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posted Wednesday, 24 August 2011

These graphs show the ground motion recorded by a seismometer in Central Park (located just off the 96th Street Transverse). One graph for each of the orthogonal directions: up-down, north-south, and east-west. The horizontal axis shows the time after the earthquake started in Virginia in seconds.

Source: Lamont-Doherty Earth Observatory of Columbia University

You can see it took about 60 s for the energy to reach New York City. The little wiggles at the beginning came from the fast traveling P waves. “P” stands for “primary”, since these waves arrive first, or “pressure”. When a P wave travels through a medium, the medium is alternately stretched and compressed in a direction parallel to the propagation of the wave. Waves in which the disturbance is parallel to the propagation are called longitudinal waves. They can propagate through any medium — solid liquid or gas — since any medium can be compressed or stretched. P waves don’t carry a lot of energy, but you can hear them. They are essentially sound waves.

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After 110 s the S waves arrive. “S” stands for “secondary”, since these waves come after the primary waves, or “shear”, since they shear the surface. A shear is a type of deformation where one part of a solid is moving one direction while an adjacent area is moving in the opposite direction. When an S wave passes by you, the ground on one side is moving up while the ground on the other side is moving down. Waves in which the motion of the disturbance is parallel to the propagation are called transverse waves. They can only propagate through solids since liquids and gases can’t tolerate shearing. They flow instead. This is how geologists were able to determine that the outer core of the earth is molten. When they examined the records of lots of earthquakes they found that seismometers that were beyond a certain distance from the earthquake stopped detecting S waves. This indicated that the S waves were being blocked by a liquid layer deep within the earth that cast a “shadow” on the side of the earth opposite the source of the earthquake. S waves carry more energy than P waves.

A transverse wave in action

The ground really started moving after 130 s. This is when the surface waves arrived. P and S waves are so-called “body waves” since they propagate through the earth in all directions. Surface waves, as the name suggests, propagate over the surface of the earth. Surface waves are also said to be “complex” since they posses the characteristics of both longitudinal and transverse waves. This combination of oscillatory motions at right angles to one another results in an overall circular motion of the surface. This is what produces the characteristic rolling sensation of earthquakes. Surface waves are like ocean waves in stone. Surface waves carry the most energy, but they do not propagate as far as body waves.

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