Oregon’s ocean shore is subject to a wide range of geologic forces and climatic conditions that continually shape the coast and put life and property at risk. As development progresses into steeper slopes, ocean bluffs, and dune areas, the level of risk increases, as does the need to make better decisions about where to build and what to do to protect existing development.
haz.ard (haz'erd), n. 1. a chance happening: ACCIDENT. 2. a chance of being harmed or injured: DANGER. 3. a possible source of danger
(Webster's New College Dictionary, 1995)
About Coastal Hazards in Oregon
Oregon’s ocean shore is subject to a wide range of geologic forces and climatic conditions that continually shape the coast and put life and property at risk. As development progresses into steeper slopes, ocean bluffs, and dune areas, the level of risk increases, as does the need to make better decisions about where to build and what to do to protect existing development.
Climatic conditions, sea level rise, storms and cyclical events such as El Niño exacerbate chronic ocean shore erosion problems, resulting in dramatic losses of beach and property and heightening the focus on hazard issues. Dynamic geologic processes of crustal uplift and subsidence, earthquakes, and volcanic activity are potentially catastrophic forces that are also constantly present - though they occur over a longer time period. These forces have been causing dramatic changes in the coastal landscape long before humans settled and developed along the ocean shore. The vulnerability of coastal communities to the chronic and catastrophic forces at work is a continual concern to those who live, work, and recreate in those communities.
There are two general categories of coastal hazards. Catastrophic hazards are regional in scale and scope. Cascadia Subduction Zone earthquakes, and the ground shaking, subsidence, landsliding, liquefaction, and tsunamis that accompany them, fall into the category of catastrophic hazards. Chronic hazards are local in nature, and the threats to human life and property that arise from them are generally less severe than those associated with catastrophic hazards.
Chronic hazards
Chronic hazards are those we can see clear evidence of along the shore – beach, dune, and bluff erosion, landslides, slumps, gradual weathering of sea cliffs, and flooding of low-lying lands during major storms. The damage they cause is usually gradual and cumulative.
The regional, oceanic, and climatic environments that result in intense winter storms determine the severity of chronic hazards along the coast. The short term, chronic events resulting in coastal flooding, erosion, and landslides are categorized as wave attack, mass wasting, and human activities. These processes operate over relatively short time periods in limited geographic areas and affect shoreline stability. The importance of these factors vary from setting to setting, but the wide distribution and frequent occurrence of chronic hazards makes them a more immediate concern. Due to the relative uncertainty and infrequency of catastrophic events, Oregon coastal communities generally focus planning efforts on the chronic coastal hazards of flooding, erosion, and landslides.
Catastrophic hazards
Earthquakes and resulting tsunamis occur over larger geographic areas and time frames than chronic coastal hazards. Although not as frequent in occurrence, the damage caused by these catastrophic events is immediate and life threatening. Off the coast of Oregon, subduction zone earthquakes can be generated along the sloping boundary between the descending Juan de Fuca plate and the North American plate. This area - known as the Cascadia subduction zone - could produce an earthquake of magnitude of 8.0 to 9.0, or greater. Geologists have found evidence that earthquakes of this magnitude have occurred once every 300 to 600 years.
The last major earthquake of this magnitude occurred in late January of 1700 A.D. The resultant tsunami was recorded locally and caused damage in Japan. An earthquake of this size would cause enormous damage to the coast and large portions of Western Oregon. In many areas, especially on the coast, liquefaction and landslides could damage buildings and their foundations, destroy bridges and cause massive loss of life. A great subduction earthquake could last as long as four minutes. Damaging tsunamis would be expected to arrive along the Pacific Northwest coast including Oregon within 5 to 30 minutes and elsewhere in the Pacific Rim hours later.