Areal Sources
Areal sources are based on historical seismicity and are most commonly used in areas that do not have known faults or to account for background seismicity. Areal sources are used to model the spatial distribution of seismicity in three dimensions, an area in map view and a depth. Subduction zones can also be modeled as areal sources by dividing out those earthquakes that occurred on the interface or the intraslab.
Compilation of an Earthquake Catalog
The first step in determining the input parameters required to conduct a SHA is to compile an earthquake catalog. Sources on the internet that provide this information are listed below. It is also important to include pre-instrumental records (often found in literature) and any local seismic networks that may not be included in ANSS or NEIC records.
USGS Peterson Catalog: The USGS published a compile catalog (duplicates removed, declustered and consistent magnitude scale) that was used to develop the 2014 National Seismic Hazard Maps (NSHM). This catalog is separated into the Western and Eastern United States. This catalog contains the magnitude in expected moment magnitude E[MW],magnitude uncertainty, and a counting factor termed N* (or nstar) for each event. The counting factor N* can be used to compute unbiased earthquake rates following guidance presented in CEUS-SSCn.[1] Earthquake recurrence parameters are then computed using the maximum likelihood approach by using the N* factor instead of the observed counts. This approach has been shown to work well for catalogs with variable levels of catalog completeness as a function of magnitude.[1]
Defining the Volume of Areal Sources
In order to define the volume of the areal sources it is important to plot the data and view it in three dimensions. This is especially important in areas where there are subduction zones. Any event that is not accurately located in each dimension should be removed prior to this. For example, several catalogs set a default depth if a depth cannot be accurately located, these events should be removed. After the volume of the Areal source is defined it is OK to include the event in the data set, but the volume should not be set based on events that are not located properly.
Defining the geometry of areal sources is more of an art than a science. The goal of defining an areal source is to group together events with similar tectonic regimes (active crustal, subduction, intraslab, and stable continental) and earthquake distribution all the while keeping in mind the regional and local geology.
Examples
USGS Advanced National Seismic System (ANSS)
USGS National Earthquake Information Center (NEIC)
Best Practices Resources
Citations:
Revision ID: 3825
Revision Date: 09/30/2022