Subsidence in the Pacific Northwest
Introduction
Subsidence is the lowering of a portion of the earth's crust, which causes a loss of surface elevation due to the removal of subsurface support. Subsidence can be caused by both natural and human activities. Natural factors contributing to subsidence occurrences are earthquakes, soil compaction, and watering of loess (wind) deposits. Common human activities that influence subsidence are mining operations and overpumping of groundwater. The major influence in the Cascadia region with respect to subsidence is earthquakes.
What are the Characteristics of Subsidence?
During earthquakes, large areas of the surface can drop several feet almost instantaneously. Although this is a rare occurrence, when it does happen the effects can be devastating. The most significant subsidence events along the Pacific Northwest are associated with CSZ earthquakes. Subsidence of up to 2 meters (6.5 feet) could be common during a subduction zone earthquake along certain portions of the Washington and Oregon coasts. However, these events do not occur often (the last one occurred approximately 300 years ago). Therefore, the frequency of subsidence along the Pacific Northwest coast is not as high as other seismic hazards.
The image to the right shows flooding in the town of Portage, Alaska, due to subsidence of 1.8 meters (6 feet) during the 1964 Alaskan Earthquake. Flooding events due to subsidence and tsunami inundation could produce similar results along portions of the Washington and Oregon coasts. Tsunami inundation can be greatly exacerbated if subsidence occurs prior to the tsunami impact.
Source: Federal Emergency Management Agency (FEMA). Multi-hazard Identification and Risk Assessment: A Cornerstone of the National Mitigation Strategy. Washington, D.C., 1995 pg. 369