19-4. Geotechnical investigations of submarine and subaerial slope stability in the Cascadia Subduction Zone: A Subduction Zone Science Team Project

Submarine and subaerial landslides can have significant impacts to the built environment, trigger or contribute to damaging tsunami-generation, and mobilize large volumes of sediment throughout the landscape. While numerous factors precondition slopes for failure (e.g., high sedimentation rates, compaction, elevated pore pressures), earthquakes are a frequent trigger of slope failures in both subaerial and submarine environments along subduction zones. Deep sea turbidite records are a critical component of estimated Holocene Cascadia Subduction Zone (CSZ) earthquake recurrence rates. However, the sources, triggering mechanisms, and pathways of these turbidity flows are poorly constrained, and lead to substantial uncertainty in the inferred earthquake recurrence and seismic hazard of the CSZ. Turbidite records from lake basins can also provide robust and independent records, but also require knowledge of both local subaerial and lacustrine slope stability. Improvements to our understanding of submarine and subaerial landslide triggering in response to long-duration subduction zone earthquake shaking, particularly those instabilities leading to submarine and lacustrine turbidite or landslide deposits, may better constrain the magnitude, timing, and shaking intensity of past CSZ earthquakes and improve our assessments of submarine landslide related hazard and risk. This work can also aid in linking failure source regions to deposits onshore and offshore, which will provide much needed context for interpretation of failure deposits as proxies for earthquake recurrence and is relevant for identifying potential sources of landslide generated tsunamis. This work is supported by active USGS and academic initiatives to compile margin-wide datasets, including high-resolution multibeam bathymetry data, onshore-offshore crustal scale geophysical data, sediment cores, submarine landslide mapping, and observed and simulated ground motions. 

We encourage innovative research proposals designed to address one or more of the following research themes focused in the Cascadia Subduction Zone. Integration and synthesis of datasets, including data from other subduction zones, along with novel approaches to addressing the complex interrelationships among these research areas as well as research proposals that aim to develop geotechnical approaches, methods, and techniques that can be applied to both the onshore and offshore environments are highly encouraged. Possible research themes include, but are not limited to:  

Characterization of the role of ground shaking on the initiation of submarine and subaerial landslides 

Understanding the relationships between earthquake ground motion amplitude, duration, and frequency content, and landslide failure style or size. In addition to the inherent properties of seismic waves generated during earthquake rupture, local or regional lithology and topography can lead to significant amplification or modification of the waves; thus, developing models for site response is necessary for more accurate reconstructions and/or predictions of seismic hazard. Characterizing site response may be important in distinguishing earthquake generated failures from other types of triggers (e.g., oceanographic or climatic) that complicate interpretations of the recurrence record. 

The influence of sediment properties and stratigraphic patterns (e.g., lithology, grain size, slope morphology, presence of weak layers) on preconditioning slopes for failure 

Determining which types of sedimentary environments are prone to generating mass failures and downslope transport during large (M>7) earthquakes is critical to hazard assessments. Spatially continuous, parallel bedded strata can allow lateral migration and vertical confinement of pore fluid leading to generation of super-hydrostatic ambient pore fluid pressures (overpressure) that can reduce shear strength and promote failure along bedding planes. In many cases, these bedding planes are comprised of ‘weak layers’ of sediment with particular characteristics such as lithology, grain size and shape that can influence the shear strength and cohesion of slope sediment. Strain hardening or weakening of lacustrine or seafloor sediments may also complicate the response to shaking and need to be considered. The morphology of the pre-failure seafloor or subaerial topography and internal structure may also play a significant role in the type of landslides triggered.  

The role of fluids, elevated pore pressures, liquefaction, and/or cyclic softening of materials in failure generation 

Elevated pore fluid pressure, from a variety of causes, often plays an important role in slope failures by reducing friction, and subsequently sediment shear strength. The role of cyclic softening and liquefaction on the initiation of submarine landslides by developing or adopting physical, numerical or theoretical models of other related shaking-triggered failure phenomena (e.g., as for subaerial landslides, dynamic earthquake triggering, and liquefaction generally) could be explored.  

While research is the focus of this project, the Fellow would be part of a Subduction Zone Science (SZS) Mendenhall Fellow Team with the goals of developing skills needed to do collaborative and integrative science, and of facilitating communications about SZS within the USGS and broader SZS community. This Team would participate in SZ4D Working Groups (www.sz4d.org) and develop and implement mechanisms to facilitate exchange of information among USGS SZS scientists.  

Interested applicants are strongly encouraged to contact the Research Advisor(s) early in the application process to discuss project ideas. 

Proposed Duty Stations: Moffett Field, CA; Santa Cruz, CA; or Seattle, WA.  

Areas of PhD: Geotechnical engineering, geomorphology, seismology, or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered). 

Qualifications: Applicants must meet one of the following qualifications: Research Civil Engineer, Research Geologist, Research Geophysicist.  

(This type of research is performed by those who have backgrounds for the occupations stated above.  However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.) 

Human Resources Office Contact: Beverly Ledbetter, 916-278-9396, bledbetter@usgs.gov 

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