Seismic Waves

When earthquake energy is released, seismic waves radiate from the focus (Stein, pp 53-118).   Waves of several types are set into motion.   The different wave types have different travel velocities and modes of vibration.   Initially, two types of waves radiate from the focus:

• Primary Waves (or P-Waves) - These are compressional waves, also known as longitudinal waves and are the first seismic waves to arrive at a location on the Earth.   This type of wave has the highest velocity for traveling through Earth materials.
   
• Secondary Waves (or S-Waves) - These are shear waves, also known as transverse waves and are the second to arrive at a location on the Earth.   This type of wave has the second highest velocity for traveling through Earth materials.

The P-Waves and S-Waves generate a third type of seismic wave known as large waves (or L-Waves).   They radiate along the Earth's surface from the epicenter of an earth quake and are therefore also known as surface waves.

Some Useful Links

Following are some hyperlinks that can further assist with the evaluation and description of seismic waves:

• Seismic Waves (Nevada Seismological Laboratory of the University of Nevada, Nevada, USA):   Descriptive text and drawings of seismic deformation, seismic waves, and seismograms.

• What is Seismology and What are Seismic Waves? (Michigan Technological University, Michigan, USA website):  Good discussion of the various types of seismic waves with illustrations.

• Seismic Waves (The University of Liverpool, UK website):   Emphasis on "P" and "S" waves, how they travel, and how they are detected and recorded.

• USGS Earthquake Hazards Program - Animation of Seismic Waves.

• USGS maps, products, and publications - Online information resource on the U.S. Geological Survey website.