Earthquake Risks II: Co-seismic Landslides in Western Washington and Oregon
Prepared by:
James McLeod
ES473 Environmental Geology, Spring 2009
ABSTRACT
Co-seismic landslides are associated with significant
infrastructure damage and human casualties in earthquakeprone areas of the world. These types of mass-wasting
processes are induced by the ground movement associated with
earthquakes. Factors contributing to co-seismic landslides
include slope gradient and water-saturation state of surface
materials. Correlating historic co-seismic landslide data in the
Pacific Northwest with those from similar geologic terrains
elsewhere enable better prediction of slope failure during future
Cascadia earthquake events.
Evidence of past co-seismic landslides is commonly hidden
by overlying sediment and vegetation, or has been removed over
time by erosion. As western Washington and Oregon have
become more populated over the past several decades, the risks
of earthquake-related losses are greater than at any time in the
past. By examining records and using new technologies like
LIDAR, researchers are better able to predict where co-seismic
landslides will occur in the future and prevent loss of life. This
paper examines trigger mechanisms for seismically induced
landslides and provides an overview of hazard-prone localities in
the Pacific Northwest.
IDENTIFYING PAST CO-SEIMIC LANDSLIDES
Analysis of data from previous landslides came help us
predict where they might happen in the future. There is
evidence that shows that landslides will occur in the same area
more than once, so being able to recognize where these past
landslides have occurred can help prevent loss of property and
lives.
Problems with recognizing past landslides is that they can be
hidden by overlying sediment and vegetation or the debris and
sediment that was left behind from the slides have been eroded
away over time.
Figure 5. Combination of high resolution photos and LIDAR to show past
Landslides
NEW TECHNOLOGIES
INTRODUCTION
Co-seismic landslides are triggered by the ground movement
associated with earthquakes and are common in areas of high
slope gradients and water-saturated state of soil at the surface
of the earth. These types of landslides can cause serious
damage to homes and property along with human casualties
Figure 2. shows a entire houses being moved down slope with the slide
severely damaging them and the road
DAMAGES
You can insert images throughout poster
The main source of damage cause by co-seismic landslides will
be down along the coast highway and roads that run along
rivers. These areas are more prone to landslides because of the
steeper gradients and amount of water in the soil. Homes and
property are at greater risks in landslide prone areas. This risk
has greatly increased over the past decades due to population
growth. People have moved in and built on ancient landslides
deposits that have been covered up or eroded away.
There are new technologies that have emerged in the last 5 or 6
years are making it much easier to find past landslides that are
covered up by vegetation. This new technology is called LIDAR.
LIDAR uses lasers that bounce back and forth between the
machine and ground recording data points. Once these points
are analyzed they can strip away vegetation and building off the
landscape giving us very detailed maps of the ground. Along
with LIDAR there are much higher resolution photos that can
get to .5 meter resolution giving us very clear pictures of the
landscape.
CONCLUSIONS
Co-seismic landslides are one of the major hazards that will be
associated with Cascadia Subduction Zone earthquake. They
will cause damage to homes, buildings, and roads. Coastal
communities all along the Oregon and Washington coast could
be isolated for days from HWY 101 being blocked by these
landslides. Looking at past landslides combined with the new
technologies like LIDAR will help us predict where these coseismic land slides will occur in the future and help prevent
loss of property and life in such an event ever occurring.
References Cited
Figure 1. Seismogram comparison of a 4.0 magnitude earthquake to a
9.0 magnitude earthquake
Hofmeister, R. “Earthquake-induced slope instability: A relative
hazard map for the vicinity of the Salem Hills” Oregon Geology,
Volume 61, Number 3, May/June 1999
EARTHQUAKE MAGNITUDE
A magnitude 4 earthquake is the estimated minimum value for
causing a landslide. There have been 23 known earthquakes
with a magnitude 4.7 or greater in Washington with 14 of those
that having direct links to causing landslides. The predicted
magnitude of an earthquake associated with the Cascadia
Subduction Zone is 9.0 or greater.
Schuster, Robert and Chleborad, Alan. “Earthquake-Induced
Ground Failure in Western Washington” 19xx
Figure 3. Shows roads being
covered by landslides
Figure 4. A house has been moved off of its foundation
as a landslide moved from the slope on the backside of
the property
Schuster, Robert and Howard. “Ground Failure Associated With
the Puget Sound Region Earthquakes of Aprill 13, 1949, and
April 29, 1965” 1996