Today in Earthquake History
Earthquake History for July 31st
From Significant Earthquakes of the World 2002.
In 1897, Oldham noticed that seismograms from earthquakes consistently showed three different disturbances, the first and second "preliminary tremors" (now known as P waves and S waves, respectively) and the "large waves" that followed the preliminary tremors, and that the difference in arrival time between the "large waves" and the "preliminary tremors" increased in a predictable fashion with increasing distance from the earthquake.
In 1900, he established that the "preliminary tremors" (P and S waves) have travel paths that take them through the body of the Earth (we now call them body waves), and that the "large waves" (now called surface waves) travel along the Earth's surface.
In 1906, Oldham used evidence from eathquake waves to demonstrate the existence of a large central core at a depth of about 3,821 km beneath the Earth's surface.
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