Earthquakes

An earthquake is a sudden shaking of the ground. They generate seismic waves which can be recorded on a sensitive instrument called a seismograph.

The record of ground shaking recorded by the seismograph is called a seismogram.

Modern seismographs are electromagnetic and operated by suspending a magnetic mass, or pendulum, within an electric coil.

Because a magnet moving inside a coil creates a current within the coil, the movement of the ground during an earthquake can be converted into an electrical signal.

This signal could then be used to modify the projection of light onto photographic paper, or to move a needle across paper and trace out the wiggles of the Earth's shaking.

Today's seismometers are called broadband because they are able to sense ground motion over a wide range of frequencies, from thousands of seconds to less than a hundredth of a second.

Seismic Waves

Earthquakes generate seismic waves which travel all around the world.

This was first discovered in 1889 by E. von Rebleur Paschwitz who noted that waves recorded on a horizontal pendulum in Potsdam, Germany were generated by an earthquake far away in Tokyo, Japan.

Seismic Deformation

When an earthquake fault ruptures, it causes two types of deformation: static; and dynamic. Static deformation is the permanent displacement of the ground due to the event. The earthquake cycle progresses from a fault that is not under stress, to a stressed fault as the plate tectonic motions driving the fault slowly proceed, to rupture during an earthquake and a newly-relaxed but deformed state.

Typically, someone will build a straight reference line such as a road, railroad, pole line, or fence line across the fault while it is in the pre-rupture stressed state. After the earthquake, the formerly stright line is distorted into a shape having increasing displacement near the fault, a process known as elastic rebound.

Seismic Waves

The second type of deformation, dynamic motions, are essentially sound waves radiated from the earthquake as it ruptures. While most of the plate-tectonic energy driving fault ruptures is taken up by static deformation, up to 10% may dissipate immediately in the form of seismic waves.

The mechanical properties of the rocks that seismic waves travel through quickly organize the waves into two types. Compressional waves, also known as primary or P waves, travel fastest, at speeds between 1.5 and 8 kilometers per second in the Earth's crust. Shear waves, also known as secondary or S waves, travel more slowly, usually at 60% to 70% of the speed of P waves.

P waves shake the ground in the direction they are propagating, while S waves shake perpendicularly or transverse to the direction of propagation.

Slinky ?

The Slinky and The Rope ! The P waves move in a compressional motion similar to the motion of a slinky

P Waves

While the S waves move in a shear motion perpendicular to the direction the wave is travelling.

S Waves