A Simple Description of Seismic Hazard Maps
I need a simple description of what the maps are about.
These maps depict earthquake hazard by showing, by contour values, the earthquake ground motions that have a common given probability of being exceeded in 50 years.
The ground motions being considered at a given location are those from all future possible earthquake magnitudes at all possible distances from that location. The ground motion coming from a particular magnitude and distance is assigned an annual probability equal to the annual probability of occurrence of the causative magnitude and distance.
The method assumes a reasonable future catalog of earthquakes, based upon historical earthquake locations and geological information on the recurrence rate of fault ruptures.
When all the possible earthquakes and magnitudes have been considered, one can find a ground motion value such that the annual rate of its being exceeded has a certain value. Hence, on a given map, for a given probability of exceedance, PE, locations shaken more frequently, will have larger ground motions.
For a LARGE exceedance probability, the map will show the relatively likely ground motions, which are LOW ground motions, because small magnitude earthquakes are much more likely to occur than are large magnitude earthquakes. For a SMALL exceedance probability, the map will emphasize the effect of less likely events: larger-magnitude and/or closer-distance events, producing overall LARGE ground motions on the map.
The maps have this format, because they are designed to be useful in building codes, in which we assume that, for the most part, all buildings would be built to the same level of safety. For other applications, maps of another format might be more useful.
For instance, many buildings across the US are built more or less the same, regardless of earthquake hazard. If we knew that a particular type of building was likely to fail at a particular ground motion level, we could make a map showing contours of the likelihood of that ground motion value being exceeded, due to earthquakes.
