer crust is made up of plates of various sizes that move in relation to one another. There is so much movement in the crust that several million earthquakes may occur each year, many of these go undetected. Most earthquakes are apparently the result of large slabs of underground rock breaking and snapping into new positions when under stress. Usually these disturbances occur along fractures in the earth’s outer shell. These fractures are known as faults.
Scientists try to explain by using the theory of plate tectonics. They say that earth’s crust lies on some 20 rigid plates, or slabs, of rock that move slowly, sliding past and at certain points under one another. also about 90 percent of all earthquakes occur along faults at the perimeter of plates. Although rare, sometimes very destructive quakes also occur within the plates.
The severity of an earthquake may be measured by its magnitude or its intensity.
“Magnitude” refers to the moment magnitude scale. This scale is based directly on the slip of the rock along a fault. The Richter scale measures the amplitude of seismic waves and is therefore an indirect measurement of the severity of an earthquake.
An explanation is to be found in plate tectonics, a concept which has revolutionized thinking in the Earth’s sciences,” says the National Earthquake Information Center (NEIC). “We now know that there are seven major crustal plates, subdivided into a number of smaller plates, all in constant motion relative to one another, at rates varying from 10 to 130 millimeters [three eighths of an inch to five inches] per year. Most earthquakes, are confined to narrow belts that define the boundaries of the plates. This is where 90 percent of major earthquakes are likely to occur.
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