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The Effects of Soil Type on Earthquake Damage

June 18, 2012
soil types map

Soil Type Mapping

When thinking about the dispersion of effects of an earthquake, many people think of it like dropping a pebble in a lake. The pebble hits the water and it creates a uniform ripple effect getting weaker as it travels from the center. The earth’s surface, however, is not uniform like the water in this comparison. During an earthquake one area can experience over ten times the effects as a neighboring area that is the same distance from the fault line. This is because of what are called “site effects”. Site effects are variations that occur in the geologic conditions of a particular location. There are two main conditions that account for these variations: The softness of the soil or rock, and the total thickness of the sediment above the bedrock. The softer and thicker the sediment is, the greater the effects of an earthquake will be amplified.

As seismic waves travel though the ground, they travel faster through hard rock than soft soil. As a result, when the waves move from hard rock to soft soil, the amplitude (largeness) of the waves needs to increase to be able to carry the same amount of energy, creating stronger shaking. This same principle accounts for the site effects of sediment thickness. The deeper the sediment above bedrock, the more soft soil there is for seismic waves to travel through, therefore creating stronger amplifications.

The National Earthquake Hazards Reduction Program (NEHRP) has defined six different soil and rock types based on their shear-wave velocity, in order to determine amplification effects:

  • Type A, hard rock (igneous rock).
  • Type B, rock (volcanic rock).
  • Type C, very dense soil and soft rock (sandstone).
  • Type D, stiff soil (mud).
  • Type E, soft soil (artificial fill).
  • Type F, soils requiring site-specific evaluations.

Type A having the least amplification and Type E the most.

Although soil type is a significant indicator of how specific locations will be affected by an earthquake, other factors can also contribute to “hotspots”, which are unique to each earthquake. The orientation of the fault, irregularities in the rupturing fault surface, and dispersion of waves as they hit subsurface structures can all have an influence in creating hotspots.

Do you know what type of soil your building is standing on?

WSRB‘s PropertyEDGE™ software provides you with soil type maps for Washington State. To learn more, leave us a message in the comments section below or contact Tracy Skinner at 206.273.7146.

 

Article by: April LaRita Green

Source: Southern California Earthquake Center – Earthquake Shaking – Accounting for “Site Effects”, The National Earthquake Hazards Reduction Program – Effect of Soil and Rock Types

3 Comments leave one →
  1. Anonymous permalink
    November 20, 2014 4:04 pm

    How should I site this page?

    • November 21, 2014 9:04 am

      A link and a date would be fine, thanks!

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