Seismic hazard is the hazard associated with potential earthquakes in a particular area, and a seismic hazard map shows the relative hazards in different areas. The maps are made by considering what we currently know about:
- Past faults and earthquakes
- The behavior of seismic waves as they travel through different parts of the U.S. crust
- The near-surface site conditions at specific locations of interest
Hazard maps can be used for land-use planning, mitigation, and emergency response.
The different maps show different probabilities that 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.
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.