CSZ and Skamania County

from Thomas Pierson’s, U.S.G.S, presentation on April 7, 2015

Possible Incidents and Events in Skamania County, Washington, That Could Result from a Magnitude 9 (M9) Cascadia Subduction Zone (CSZ) Earthquake

It is important to note that there are large uncertainties inherent in ground-motion prediction during earthquakes. The biggest variables are the magnitude (energy release) and the location (epicenter and depth) of the earthquake. Another variable is the material through which the seismic waves pass: hard rock, such as the Columbia River Basalt, does not significantly dampen high frequency ground motion, while softer sedimentary rock units typically do.
Strong ground motion in Skamania County is possible, according to Tim Walsh (WA DNR), not only from the main earthquake itself, but also from possible reactivation of shallow crustal faults located in or near the county. Peak ground accelerations of about 0.2 to 0.3 g can be expected in Skamania County for an earthquake with a 2% exceedance probability in 50 years (USGS, Earthquake Hazards Program, Seismic Hazard Map for Washington State). Such shaking is considered to be “very strong”, and peak accelerations of 0.2–0.3 g typically cause a degree of damage described by the Modified Mercalli Intensity Scale (level VII) [http://earthquake.usgs.gov/learn/topics/mercalli.php]:

“Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.”

To see how buildings respond to earthquakes, go to

In general, buildings constructed of wooden framing built to modern building codes withstand earthquake shaking better than unreinforced masonry buildings. In addition to the main CSZ earthquake, there may be aftershocks large enough to cause additional damage in the days to weeks following the initial earthquake.

Much of Skamania County faces the hazard of earthquake-induced landslides, in addition to the direct hazard of ground shaking. A study of earthquake-induced landslides from around the world (Keefer, 1984: Geol. Soc. Amer. Bull. 95:406-421) documents that landslides can be triggered more than 500 km (300 mi) from the epicenter of an M9 earthquake. Skamania County is about 200 km east of a potential rupture zone in a CSZ earthquake, so landslides could be expected. Furthermore, much of the north side of the Columbia River Gorge in Skamania County is composed of old landslides (Pierson and others, in press, Landslides in the Western Columbia River Gorge, Skamania County, Washington: U.S. Geological Survey Scientific Investigations Map), including the terrain on which the town of Stevenson is built. Some of these slides have been active historically, and some are currently active (creeping), suggesting that parts of this terrain are only marginally stable. It is reasonable to assume that additional slide movement could be triggered by a CSZ earthquake, particularly if the earthquake were to occur in late winter or spring when water tables are high due to winter rainstorms. Some low-lying areas along the floodplain of the Columbia River are likely underlain by saturated sandy sediment, which can liquefy during earthquakes. Liquefied sediment can spread laterally, and structures built on such deposits can pull apart or sink into the ground.
Very large (M9) CSZ earthquakes have an average recurrence interval of about 500 years [http://pnsn.org/outreach/earthquakesources/csz], although the actual intervals between such earthquakes in the past have ranged from less than 200 years to more than 1000 years (Clague, 1997: Reviews of Geophysics 35:439-460). The last one to occur in the Pacific Northwest was on January 26, 1700 (Atwater, 2005: US Geological Survey Prof. Paper 1707). Given the occurrence of the next M9 CSZ earthquake, some of the following events/incidents could occur in response to the expected ground shaking:

  1. Rockfalls would likely occur at numerous locations, particularly along Hwy 14 where high, steep, rock-face road cuts presently exist. Some of these could completely block traffic, and some might result in casualties if rocks fall on vehicles or on houses.
  2. About 8 km2 of the Red Bluffs landslide (just west of Stevenson) is now (in 2015) active, moving at about 15-25 cm/year, and is slowly deforming the natural gas pipeline that is buried in the toe of this slide. A CSZ earthquake might accelerate movement on this slide, leading to a rupture of the gas pipeline. Landslide acceleration could possibly also cause one or more of the BPA high-voltage transmission towers (in the same right-of-way as the pipeline) to tip or move downhill.
  3. A number of old landslides could be reactivated throughout Skamania County by a CSZ earthquake, especially during a very wet winter season and possibly resulting in damage to or destruction of homes, power outages, breakage of water and/or sewer lines, and break-up or blockage of roads and other infrastructure.
  4. Liquefaction of sandy fill could cause Hwy 14 to break up and become impassible at the Ash Lake causeway, low-lying areas around North Bonneville, and the flats just east of the mouth of Wind River. These areas have been designated as moderately susceptible to liquefaction failure on the WA DNR map of Liquefaction Susceptibility of Skamania County. Localized areas where the sandy fill is relatively uniform in size, fine grained, and fully saturated are particularly susceptible.
  5. Cave-ins within railway tunnels along the BNSF line could block rail traffic.
  6. A cave-in might be triggered in one of the weak zones in the Spirit Lake drainage tunnel at Mount St. Helens, resulting in blockage of the Spirit Lake outlet and a gradual rise in the water level at Spirit Lake. If the blockage were not removed, lake level would rise to a critical level and result in a catastrophic outbreak of Spirit Lake, threatening Kelso, Longview, and Interstate 5 with a large flood or lahar.

