New map outlines landslide risks in western Oregon

According to the new map, the highway corridors to the coast which will face comparatively less risk from landslides will be Oregon Highway 36 from near Eugene to Florence; Oregon Highway 38 from near Cottage Grove to Reedsport; Oregon Highway 18 from Salem to Lincoln City; and large portions of U.S. Highway 30 from Portland to Astoria. However, landslides or other damages could occur on any road to the coast or in the Cascade Range due to the anticipated high levels of ground shaking.

The new research, along with other considerations, will help ODOT and other officials determine which areas merit the most investment in coming years as part of long-term planning for the expected earthquake. Given the high potential for damage and minimal resources available for mitigation, experts may choose to focus their efforts on highway corridors that are expected to receive less damage from the earthquake, Olsen said.

The research reflected in the new map considered such factors as slope, direction of ground movement, soil type, vegetation, distance to rivers, roads and fault locations, peak ground acceleration, peak ground velocity, annual precipitation averages, and other factors.

The release notes that ODOT, Oregon State and DOGAMI have been state leaders in research on risks posed by the Cascadia Subduction Zone, earthquake and tsunami impacts, and initiatives to help the state prepare for a future disaster that scientists say is a certainty.

Officials said it is important to consider not just the damage to structures that can occur as a result of an earthquake, but also landslide and transportation issues.

“ODOT recognizes the potential not only for casualties due to landslides during and after an earthquake, but also for the likelihood of isolating whole segments of the state’s population,” one ODOT official said. “Thousands of people in the coastal communities would be stranded and cut off from rescue, relief and recovery that would arrive by surface transport.”

ODOT recently completed a seismic vulnerability assessment and selected lifeline corridor routes to prioritize following an earthquake.  ODOT also maintains an unstable slopes program, evaluating the frequency of rockfalls and landslides affecting highway corridors.

DOGAMI recently released another open file report as part of the Oregon Resilience Plan, which evaluated multiple potential hazards resulting from a Cascadia subduction zone earthquake, including landslides, liquefaction, and tsunamis.

Some recent efforts at OSU have also focused on understanding the different concerns raised by a subduction zone earthquake compared to the type of strike-slip faults more common in California, on which many seismic plans are based. Subduction earthquakes tend to be larger, affect a wider area and last longer.

— Read more in Rubini Mahalingam et al., Landslide Susceptibility Analysis of Lifeline Routes in the Oregon Coast Range, DOGAMI Open-File Report O-15-01 (February 2015); and Michael J. Olsen et al., Impacts of Potential Seismic Landslides on Lifeline, ODOT Research Report SPR-740 (February 2015)