Gravity tries to make the earth as compact and round as possible. Earthquakes cause the earth to become more compact and spin slightly faster. Knowing the precise time of arrival and the time of an earthquake, each wave's average velocity along a specific path can be calculated. After many earthquakes and knowing the average velocity along tens of thousands of different paths, a computer can estimate the wave speed at every point inside the earth. Higher than normal speed implies either colder or denser rock at that point.
Earthquake waves travel faster under continents.
Some increases in speed are too great to be caused entirely by colder temperatures. Earthquakes often occur near sloping planes, called Benioff zones , that intersect a trench. Hydroplate Explanation for Trenches. Before the flood, the weight of rock and water, pushing down on the subterranean chamber's floor, balanced the floor's upward pressure. The rupture destroyed that equilibrium.