| Geologic Hazards on the Oregon Coast |
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The Oregon coast has a justly deserved, worldwide reputation for its spectacular scenery. Because the coast lies at the interface between land and the Pacific Ocean, it is a zone of great instability and vulnerability, creating geologic hazards that can put people and property at risk. The Oregon Department of Geology and Mineral Industries (DOGAMI) has worked for years with coastal communities to identify these coastal hazards. In 2000, DOGAMI opened a Coastal Field Office in Newport to expand on the important ongoing research and mitigation efforts that will help minimize the effects of coastal geologic hazards.
CATEGORIES OF COASTAL GEOLOGIC HAZARDS
Chronic hazards:
Chronic hazards are those hazards that constantly affect the coast: beach, dune, and bluff erosion; slides, slumps, and gradual weathering of sea cliffs; and flooding of low-lying areas during major storms. Some areas have periodic sand buildup as well. Chronic hazards come from winter storms, associated storm surges, and wave setup; strong nearshore currents; high winds, rain, runoff, and associated lowland flooding; and elevated sea levels caused by seasonal effects and periodic El Ninos.
Waves, currents, tides, and storms are constantly affecting beaches and headlands, causing erosion, landslides, and flooding. Many coastal features such as beaches and sand spits are constantly changing. Attempts to stabilize such ephemeral features so they cannot change in most cases are ultimately futile, because the forces that have shaped this land for millions of years are not easily overruled. For example, cliffs with marvelous views are there because land in front of them has slid away. And areas that have had one landslide are likely to have others. The coast of Oregon is constantly under attack by the ocean.
Learn more about coastal landslides
Catastrophic hazards:
Catastrophic hazards are associated with earthquakes and related tsunamis. The eastward-moving Juan de Fuca tectonic plate dives under the westward-moving North American plate just off the Oregon coast at the Cascadia Subduction Zone. Powerful earthquakes up to magnitude 7 can take place in either the North American or the Juan de Fuca plates. The Cascadia Subduction Zone, however, is capable of generating even larger earthquakes — up to magnitude 9.
These larger earthquakes would occur under the ocean and can cause desturctive tsunamis that can strike the coast between 10 and 30 minutes after the earthquake. The geologic record shows that these large Cascadia Subduction Zone earthquakes and accompanying tsunamis occur every 500 years, plus or minus 200 years, with the last one having occurred about 300 years ago.
Catastrophic hazards on the coast associated with earthquakes include severe ground shaking lasting up to 5 minutes; liquefaction of saturated, unconsolidated soils such as sand or silt; numerous landslides; land subsidence and flooding; and tsunamis.
Tsunami hazards:
Undersea earthquakes can cause tsunamis. Oregon is vulnerable to two types of tsunamis—distant and local. Local tsunamis are most generally associated with Cascadia subduction zone earthquakes. Tsunamis from distant undersea earthquakes can take place anywhere in the Pacific Rim and will take several hours to reach the Oregon coast.
Because the Cascadia Subduction Zone is so close to the Oregon coast, tsunamis caused by earthquakes along this rift can strike the southern Oregon coast within 10-15 minutes of the earthquake and the north coast within 20-30 minutes. In many communities, the only warning will be the earthquake itself.
Distant tsunamis have struck the Oregon coast in the past, most recently in 1964. A magnitude 9.2 subduction zone earthquake struck the coast of Alaska with the resulting tsunami traveling down the pacific coast where it killed 4 people at Beverly Beach and caused damage in many Oregon coastal communities.
Senate Bill 378 passed by the 1995 Legislature placed restrictions on the construction of certain types of critical and essential facilities within tsunami inundation zones along the coast. DOGAMI developed a series of 38 maps showing inundation zones along the whole coast. Local communities have used these maps to develop tsunami evacuation routes, and have put up signs showing hazard zones and evacuation routes.
Increasing public awareness of tsunami hazards is essential to the safety of coastal residents. Coastal schools are now required to hold tsunami evacuation drills. DOGAMI has installed historical markers describing tsunamis along the coast at Seaside, Newport, Reedsport and many state parks along the coast. Brochures, bookmarks and other educational materials are available free to the public about the dangers of tsunamis.
Learn more about tsunami hazards
Tsunami hazard maps for the Oregon coast and evacuation brochures
EARTHQUAKE HAZARDS:
As mentioned above, coastal Oregon is particularly vulnerable to Cascadia subduction zone earthquakes and the associated tsunamis. DOGAMI has produced tsunami innundation zone maps for the coast and relative earthquake hazard maps for most larger coastal communities.
In 1993, western Oregon was changed from Seismic Zone 2B to Seismic Zone 3. Subsequently, the southern Oregon coast was upgraded to Seismic Zone 4, the highest seismic design standards in the country.
This means that new buildings are being built to higher standards so that they will be better able to withstand earthquakes. Many older buildings along the coast were constructed before this code change and are therefore not as safe. The Seismic Rehabilitation Task Force was appointed by the legislature in 1997 to help recommend ways to address the problem of how to rehabilitate these older buildings. Copies of the Task Force recommendations are available from Nature of the Northwest.
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