Analysis of precipitation data associated with historical landslide events in Seattle has resulted in the identification of precipitation thresholds for the initiation of landslides. Also, an air-temperature index for multiple landslide events is identified, and in conjunction with the precipitation thresholds, is used to develop a method for anticipating the occurrence of landslides during the December-March wet season.
The precipitation thresholds are based on 3-day cumulative precipitation that occurred immediately prior to landslide events and antecedent 15-day precipitation that occurred prior to the 3-day amounts. The data indicate that 15-day cumulative precipitation influences the amount of subsequent 3-day precipitation required to initiate landslides.
Results of the analysis also indicate that air-temperature data can be used to help identify times when conditions are conducive for the initiation of landslides. It is shown that a high percentage of precipitation-related landslides occurred on days when the daily maximum air temperature was between 46? and 56? F (9? and 13? C) and that nearly all of the slides occurred on days when the daily maximum air temperature was above 43? F (6? C).