Tsunami mapping identifies areas that could be flooded by a tsunami. The California Geological Survey (CGS) is responsible for creating our tsunami maps. CGS identifies all possible tsunami sources and models the flooding extent. The models are combined to identify a WORST-CASE line. For Humboldt and Del Norte Counties, the largest tsunami is caused by an earthquake on the Cascadia subduction zone, estimated to be around a magnitude 9.0. In addition to the use of the worst-case scenario earthquake, several assumptions are factored into these maps. It is assumed that the tsunami arrives at high tide and that some land level subsidence (dropping down of the land) occurs during the earthquake. This provides a landward extent of the tsunami flood model that is further extended to the nearest elevation contour plus an additional 1-meter vertical elevation is added to determine the boundary between the yellow and green zones.