EARTHQUAKES

Earthquake Behavior

The earthquake is a dynamic event. When the fault ruptures, energy is released. The energy travels through the earths layers in various wave forms similar to the wavelengths of light coming from the sun that warm our bodies, or wavelengths of sound that reach our ears from some distant sound source. Pictorial descriptions of these waves are shown below. Imagine that a house is planted on the surface of each wave and that it must stay intact during the passing energy wave.

EARTHQUAKE WAVE PROPAGATION

EARTHQUAKE WAVES

The original disturbance of the soils and rocks during an earthquake are a mixed bag of movements, fractures, and energy transfer mechanisms. But, the energy waves traveling away from the center of the earthquake (Hypocenter), quickly becomes a set of waves. These waves are the mechanisms that travel outward and ultimately reach structures and do damage. There are four wave types, two body waves and two surface waves. The two body waves are called Primary (P) waves and Secondary (S) waves. The two surface waves are called Love waves and Rayleigh waves. Ironically, much of the damage comes from the Love waves. See the page in this website that provides examples of the four different types of wave behavior. See examples this page.

WAVE SPEEDS

in Feet per Second

Material P Waves S Waves

Basalt 21,000 11,000

Granite 19,000 10,000

Wet Sand 4,900 1,975

Dry Sand 1,980 780

Location

Mantle 26,000 11,600

Crust 21,000 14,800

Measured at Northridge

At 328 feet Deep 2,130