One of the benefits of probabilistic seismic hazard assessment is that it sums the hazard to a site from all relevant earthquake sources. However, this summation may hamper the application of PSHA to cases that need scenario earthquakes, such as those that require time histories. Performing geographic hazard deaggregation allows us to determine predominant sources of seismic hazard. From these, plausible scenario earthquakes are identified, and time histories may be accessed or constructed.

Two widely used methods of deaggregation of hazard should be mentioned. 

The first method separates the contributions into a limited number of bins of (annular) distance, magnitude, and ground-motion uncertainty (see McGuire, 1995). This method is most appropriate where sources are distributed reasonably symmetrically around the site (Cornell, 1968). 

The second method, geographic deaggregation of hazard, separates the contributions into bins of location, magnitude, and ground-motion uncertainty (Bazzurro and Cornell, 1999). Geographic deaggregation allows us to identify sources with specific faults or with seismically active regions. 

For the second method, equal-area bins, with constant increments in radius, dR, and variable (decreasing with R) azimuth angle, dh will be used. Deaggregation of ground motion uncertainty is not addressed in this article, we accumulate the exceedance probability for all ground motions greater than u including those greater than two standard deviations above the median. It is important to note that probabilistic ground motion at a site is not in general equal to the median ground motion from any given (R, M) pair associated with the site during a seismic hazard deaggregation. 

By averaging contributions within a geographic cell, the resulting M is potentially different from the main component source magnitudes. In those cases, the geographically deaggregated hazard does not yield a “scenario earthquake.” Further examination of the contributors to dominating cells may then be required to better specify “scenario earthquake” parameters.

The seismic hazard assessment for the 1996 national seismic hazard maps have been geographically deaggregated (Geographic seismic hazard deaggregation) to assist in the understanding of the relative contributions of sources. These deaggregations are exhibited as maps with vertical bars whose heights are proportional to the contribution that each geographical cell makes to the ground-motion exceedance hazard. Bar colors correspond to average source magnitudes. Harmsen and Frankel (2001), Akinci et al., (2009) also extend the deaggregation analysis to the western conterminous United States. In contrast to the central and eastern United States (CEUS); the influence of specific faults or characteristic events can be clearly identified.

Examination of these maps of hazard contributions enables the investigator to determine the distance and azimuth to predominant sources, and their magnitudes. This information can be used to generate scenario earthquakes and corresponding time histories for seismic design and retrofit. Where fault density is lower than deaggregation cell dimensions, we can identify specific faults that contribute significantly to the seismic hazard at a given site.

Detailed fault information enables investigators to include rupture information such as source directivity, radiation pattern, and basin-edge effects into their scenario earthquakes used in engineering analyses. Geographic deaggregation of probabilistic seismic hazard aids in the determination of the location of the most probable source contributing to that hazard. This determination can be specified as a distance, average magnitude, and site-to-source azimuth, and can be further analyzed with regard to ground-motion uncertainty. 

Geographic deaggregation of seismic hazard (GDSH) and magnitude distance epsilon deaggregation (MDED) were utilized to identify the design earthquake and the main earthquake scenario.