Recent attention has been focused toward determination of threshold rainfall levels in the estimation and prediction of mass wasting events (such as landslides, mudflows, debris flows, sloughs, earth movements, and so forth). The literature indicates use of a cumulative rainfall value as a threshold that correlates mass wasting occurrence versus rainfall depth occurrence. In the current article, the threshold concept is extended to use of rainfall return period estimates. Return period estimates are developed for the peak 1-day, 2-day, and successively larger continuous peak durations of rainfall, to duration sizes of 1 year. Such a return period peak duration rainfall analysis is prepared for each rainfall season in the rain gauge record, resulting in a historic summary of peak duration return frequency based on every possible peak rainfall duration. By using return period as a measure of the rainfall depth for each peak duration, one can correlate the rareness of rainfall occurrence to the rareness of the mass wasting events for those events substantially triggered by rainfall. A similar type of analysis can be developed for estimates of infiltration by using a selected infiltration relationship and then synthesizing the resulting estimates in the same way that the rainfall data are analyzed. Once the entire history of the rain gauge is analyzed for infiltration, return period estimates can also be developed for use in possible correlation to mass wasting events. A case study involving a catastrophic landslide in La Conchita, California, is considered as an example application.