Unprepared: Will We Be Ready For The Megaquake In Oregon?

What Is A 9.0 Earthquake?

By Ryan Haas (OPB)
Jan. 14, 2015 8 a.m.

Monday, January 26, marks 315 years since a major Cascadia Subduction Zone earthquake shook Oregon. Geologists believe that quake was around a magnitude 9.0 — and that another one is looming for the region. Understanding the level of destruction associated with these types of temblors makes it clear why geologists are so concerned.


What is a subduction zone earthquake?

A subduction zone is a large area where two plates of the Earth's crust meet and one is forced under the other. The plates sliding past each other cause extreme amounts of force to build up as friction restricts the movement. When the amount of force built up exceeds the friction holding it back, the plates slide past each other, causing the ground to rumble.

Subduction zone earthquakes typically break across a much larger area than fault-line "bullseye" quakes (like the San Andreas), and can affect an area hundreds of miles long.

How do we know the subduction zone exists here?

Mostly because of geological data left behind by subduction zone earthquakes, known as megathrust earthquakes. Subduction zones are the only type of plate meeting that can cause earthquakes greater than magnitude 8.5, according to the Pacific Northwest Seismic Network. The last one off the Oregon coast happened January 26, 1700.

Why do scientists worry that we'll have a 9.0 earthquake off the Oregon Coast?

Geological evidence shows that earthquakes at or near a magnitude 9.0 have been happening along the Cascadia Subduction Zone for thousands of years. Scientists have even been able to figure out how often they happen along the fault section that spans the Oregon Coast — an average of every 234 years. Since the last one happened 315 years ago, Oregon is due for a major event.

A timeline of documented earthquakes along the Cascadia Subduction Zone throughout history.

A timeline of documented earthquakes along the Cascadia Subduction Zone throughout history.

Oregon State University and DOGAMI

How is a 9.0 different from a 6.0, or a 7.0?

There is a significant difference in the damage caused by a magnitude 9.0 earthquake and a more common magnitude 6.0 or 7.0. In the latter, the shaking does not last as long and it may only damage poorly built structures. Sometimes a magnitude 7.0 earthquake can be strong enough to damage earthquake-resistant structures, however.

A magnitude 9.0 earthquake can last for five minutes or longer, and the amount of energy released is about 1,000 times greater than that of a 7.0. According to the U.S. Geological Survey, the most powerful quakes could leave few if any masonry buildings standing, destroy bridges and toss objects into the air.