What is earthquake magnitude?

Earthquake magnitude is a measure of the amount of energy released by an earthquake.  The most common magnitude is the “moment magnitude”.  Moment magnitude is calculated from the amount of slip on the fault causing the earthquake and the area of the fault surface which breaks.

Magnitude values range from 0 to 9+, on a logarithmic scale.  This means that about thirty times more energy is released for each unit increase in magnitude.  For example, a M8 earthquake releases thirty times more energy than a M7, and a M9 earthquake releases 900 times more energy than a M7 (30x30=900).

What is the Cascadia Subduction Zone (CSZ)?

The CSZ is a geologically complex area within approximately forty miles of the Pacific Northwest coast.  It extends from Northern California to the Northern end of Vancouver Island.  In Southern Oregon and near Cape Mendocino, California the CSZ is within a few miles of the coastline.  The CSZ is an area were several pieces of oceanic crust are being pushed under the North American crust.  This process is responsible for most of the earthquakes in the Northwest as well as the Cascade volcanoes.

What is the “big one”?

In the Pacific Northwest the “big one” refers to a M9 earthquake that may be produced by complete rupture of the Cascadia Subduction Zone (CSZ). The impact in the Portland area according to the USGS map will be low to moderate due to the distance from the fault.

How much ground shaking will the big one produce in Portland?

The intensity of ground shaking depends upon earthquake magnitude, the distance and depth to the fault rupture, and local soil characteristics.  The USGS and others have done extensive modeling of ground motions that may be produced by a M9 CSZ earthquake.  USGS ShakeMaps for the “big one” and for the 2011 Tohoku, Japan earthquake are shown below:


Maps show earthquake shaking in Japan's 2011 magnitude-9 earthquake, left, and for a quake of the same force expected off Oregon's coast. Green indicates light shaking. Yellow shows stronger shaking. Dark red indicates shaking strong enough to seriously damage even well-built structures. (Oregon Department of Geology and Mineral Industries)

Damage assessment from the Tohoku earthquake shows that very few lives were lost, and there was relatively little building damage in the yellow areas on the above map.

Research that is currently underway at the University of Washington indicates that ground shaking at a particular location depends on the direction of rupture propagation.  For example, the UW research shows that Seattle will experience less severe shaking if the epicenter is located near Seattle compared to an epicenter that is located further offshore.  “The reason is because the rupture is propagating away from Seattle, so it’s most affecting sites offshore.  But when the epicenter is located pretty far offshore, the rupture travels inland and all of that strong ground shaking piles up on its way to Seattle.” (Erin Wirth)

When with the “big one” happen?

No one knows.  Earthquake predictions primarily rely on statistical analysis of how often earthquakes have occurred in the past.  Over the past 10,000 years there is evidence that up to forty-three large CSZ earthquakes have occurred.  The most recent analysis by Chris Goldfinger at Oregon State University indicates that there is up to a twenty percent probability that a M9 CSZ earthquake will occur in the next fifty years. 

Are there faults near Portland?

There are numerous faults in the Portland metropolitan area.  The three main faults that have been identified by scientists are shown below. 

Are Portland’s faults active?

In 2001, Ian Madin of the Oregon Department of Geology and Mineral Industries observed the first evidence that two earthquakes may have occurred on the Portland Hills Fault within the past 12,800 to 15,000 years.  Madin observed deformed Missoula flood deposits in a shallow trench at Rowe Middle School in Milwaukie.  There is no other evidence of geologically recent fault movement on any of the other major faults.

How much shaking would be produced by a local earthquake?

Of the three main faults in the Portland area, the Portland Hills Fault is the longest and therefore, has the potential for producing the largest earthquake.  Based upon the size of the fault is estimated that a M7 is the largest earthquake that can be produced by the Portland Hills Fault.  A M7 earthquake on the Portland Hills Fault may produce ground shaking in excess of 0.4 g in downtown Portland.  This level of shaking is approximately twice the level that is predicted by the “big one”.

What is the probability that an earthquake will occur on the Portland Hills Fault?

Earthquake probability cannot be determined because there is too little historical data.  Analysis using assumed slip rates associated with tectonic plates in the region suggest that the recurrence interval for earthquakes on the Portland Hills Fault may range from 2,000 to 16,000 years. (Wong et al., 2001)