Waveforms

Seismographs

Seismographs are instruments used to record the motion of the ground during an earthquake. They are installed in the ground throughout the world and operated as part of a seismographic network. Seismograph Info . A seismogram is the recording of the ground shaking at the specific location of the instrument. On a seismogram, the HORIZONTAL axis = time (measured in seconds) and the VERTICAL axis= ground displacement (usually measured in millimeters).

When there is NO EARTHQUAKE reading, there is just a straight line except for small wiggles caused by local disturbance or "noise" and the time markers. Seismograms are digital now - there are no more paper recordings.

Causes of Earthquakes

The tectonic plates that make up the earth's crust are moving constantly. As the edges of these plates slide against each other in fault zones, friction can slow them down, leading to the buildup of pressure over long periods of time and releasing the pent-up pressure in the form of seismic waves.

What can Earthquakes tell us?

Earthquakes are very useful to humans because they provide a picture of what's going on underground. ... Earthquakes can also tell us about the internal structure of the Earth. By measuring the time it takes the seismic waves to travel through the Earth we can map out the structure of the Earth down to the core.

Two types of Internal Earthquakes

A P wave moves by alternately compressing and dilating the material through which it passes, somewhat analogous to stop-and-go traffic on the freeway during rush hour. An S wave moves by shearing the material from side to side, analogous to flipping a rope tied to a tree.