21-40. Geomorphic approaches to characterizing megathrust processes across the Cascadia Subduction Zone

Please communicate with individual Research Advisor(s) on the right to discuss project ideas and answer specific questions about the Research Opportunity.

How to Apply

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Despite a ~10,000-year history of great M8-9 earthquakes along the Cascadia Subduction Zone (CSZ) inferred from onshore and offshore geologic evidence, there are no seismic recordings of these earthquakes to directly measure shaking intensities or to characterize rupture characteristics. While variations in inferred coseismic coastal subsidence caused by the last great CSZ earthquake in 1700 allow for estimates of coseismic fault slip, a huge range of fault characteristics, or even multiple earthquakes, can be used to explain these limited observations. The lack of instrumental recordings, and sparse nature of the onshore paleoseismic record, contribute to large uncertainties in our understanding of fundamental rupture characteristics of the CSZ. In turn, this limits our ability to accurately characterize seismic hazard across much of the U.S. West Coast. Examples of current uncertainties include: the distribution of locking along the seismogenic zone; coseismic slip, and in turn coseismic uplift and subsidence along-strike during past CSZ earthquakes; the location and frequency of partial-margin ruptures; the potential for multi-earthquake sequences closely spaced in time; and the persistence of earthquake rupture characteristics in space and time across multiple earthquake cycles. This research opportunity seeks proposals that will derive constraints on some of these listed uncertainties based on topographic expression in the Cascadia Subduction Zone, which can be complementary to, and augment, similar studies with modern geodetic and seismic data over different time scales.  

There exist strong connections between forearc topography and the tectonic and geodynamic processes that shape it. At local scales within Cascadia, recent studies have shown that geodetic and geomorphic analyses can provide insights into megathrust characteristics and the seismic cycle. For example, long-term denudation rates combined with modern geodetic data have allowed for a better understanding of the connection between short-wavelength deformation at scale of earthquake cycles, shear stress dissipation along the megathrust, and the accumulation of topography (Michel-Wolf et al., 2022). Similarly, a topographic proxy for rock uplift rate utilizing the geometry of debris-flow networks found a discrepancy between long-term and modern inter-seismic uplift rates that can be exploited to estimate both earthquake recurrence as well as the inland extent of coseismic deformation (Penserini et al., 2017). These techniques have only been applied to select regions of Cascadia, however, and it remains to be seen what a margin-wide analysis using available high-resolution topographic data might reveal about discrepancies between modern and long-term deformation, and the nature of the geomorphically-integrated displacement field across the subduction zone. Recent work has also identified the morphology of the marine shelf break as an indicator of downdip locking depth and persistence in subduction zones more generally (Malatesta et al., 2021), and the pattern of emergent coastal peninsulas as indicators of rapid uplift and aseismic creep in zones that limit seismic rupture (Saillard et al., 2017). To what extent these features can be utilized to understand rupture processes and limits in Cascadia remains an open question.  

We seek proposals that develop, or apply, geomorphic techniques to address one or more of the above uncertainties in Cascadia Subduction Zone fault characteristics over the time scales of one-to-many earthquake cycles (10^2 ~ 10^4 years). Possible research topics include, but are not limited to: 

• Is the topographic proxy a1 (Penserini et al., 2017), signified by the rollover from debris-flow to fluvial channel processes in slope-area space, a robust indicator of rock uplift across the margin? And if so, what do the patterns tell us? 

• Do other geomorphic proxies for rock uplift, such as hilltop curvature, provide a more robust and finer-scale indicator of inelastic deformation across the margin over multi-earthquake cycles? 

• Is any information regarding megathrust processes encoded in the topography of bedrock or alluvial coastal channel networks?  

• Might the distribution of peninsulas and marine terraces across the CSZ tell us about potential rupture barriers and zones of aseismic creep akin to other subduction zones? 

• Can shelf break morphology, together with forearc topography, reveal information about the persistence of locking depth along the CSZ margin?  

Proposals beyond these suggested topics are welcome, and we encourage applicants to consider the research frameworks and priorities described in the USGS Subduction Zone Science Plan (Gomberg et al., 2017) and the SZ4D Draft implementation plan. Interested applicants are strongly encouraged to contact the Research Advisors early in the application process to discuss project ideas. 

References:  

Gomberg, J.S., Ludwig, K.A., Bekins, B.A., … & Wein, Anne, 2017, Reducing risk where tectonic plates collide—U.S. Geological Survey subduction zone science plan: U.S. Geological Survey Circular 1428, 45 p., https://doi.org/10.3133/cir1428. 

Malatesta, L. C., Bruhat, L., Finnegan, N. J., & Olive, J. A. L. (2021). Co‐location of the downdip end of seismic coupling and the continental shelf break. Journal of Geophysical Research: Solid Earth, 126(1), e2020JB019589. 

Michel-Wolf, L., Ehlers, T. A., & Bendick, R. (2022). Transitions in subduction zone properties align with long-term topographic growth (Cascadia, USA). Earth and Planetary Science Letters, 580, 117363. 

Penserini, B. D., Roering, J. J., & Streig, A. (2017). A morphologic proxy for debris flow erosion with application to the earthquake deformation cycle, Cascadia Subduction Zone, USA. Geomorphology, 282, 150-161. 

Saillard, M., Audin, L., Rousset, B., Avouac, J. P., Chlieh, M., Hall, S. R., ... & Farber, D. L. (2017). From the seismic cycle to long‐term deformation: Linking seismic coupling and Quaternary coastal geomorphology along the Andean megathrust. Tectonics, 36(2), 241-256. 

Proposed Duty Station(s): Seattle, Washington; Portland, Oregon; or Moffett Field, California  

Areas of PhD: Geology, geomorphology, geophysics, 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 Geologist or 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:  Paj Shua Cha, 650-439-2455, pcha@usgs.gov

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