John: Hi, my name is John Manke and I worked on a project very similar to Bryan’s over
the summer. My talk is titled, “Locating a new strand of the Seattle fault in southeast
Seattle using topographic analysis, borehole analysis, and ground penetrating radar.”
Getting right into it, we have crustal shortening in the Puget lowland because the Pacific plate
is moving this way and the Juan de Fuca plate is moving that way resulting in rotation and
shortening. We have a series of thrust faults throughout the Puget lowland. We also have
a very glacial landscape here. We have evidence of at least six large scale periods of glaciation.
These ice sheets are very good at depositing large amounts of sediment while bulldozing
through that sediment as they come by. So it’s difficult stratigraphy to identify.
The Seattle Fault Zone is a south-dipping thrust fault that extends from the Kitsap
Peninsula to the Cascade Foothills. It’s been active for about 40 million years and
there’s about 7 kilometers of throw. So this is my study area. I began by denoting
lineaments in the topography. On the left, we have LiDAR DEM and on the right we have
a hillshade image of the same area and I denoted lineaments that were not related to the glacial
fabric in the area. I further narrowed my study area by finding a scarp in the Lake
Washington bathymetry. So this is the raw data from NOAA. I multiplied it by a Boolean
raster to eliminate the topography above sea level and I came out with something like that
which we’ll zoom into here. So I want you to notice this scarp in the lake. Very
clearly shown there. That coincides with another feature in the aeromagnetic data so I’m
going to focus sort of in the land where that comes in. So we have over 80,000 borehole
logs in the Seattle area and these are – they only represent spatial data including location,
depth, and elevation. But we need to combine it with layer data using these exploration
IDs. So I further narrowed my study area, I chose almost 600 points based on location
confidence, depth, and whether or not they have layers. And you can see it really reduces
the point density which became a problem. So I joined these two tables together to form
a table which I exported into this RockWorks software. I chose a transect along these that
I thought was going to work well after sifting through all the data. I came up with this.
So here we are back in ArcGIS. I used ArcGIS to create the surface profile of this transect.
I used the interpolate shape tool with this LiDAR DEM as the input surface and I came
up with this profile – 10 times exaggerated. I want you to notice the pink layers – those
represent the sedimentary bedrock which ceased right there where I drew the line that also
coincides with one of the lineaments I denoted earlier. So we have bedrock to the south and
no bedrock to the north. I was going to check this with GPR. So this is an antenna that
emits an 80 megahertz radiowave into the ground and reflects back. We measured the two-way
travel time which is only effective to about 20 feet. These are the transects that we did.
The four up north are crossing where I think the fault is. The one there on Seward Park
was used as a control because it has gravel trails, no buried utilities, and it crosses
a well-defined geologic contact. We crossed from the tertiary bedrock into the Quaternary
Vashon Till at this location. I georeferenced a geologic map and also a trail map to get
the surface. And notice the defraction pattern in the Till. So, I don’t really see a whole
lot in these transects but potentially there is the same sort of defraction pattern in
the Till where I know there is till in this B-B transect line. My results are that I propose
that there is a new strand of the Seattle fault represented there. It follows the scarp
in the lake, the aeromagnetic data, and also the lineaments that I mapped. There are north-dipping
beds in the Blakely Formation and I think it matches with this interpretation made by
Brocher that there is splay thrust that comes back out from the roof thrust. It seems to
be consistent. This is a figure that you just saw. We have challenges in the Pacific Northwest
dealing with this glacial stratigraphy, paleotopography, all this vegetation and water, and then working
in the city with inconsistent and too shallow boring logs. Here are a lot of references.
I’d like to thank Bryan and Rebecca for helping me with this project and Kathy Troost
and Bryan Sherrod for pointing me in the right direction.