Cascadia Subduction Zone

The Cascadia Subduction Zone is an approximately 650 mile long fault stretching from Northern California to Southwestern Canada. This fault is capable of producing very large (magnitude 8.0 and greater) earthquakes having the potential to cause significant damage to developed areas, tsunamis, landslides, and other detrimental impacts.

The Cascadia Subduction Zone fault occurs at the intersection of the Juan de Fuca Plate and the North American Plate. Through tectonic stresses, the Juan de Fuca Plate (oceanic crust) is being subducted beneath the North American Plate (continental crust). The fault is currently locked by friction, and stress is building along the fault zone. Rupture of the fault and release of this stress will cause a powerful, damaging earthquake and subsequent tsunami. Both the earthquake and tsunami will have widespread impacts across the region.

 

1. Recurrence Intervals

Geologic evidence has shown that there have been 40 earthquakes with a magnitude of 8.0 or greater over the last 10,000 years. The recurrence interval for these earthquakes ranges from about 200 to 600 years with an average recurrence of about 250 to 300 years. Geologic evidence, along with recorded history of a tsunami reaching Japan, shows the last major Cascadia earthquake event occurred over 300 years ago on January 26, 1700. The estimated magnitude of the year 1700 earthquake was 9.0.

 

2. Risk and Impacts

A powerful and damaging earthquake could occur at any time along the Cascadia Subduction Zone fault. Estimates for the probability of a large earthquake vary, with the currently accepted estimate at about a 30% chance of a magnitude of 8.0 or greater earthquake in the next 50 years. It is projected that an earthquake of this magnitude would impact over 13 million people from Northern California to Southern British Columbia.

Large, deep earthquakes, as will be produced from a Cascadia event, result in long duration shaking. Up to 4 to 6 minutes of ground shaking is expected. This ground shaking will cause significant damage to infrastructure, including: communications, power, water, sewer, roads, bridges, and buildings. Brick and mortar structures will incur the most damage. Poorly compacted fills, along with loose, natural soils will lose strength and behave like a fluid in a process referred to as liquefaction. Any buildings or other structures supported on soils susceptible to liquefaction will be significantly damaged and may collapse. Depending on the epicenter of the earthquake, and the location along the coast, 15 to 30 minutes after the earthquake a tsunami will impact the shoreline. As seen by recent tsunami events triggered by earthquakes in Japan and Chili, a tsunami will be extremely destructive and will cause widespread damage and potentially loss of life in coastal communities.

Internet Resources

The following links provide additional information on the Cascadia Subduction Zone and the impacts of a large earthquake event:

While a Cascadia Subduction Zone earthquake will be a major natural disaster, it will be survivable if you are prepared. It will likely take one or two weeks for first responders to reach many areas so you must be prepared to care for yourself and your family. The links on this website provide information and resources to help guide in your workplace and personal preparedness. Don’t be a victim! Be prepared!

 

3. Workforce Preparation

Every employee, from top managers to part-time and temporary workers, need to learn what to do during an earthquake. Orientations should emphasize safe places to "drop, cover, and hold on" during earthquake shaking and safe locations where people can rendezvous when the shaking has stopped and it is safe and advisable to evacuate your facilities.

It is recommended that agencies hold periodic, mandatory earthquake drills to give employees opportunities to practice what they have learned and condition themselves to react spontaneously and safely when the first jolt or shaking is felt. To help protect workers in the immediate aftermath of earthquakes or other disasters, arrange for employees to be trained now in first aid, cardiopulmonary resuscitation (CPR), and the use of fire extinguishers. Earthquake training should be thoroughly integrated into the organization’s emergency preparedness, response, and recovery planning.

Internet resources for preparing your workplace are listed below:

 

4. Personal Preparedness

It is important to make sure that the entire family is prepared and informed in the event of a disaster or emergency. You may not always be together when these events take place and you should have plans for making sure you are able to contact and find one another.

Following are links that will help you, your family, and neighborhood know what to do during an event and to prepare for an earthquake