Seismic Waves; Types and Categories

The sudden release of energy from the earthquake focus, also known as the hypocenter, causes two types of seismic waves to travel through the earth crust. The main types of seismic wave Body Waves and Surface Waves can be broken down further into waves with different types of motion.

Body Waves travel directly through the earth crust.  They generally have a short wave length and affect the areas nearest the epicenter.  The types of Body Waves include:

• P-Waves (Primary Waves) travel the fastest of the specific waves and cause the least damage.  These waves are characterized by an expansion and compression of material as they travel.
• S-Waves (Secondary Waves) have side-to-side and up-and-down motion and arrive to a location behind P-Waves.  The transverse shearing behavior of these seismic waves causes extensive damage.

Surface Waves travel along the surface.  They have a longer wave length than those listed above, and their affects can occur further from the epicenter.  The types of Surface Waves include:

• Love Waves are lateral shear waves with a side-to-side motion.  These seismic waves behave only on the horizontal plane.
• Rayleigh Waves are long, rolling waves that are defined by vertical rotary movement.

For purposes of the exam, it is very important to be familiar with the different types of waves and to be able to quickly apply logic when only a name is referenced in the question.  It is also important to remember that the epicenter of an earthquake is the point on the earth surface directly above the focus, and that the distance between the two is known as the focal depth.

Find more information on seismic waves in the Seismic Design Review Workbook and Fundamentals of Seismic Analysis and Design of Buildings, which can be found on our Resources page!

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