When it comes to landslides and their more fluid cousins, debris flows, there are a number of things that residents and homeowners can do to reduce their risk. These are covered in a free, online USGS publication: The Landslide Handbook—A Guide to Understanding Landslides [http://pubs.usgs.gov/circ/1325/]. A modified list of the basic steps from this publication for lessening risk is given below:

What Can You Do If You Live On or Near Landslide-Prone Hillslopes?

Prior to Intense Storms:
  • Become familiar with the land around you. Learn whether and where landslides or debris flows have occurred in your area by contacting local officials, emergency management officials, State geological surveys or departments of natural resources, and university departments of geology. Slopes where landslides or debris flows have occurred in the past are likely to experience them in the future.
  • Support your local government in efforts to develop and enforce land-use and building ordinances that regulate construction in areas susceptible to landslides and debris flows. Wherever possible, buildings should be located away from steep slopes, hummocky ground that indicates past instability, stream or river banks, “dry” creek beds (intermittent-stream channels), and the mouths of small canyons. Where landslides are a potential hazard, a licensed engineering geologist should be consulted for advice on building placement and on any regrading of the property.
  • Watch the patterns of storm-water drainage on slopes near your home and note especially the places where runoff water soaks into the ground or converges to flow over your property. If possible, divert such water into drain pipes so that it cannot soak into the ground.
  • Don’t allow water from gutter downspouts to soak into the ground; divert to creeks or storm sewers. Don’t water lawns or shrubs excessively. Promptly repair any breaks to water lines.
  • Watch the hillsides around your home for any signs of land movement, such as small slumps, especially soggy ground, the formation of ground cracks, or the progressive tilting of trees.
  • Contact your local authorities to learn about the emergency-response and evacuation plans for your area and develop your own emergency plans for your family and business.
During Intense Storms:
  • Stay alert and stay awake! Many landslide and debris-flow fatalities occur when people are sleeping. Listen to a radio (NOAA Weather Radio) for warnings of intense rainfall. Be aware that intense short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather. Periods following prolonged very wet weather can also be times when landslides occur.
  • If you are in an area susceptible to landslides and debris flows, consider leaving if it is safe to do so. Remember that travel during an intense storm is hazardous.
  • Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together, creaking or groaning of your house, or a deep rumbling sound. A trickle of flowing or falling mud in a channel or over the ground surface may precede larger slides or flows. If you are near a stream or channel, be alert for any sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate a debris flow upstream that is only minutes away, so be prepared to move quickly. Don’t delay! Save yourself, not your belongings.
  • Be especially alert when driving during heavy rain. Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, falling rocks, or streams overflowing culverts.
What to do if you suspect imminent landslide danger:
  • Evacuate! Most houses cannot survive landslides or debris flows. Avoid crossing the potential path of a landslide or debris flow, if possible. Evacuation on foot may be your only option.
  • Contact your local fire, police, or public works department to report the situation.
  • Inform/warn affected neighbors. Assist neighbors who may require special assistance—infants, elderly people, and people with disabilities.
  • After landslides occur:
  • Stay away from the slide area, if possible. Landslides and debris flows commonly occur as multiple pulses of debris (and the first may not be the largest.
  • Check for injured and trapped persons in or near the slide area. Give first aid if trained, and call for help.
  • Listen to a battery-operated radio or television for the latest emergency information.
  • Remember that flooding may occur after a debris flow or a landslide.
  • Check for damaged utility lines. Report any damage to the utility company.
  • Check the building foundation, chimney, and surrounding land for damage.

Thomas C. Pierson, Ph.D. LG
U.S. Geological Survey
April 7, 2015