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Accounting for Site Effects in Probabilistic Seismic Hazard Analyses of Southern California - The SCEC Phase III Report
Non-technical Overview: Earthquake Shaking - Accounting for "Site Effects"
Fact Sheet: Download the pdf version
Press Briefing: SCEC InstaNET story about the event which announced the Phase III results
Press Release
Poster: Earthquake Ground-Motion Amplification in Southern (Download the pdf version)
Technical Resources: Attenuation relationships, basin depth calculator, images, abstracts, etc.

The most intense shaking experienced during earthquakes generally occurs near the rupturing fault, and decreases with distance away from the fault. In a single earthquake, however, the shaking at one site can easily be 10 times stronger than at another site, even when their distance from the ruptured fault is the same. Scientists have assumed that local geologic conditions are the cause of this difference in shaking intensity, but they have not been certain of the particular conditions that are most responsible, and the degree to which they affect earthquake shaking. Combining this information with estimates of where and how often earthquakes will occur would allow for better estimates of how intense shaking will be during future earthquakes.

The SCEC "Phase III" Report, published in December, 2000, in BSSA, has quantified how local geologic conditions, known as "site effects," contribute to the shaking experienced in an earthquake. The study identified that the important geologic factors at a site are

  • the softness of the rock or soil near the surface
    (shaking is amplified in softer rock)
  • the thickness of the sediments above hard bedrock
    (shaking is amplified where sediments are thicker).

By combining these factors, a map of where site effects will amplify shaking has been produced (see overview). Even when these site effects are taken into account, however, each earthquake exhibits unique "hotspots" of anomalously strong shaking. Better predictions of strong ground shaking will therefore require additional geologic data and more comprehensive computer simulations of individual earthquakes.

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