ABSTRACT

Co-seismic landslides are associated with significant infrastructure damage and human casualties in earthquake-prone 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.

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

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.

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.

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 co-seismic land slides will occur in the future and help prevent loss of property and life in such an event ever occurring.

References Cited

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

Schuster, Robert and Chleborad, Alan. “Earthquake-Induced Ground Failure in Western Washington” 19xx

Schuster, Robert and Howard. “Ground Failure Associated With the Puget Sound Region Earthquakes of Aprill 13, 1949, and April 29, 1965” 1996

Earthquake Risks II: Co-seismic Landslides in Western Washington and OregonPrepared by:

James McLeodES473 Environmental Geology, Spring 2009

Figure 5. Combination of high resolution photos and LIDAR to show past Landslides

Figure 3. Shows roads being covered by landslides

DAMAGES

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.

NEW TECHNOLOGIES

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.

Figure 4. A house has been moved off of its foundation as a landslide moved from the slope on the backside of the property

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Figure 1. Seismogram comparison of a 4.0 magnitude earthquake to a 9.0 magnitude earthquake

Figure 2. shows a entire houses being moved down slope with the slide severely damaging them and the road