Different Probability Maps
Why are there different probability maps,
and which one do I use?
The different probabilities are selected to provide an idea of the relative range of hazard across the US. The larger probabilities indicate the level of ground motion likely to cause problems in the western US. The smaller probabilities show how unlikely damaging ground motions are in many places of the eastern US. However, basically the values chosen reflect the more recent history in earthquake engineering.
Probability from the engineering point of view
Rather than start with the idea of probability, consider approaching the issue from this direction: A structure is designed to resist earthquake ground motion having a particular value. Given this design resistance, one might ask several questions
• Under what ground motion will the building sway so much that it is uncomfortable to the persons working inside, and disrupts their work for the day? (This could occur with winds as well as with earthquakes.)
• Under what ground motion will the building bend so much that interior partitions crack and wall or ceiling fixtures drop?
• Under what ground motion will the building become permanently deformed and require expensive rehabilitation or abandonment.
• Under what ground motion will the building collapse during the shaking?
Using a hazard curve, one could determine the annual probability of occurrence of each of these ground motions. Then one could decide whether that corresponding probability is acceptable. If one of the probabilities is unacceptably high, the design would have to be revised.
The three different probability values reflect probabilities sometimes considered for design. The value 10 percent in 50 years seemed to provide values similar to those already used in design in the 1970’s in California. On the other hand, this level of probability in the eastern US produced values too low for the seismic design then under consideration to provide residual toughness in the event of possible earthquakes (unlikely in any one location, but likely in some location). The probabilities more likely to produce useful design ground motions would be near 5 percent in 50 years.
The ground motions given by three probabilistic maps span a range of probabilities considered interesting to earthquake engineers and a range of ground motions which have some intuitive understanding for the consequences.
There have been requests for maps of larger probabilities for purposes having to do with investment, insurance and banking. Eventually the web site will provide hazard curves and a means for obtaining either probabilities or ground motions from the hazard curves.
How do I know what map to choose?
How does an individual person select a map? Technical users probably have to follow predefined rules. A non-technical person may be interested in avoiding living in a location where significant shaking will cause worry, deciding on whether to carry earthquake insurance, or deciding whether to do some rehabilitation for an existing dwelling. The probability level chosen should reflect how anxious one is to avoid earthquake shaking. Here is some perspective on the 10 percent in 50 year map:
If one lives in a 100-year floodplain, there is about 1 chance in 100 of experiencing the flood in any given year. In 50 years one would expect 0.5 floods, and there is a 1 - exp(-0.5) = 39 percent chance of experiencing such a flood in 50 years. This is a higher likelihood than that of experiencing a damaging ground motion in an area where that ground motion has only a 10 percent chance of being exceeded in 50 years. In a 200-year floodplain the chance would be 22 percent, still larger than the chance for the damaging ground motion. People who are not comfortable with probabilities as large as 10 percent in 50 years for damaging earthquake ground motion should use maps with smaller probabilities. But they should also be aware that many other hazards are higher than earthquake hazards, even in California.
