Portland: Earthquake and Fault Maps

Now that we have learned the science behind earthquakes, its time to consider how they affect us. Read the following excerpt from the Oregon Department of Geology and Mineral Industries (DOGAMI) and then check out the maps.

The Portland metropolitan area and surrounding vicinity have been the most seismically active region in Oregon in historical times. Based on the relatively brief 150-year historic record, six earthquakes of Richter magnitude (ML) 5 or greater have occurred within the greater Portland area including the damaging ML 5.5 Portland earthquake of 1962 and the ML 5.6 Scott Mills earthquake of 1993 (Bott and Wong, 1993). In contrast, recent geophysical studies indicate the presence of at least three crustal faults beneath the Portland metropolitan area (Blakely and others, 1995; Pratt and others, in preparation) which could generate much more damaging crustal earthquakes of ML 6.5 or larger. An evaluation of earthquake recurrence based on the historical record suggests that crustal earthquakes of ML 6.5 and larger occur somewhere in the Portland region on average about every 1,000 years (Bott and Wong, 1993). Additionally, a convincing case has now been made to indicate that Cascadia subduction zone earthquakes up to moment magnitude (MW) 9 have occurred in the prehistoric past, as recently as the year 1700, and will occur in the future (e.g., Atwater and others, 1995; Satake and others, 1996). Thus, although in its 150-year existence the Portland metropolitan area has gone relatively unscathed by damaging earthquakes, strong ground shaking generated by either a Cascadia subduction zone earthquake or a nearby crustal event will certainly have a major future impact on the Portland area.

Earthquake Scenario Maps

Note: the different colors on the maps indicate "spectral acceleration." This is a quantitative measure of how much an Earthquake shakes a building at a particular place. The magnitude of the earthquake, the soil conditions, and the design of the building affect the spectral acceleration. The higher the spectral acceleration, the greater the shaking experienced.

• MW 9 megathrust event along the Cascadia subduction zone
  
• Crustal earthquake of MW 6.8 on the Portland Hills fault

Map Limitations: the maps are limited by geologists' knowledge of rock and soil conditions below the Earth's surface. They do not know the exact size of the faults, nor do they know the exact soil structure below each location. Thus, the maps are meant as approximate guides.

Follow Up Questions

1. The excerpt references two magnitude scales for earthquakes, "ML" and "MW." What scales to these correspond to?
2. What are the two potential sources of major earthquakes in Portland?
3. What are the three main faults that exist under the city of Portland?
4. After reviewing the maps, which of the two earthquake events (the MW 9 megathrust or the MW 6.8 Portland Hills fault) is likely to cause more damage to the city of Portland? What data on the maps lead you to your conclusion?
5. Where in Portland would you build to avoid the highest amount of shaking (i.e. spectral acceleration)?