**4. Conclusions**

We have compiled a new earthquake catalog for historical and instrumental subduction earthquakes in El Salvador and surrounding areas that represent the input data for the ITs analysis and the seismicity evaluation. Subduction seismicity patterns fit the Weibull probability distribution well for all events (including main, foreshocks, and aftershocks). The Poisson probability distribution is not suitable for the natural seismicity for small events in the subduction zone. However, such distribution fits when dependents shocks are removed from the catalog for most of the magnitude range under scrutiny but fails to explain the interoccurrence times less than 100 days for the magnitude bin 6.5–7.0. Such failure might be explained because the Gardner & Knopoff [14] method was calibrated for Southern California crustal events rather than subduction zones. Indeed, each region should have its declustering dynamic window on time and space that belongs to the tectonics of the region under study. We conclude that the catalog declustering process is necessary when employing the classical Cornell seismic hazard assessments on time-independent schemes as previous works have applied for the region [6, 29–31]. In other words, despite the Poisson probability distribution not fitting the natural seismicity of subduction earthquakes, the declustering process produces a Poisson distribution with only independent events. However, the correspondent earthquake loads might be revised when employing the whole catalog in a seismic hazard analysis.

Since all the IT analyses belong to the overall subduction zone, we expect that future research must separate interface and intraplate shocks to consider the change in the seismic activity in depth; it is clear that below 120 km, there is a decrease in the seismicity in the Cocos Plate (see **Figure 2**). The depth determinations contain a significant error in the Central America Isthmus that can make such differentiation cumbersome, especially when there is no reported focal mechanism for some events.

A *B* value of 1.0 retrieved from the Gutenberg–Richter recurrence relationship after removing dependent events validates the quality of the subduction catalog compiled in this study. Conditional probabilities that depend on the last event listed in the catalog for a certain magnitude are also given that are useful in time-dependent

*Perspective Chapter: Testing the Interoccurrence Times Probability Distributions… DOI: http://dx.doi.org/10.5772/intechopen.110584*

seismic hazard assessments. The methodology provided in this chapter would improve the quality of the following seismic hazard assessment in the country.
