Cascadia Co-Seismic Landslides
While employed at the Oregon Department of Geology and Mineral Industries (DOGAMI) one of the main projects I assisted with was a Cascadia co-seismic landslide dating project. This project is an ongoing collaboration between Oregon state and federal agencies as part of the Cascadia Earthquake Landslide Working Group. The purpose of the project is to identify large deep prehistoric landslides that potentially occurred because of the most recent Cascadia earthquake in 1700, and then date them to determine if they are, in fact, related. This information would be useful for emergency planners, community leaders, and the general public because it can shed light on what sort of landslides we can expect when the next big one hits.
As anyone who is familiar with landslides knows, dating them can be a very tricky proposition. The most common currently used method is carbon dating of plant material and wood buried within the landslide debris. However, most landslides are rather young (at least from a geologic perspective), perhaps as much as a few thousand years old, and carbon dating items from the recent past can be problematic. Furthermore, carbon dating can only provide a possible age range, and cannot give the level of exactness needed to date a slide down to a specific year. However, this project uses a novel approach to date slides, sometimes with accuracy as high as the exact year of occurrence.
This is done by finding ponds and lakes that were formed when a large landslide blocked off a stream valley. These landslide dammed lakes would have been created suddenly and flooded the forest behind them. The trees within the flooded area would have died immediately. Once these landslide dammed lakes were identified, dead snags within the water could be sampled for dendrochronological analysis. Dendrochronology is the science of finding a tree's age using patterns within the tree rings. Researchers in the field have built up a database of tree ring patterns that correspond to specific years, so it's even possible to determine with certainty which year a tree started growing and which year it died. The samples collected from the landslide-dammed flooded forest were sent in for study, and using the ring database it was possible to get the exact year that the flooding, and thus the landslide, happened.
I assisted with almost all parts of the project (except the actual dating of the samples, we sent them off to a dendrochronology expert for that). It provided me with a great deal of experience planning and executing field research. Often, the excursions lasted several days, and it was necessary to stay overnight near the study area. I helped do preliminary research on potential sites, mapped landslide features prior to going in the field, contacted property owners to request site access permission when needed, prepared GIS data for use at the site, helped identify dendrochronology sample candidates, helped collect samples for carbon dating, collected increment bores from live trees for additional dating, and lead field site research with assistants when needed.
My proudest contribution to the project was developing a GIS field data collection workflow using ArcGIS Online, Arc Collector, and ArcMap. Using these programs, it was possible to put together a shared online geodatabase that was concurrently viewable and editable by multiple users at the field research site. All the maps and data were downloadable to mobile devices, which allowed use even when there was no cell service. This was an important feature because the landslide-dammed lakes were usually very remote. Once we returned to civilization, each person could sync their edits and the shared geodatabase would be updated with all the new data. It streamlined the collection of field data immensely and was used on multiple projects.