Investigating the Eastern Extent of the Seattle Fault Zone through Depth to Bedrock and Borehole Analysis in Bellevue, Washington

This study identifies lineaments that indicate fault activity and strengthens previous interpretations of structures within the eastern extent of the Seattle Fault zone in Bellevue, WA. My investigation has compiled geotechnical subsurface data, high-resolution LiDAR imagery, and ground-penetrating radar to produce strip log sections transecting identified lineaments and depth-to-bedrock maps exposing fault structure. My work incorporates field investigation, multiple publicly available datasets, and subsurface modeling. My results include a map showing twenty-eight identified surface lineaments, five strip-log sections, and interpolated depth-to-bedrock and minimum-depth-to-bedrock maps. Several lineaments identified in the minimum-depth-to-bedrock raster are parallel to the Seattle Fault zone and suggest the presence of small splay faults beneath east Bellevue. These results strengthen previous interpretations of seismic profile data located in the study area. Another lineament identified in the minimum-depth-to-bedrock raster suggest an unmapped tear fault accommodating differential offset along fault strike between Mercer Island and Bellevue. This work also demonstrates the utility of publicly available datasets such as geotechnical subsurface explorations and LiDAR imagery in supplementing geologic investigations in the eastern extent of the Seattle Fault zone.

Locating the Seattle Fault in Bellevue, WA: Combining geomorphic indicators with borehole data

The Seattle Fault is an active east-west trending reverse fault zone that intersects both Seattle and Bellevue, two highly populated cities in Washington. Rupture along strands of the fault poses a serious threat to infrastructure and thousands of people in the region. Precise locations of fault strands are still poorly constrained in Bellevue due to blind thrusting, urban development, and/or erosion. Seismic reflection and aeromagnetic surveys have shed light on structural geometries of the fault zone in bedrock. However, the fault displaces both bedrock and unconsolidated Quaternary deposits, and seismic data are poor indicators of the locations of fault strands within the unconsolidated strata. Fortunately, evidence of past fault strand ruptures may also be recorded indirectly by fluvial processes and should also be observable in the subsurface. I analyzed hillslope and river geomorphology using LiDAR data and ArcGIS to locate surface fault traces and then compare/correlate these findings to subsurface offsets identified using borehole data. Geotechnical borings were used to locate one fault offset and provide input to a cross section of the fault constructed using Rockworks software. Knickpoints, which may correlate to fault rupture, were found upstream of this newly identified fault offset as well as upstream of a previously known fault segment.

Locating a strand of the Seattle Fault Zone in southeast Seattle, Washington through topographic analysis, borehole analysis and ground penetrating radar

Contributing to the evaluation of seismic hazards, a previously unmapped strand of the Seattle Fault Zone (SFZ), cutting across the southwest side of Lake Washington and southeast Seattle, is located and characterized on the basis of bathymetry, borehole logs, and ground penetrating radar (GPR). Previous geologic mapping and geophysical analysis of the Seattle area have generally mapped the locations of some strands of the SFZ, though a complete and accurate understanding of locations of all individual strands of the fault system is still incomplete. A bathymetric scarp-like feature and co-linear aeromagnetic anomaly lineament defined the extent of the study area. A 2-dimensional lithology cross-section was constructed using six boreholes, chosen from suitable boreholes in the study area. In addition, two GPR transects, oblique to the proposed fault trend, served to identify physical differences in subsurface materials. The proposed fault trace follows the previously mapped contact between the Oligocene Blakeley Formation and Quaternary deposits, and topographic changes in slope. GPR profiles in Seward Park and across the proposed fault location show the contact between the Blakeley Formation and unconsolidated glacial deposits, but it does not constrain an offset. However, north-dipping beds in the Blakely Formation are consistent with previous interpretations of P-wave seismic profiles on Mercer Island and Bellevue, Washington. The profiles show the mapped location of the aeromagnetic lineament in Lake Washington and the inferred location of the steeply-dipping, high-amplitude bedrock reflector, representing a fault strand. This north-dipping reflector is likely the same feature identified in my analysis. I characterize the strand as a splay fault, antithetic to the frontal fault of the SFZ. This new fault may pose a geologic hazard to the region.

Geomorphic Impacts of the 2013 Colorado Front Range Flood on Black Canyon Creek and North Fork Big Thompson River

In September 2013, the Colorado Front Range experienced a five-day storm that brought record-breaking precipitation to the region. As a consequence, many Front Range streams experienced flooding, leading to erosion, debris flows, bank failures and channel incision. I compare the effects that debris flows and flooding have on the channel bar frequency, frequency and location of wood accumulation, and on the shape and size of the channel along two flood impacted reaches located near Estes Park and Glen Haven, Colorado within Rocky Mountain National Park and Arapaho-Roosevelt National Forest: Black Canyon Creek (BCC) and North Fork Big Thompson River (NFBT). The primary difference between the two study areas is that BCC was inundated by multiple debris flows, whereas NFBT only experienced flooding. Fieldwork consisted of recording location and size of large wood and channel bars and surveying reaches to produce cross-sections. Additional observations were made on bank failures in NFBT and the presence of boulders in channel bars in BCC to determine sediment source. The debris flow acted to scour and incise BCC causing long-term alteration. The post-flood channel cross-sectional area is as much as 7 to 23 times larger than the pre-flood channel, caused by the erosion of the channel bed to bedrock and the elimination of riparian vegetation. Large wood was forced out of the stream channel and deposited outside of the bankfull channel. Flooding in NFBT caused bank erosion and widening that contributed sediment to channel bars, but accomplished little stream-bed scour. As a result, there was relatively little damage to mid-channel and riparian vegetation, and most large wood remained within the wetted channel.