Structures Vs. Earthquakes

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Structures Vs. Earthquakes

          Earthquake shaking is largely due to the release of seismic energy during periods of sudden displacement along the faults.  Predicting seismic risk in a given area is difficult because there are many parameters which are not clearly known or understood.  The extent of damage produced from a given earthquake is dependent upon many factors, including:

  • Distance from the epicenter

  • Characteristics of the rock or soil

  • Depth of the earthquake

  • Location with respect to the active fault branch

  • Magnitude of the earthquake/duration of ground shaking

  • Type of movement along the fault

         Given a moderate to severe earthquake, the most important factors affecting damage to property and loss of life are the types of building construction and density of the population.  It is difficult to actually estimate the amount of damage that might occur in any given structure as a result of an earthquake.  It can also be difficult to sort out earthquake damage from damage associated with other soil-related phenomena.  It is generally prudent planning to assume within areas of high seismic activity that during the life of a structure, ground motion will be strong enough to produce damage.  Even with the adoption of all applicable seismic code requirements, the possibility of damage simply cannot ruled out.  The benefit of seismic design is to limit the amount of damage and lower the risk of structural hazards.  Experience has demonstrated clearly that structural collapse resulting from earthquakes generally occurs in older structures without seismic design.  For more recent construction lacking construction defects, collapse is infrequent, but serious damage can occur which may result in expensive repairs.  However, the effects of large magnitude seismic events may require the demolition of recently constructed buildings.  Following the January 1994 Northridge earthquake, it was frequently determined that where partial or complete structural collapse occurred, construction defects played a significant role.

          In sorting out earthquake damage from other damage, it is important to look for patterns of damage. Patterns should be consistent with the orientation of the principal ground motion. Damage patterns may also be independent of patterns expected from other soil phenomena.  Earthquakes simply serve to worsen damage caused by other problems.

 

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