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Classifying Earthquakes

April 21, 2014

Damaged brick

Did you know that not all earthquakes are the same? Many of us in the Seattle area may be aware that earthquakes happen at different depths, known as “shallow” and “deep” earthquakes (there’s a third type, intermediate, as well) and that the Richter scale reading can give a good indication of how bad the damage might be, but different types of shaking occur during a quake as well. Earthquakes and their shaking is a complicated subject, but a little background information can help deepen your understanding beyond simple Richter scale readings.

To begin, what are the differences between earthquake depths?

Earthquakes generate two types of waves at the focus (the focus is the point in the earth where the rock broke that started the quake).

  • Body Waves: these come from the focus and spread out in all directions, including up toward the surface.
  • Surface Waves: these waves appear on the surface of the earth, above where the focus occurred. They move outward like ripples in a pond and are the ones that cause structural damage.

Body waves can be further classified into two separate categories, Primary and Secondary Waves. These travel in all directions, including up to the surface of the earth:

  • Primary Waves (P waves): these are compressional waves that shake forward and backward. To picture this, think of a slinky. If you’re holding each end (assuming you keep the middle stable) and push in with one hand, the wave that flows through the slinky to the other hand would be similar to a P wave (check out the images on slide 11 for pictures of this).
  • Secondary Waves (S waves): These are sometimes called shear waves and shake the ground in a perpendicular motion. To picture this, think of holding a rope. If you whip one end of the rope, the wave that travels to the other end would be similar to an S wave.

Once an earthquake reaches the surface, surface waves can then be felt. These waves tend to move in one of two ways:



Why is this important? The shaking of the earthquake is obviously what does the damage, but it’s not alone. Along with the building construction and height, soil types, building codes, earthquake intensity, depth, and much more, engineers, seismologists, and emergency management can utilize this information to help understand how buildings, freeways, etc., should be built and the type of damage they can expect to see after an earthquake.

As for what your insurance will cover after an earthquake, keep this in mind: a standard homeowner policy will not cover you for earthquake damage. Earthquake coverage in Washington must be purchased separately from your homeowner or business owner policy. And while it’s not cheap, neither is replacing the items that get damaged or destroyed after the shaking stops. Consult your agent if you have questions about how quake insurance works, the cost of it, and how it will cover you!

Article by: Kristen Skinner

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