Landslide Hazard - Earthquake

Landslides are the number one hazard associated with earthquakes. Earth movement (landslide) damage to businesses, homes and roads should be expected in areas of moderate to steep slopes. A popular misconception is the assumption that an “Earthquake Rider” in one’s Insurance Policy will provide compensation for damages related to earth movement.

17jap3While the landslide hazard risk assessment for Florence has a low probability and vulnerability, there are areas in Florence that are susceptible to landslide events. These include 80 acres located east of Munsel Lake Road where the road grade can extend more than 10%.

In addition, there are areas outside of Florence along Highways 101 and 126 that are vulnerable to landslide events, and if these highways were to be closed down, Florence would be isolated from the rest of the state.

Homes built on moderate to steep sand dune slopes can expect ground movement similar to that observed in the above photos. Residents can expect severe damage to streets and all underground utilities.

The below listed videos show landslides caused by heavy rains during strong winter storms – which are the “most likely” disaster scenarios coastal residents should prepare for. During a Cascadia Earthquake event the dunal sands with shallow aquifer will have ground water flowing to the surface through cracks or “mud volcanoes”. The sand on moderate to steep slopes will become unstable and can be expected to flow like is observed in these videos.

Several of the above videos show the potential for upstream flooding by landslides blocking the stream channel. Residents living near Siltcoos Lake and near Sutton/Mercer Lakes should prepare for possible flooding because both drainages have steep sand dunes on their north banks which are likely to landslide – blocking their channels.