The final section of the unit addresses the movement in the earth’s crust. The earth’s crust is always moving, even if very slowly, Slight movements of the ground are called tremors. There are more than six million tremors a year in the earth’s crust. Although many of these tremors are too slight to be felt, there is an instrument called a seismograph which can measure these movements. The Richter scale reports a measure of how much energy an earthquake releases. Earthquakes are sudden, strong movements in the earth’s crust that cause much damage. It is believed that the pressures in the earth’s crust cause earthquakes. These pressures cause the earth’s crust to break at a weak point and as the crust breaks, it also moves. One part of the crust slides or slips along the other. The break in the earth’s crust is referred to as a fault and the movement of the ground along a fault is called faulting. Earthquakes are the result of faulting. An earthquake’s point of fracture is its focus, which may be shallow, intermediate or deep (down to about 400 mi or 650 km). Two kinds of shock waves are produced by an earthquake. Body waves travel through the rock itself. Surface waves travel only on the earth’s skin similar to the way waves travel in water. Compressional waves also known as P-waves produce push-pull forces. Distortional waves refer to secondary, shear or S-waves. They move in an up and down fashion, are slower and cause rock particles to oscillate at right angles to wave direction. Wave velocity increases with rock depth and density causing waves to be reflected and bent when they reach boundaries between two layers. Long waves or L-waves occur at the surface and they can be divided into waves that vibrate horizontally at right angles to their direction, and waves that move through ground just like waves move through the ocean. Seismic stations can plot the epicenter of an earthquake by timing the arrival of these waves. L-waves are responsible for landslides, avalanches and other earthquake damage.
As we study physical science we realize that we live in a world of waves. Water waves, sound waves, light waves, shock waves and heat waves are all linked to energy since all waves are basically pulses of energy. Waves disturbances are responsible for transferring energy from place to place. Any substance through which waves can travel is a medium. As a wave travels through a medium, only energy moves from one place to another. The particles of the medium remain in place and do not move forward with the wave. Every waves has three basic features. These features are amplitude, wavelength and frequency. Amplitude refers to the height of a wave, wavelength is the distance between two neighboring crests or troughs, and frequency refers to the number of complete waves passing a point in a given time. Waves directly influence erosion and the continual changing of the earth’s surface.