Landslides are common in Seattle. They occur when there is tension between the stresses pulling down on a slope and the resistance holding it in place. The slope becomes more and more unstable as the forces of resistance and stress converge. The change in these forces is caused by dynamic factors. Some develop gradually, such as normal erosion and weathering. Others occur suddenly, such as earthquakes and torrential rains that increase water pressure within a slope.

Usually, the most catastrophic landslides occur on slopes that already have a low margin of safety (often due to weathering and erosion) and are struck by a sudden event (i.e., an earthquake, rain, or human alteration of the slope). Determining a slope's slide potential rests on discovering the inherent stability of the slope and the intensity of forces that undermine its stability. 

How is DPD Involved?
DPD assists in the City's preparedness efforts for possible landslides by providing Seattle residents with information to help homes and businesses prepare for a potential landslide; by inspecting residences and businesses to make sure these structures meet City codes and regulations upon approval of a DPD permit; and by providing rapid evaluations and expedited emergency repair permits for structures damaged by a landslide.

Seattle Landslide Information

• DPD’s Seattle Landslide Study
• USGS graph on precipitation threshold for anticipating landslides in Seattle
• New USGS landslide map of Seattle (coming soon)
• Environmentally Critical Areas Ordinance (SMC 25.09; link to City Clerk's website)
• CAM 324, Reducing Landslide and Stormwater Erosion Damage: What You Can Do
• CAM 331, Environmentally Critical Areas—Tree and Vegetation Overview
• CAM 331A, ECA: Vegetation Restoration
• CAM 331B, ECA: Hazard Trees