Description of the Research Opportunity
Turbidites form when sediments mobilize due to slope failures or stirring and entrainment of seafloor sediments caused by earthquake-shaking, tsunamigenic submarine landslides, storms, ocean currents and waves, or river floods that drain into the sea. Turbidites inferred to have formed synchronously over distances of hundreds of kilometers are assumed to be triggered by the shaking from great earthquakes, which are likely to be the only triggering process with sufficiently large spatial footprints. Thus, seismo-turbidites (turbidites inferred to have been triggered by earthquakes) have served as a primary constraint on the size and repeat time (recurrence) of past great earthquakes, particularly along the Cascadia subduction zone (Goldfinger, 2012). However, uncertainties make it impossible to rule out triggering by multiple smaller earthquakes or non-earthquake triggers occurring within intervals shorter than decades to centuries.
Recent studies of turbidites have illuminated depositional characteristics that may be uniquely attributed to the shaking from great earthquakes (McHugh et al., 2020; Kanamatsu et al., 2023). These are relatively uniform and thick mud deposits (herein ‘thick mud caps’) preserved atop conventional normally graded (i.e., fining upwards) turbidites. Nieminski et al. (2023) have observed similar deposits in their detailed evaluation of a subset of the Cascadia cores used to develop the widely-cited megathrust earthquake recurrence chronology dating back ~14,000 years (Goldfinger et al., 2012). Hugh et al. (2020) hypothesized that very low frequency shaking that characterizes only the largest earthquakes can remobilize seafloor sediments in such a way to ultimately deposit thick, perfectly homogenous mud. Nieminski et al. (2023) suggest that the thick turbiditic mud caps identified in Cascadia formed as a result of ponding in settings where only great earthquake shaking could lead to settling of mud with significant thickness and over such widespread sites. Preliminary observations also suggest that these deposits tend to be associated with a basin floor that is unusually flat, likely a reflection of the ponding process. In either interpretation, these deposits provide features that may more globally distinguish great earthquake shaking from other triggers.
Advancing subduction zone science for improving hazard assessments and forecasts, particularly in Cascadia, is a primary goal of USGS NHMA, and this RO addresses various questions that address this goal. Examples relevant to constraining the initiation and deposition of these thick mud caps include how uniquely they may be identified and measured, are they found only where ponding is likely, do they correlate and balance with evacuated masses inferred from slope/shelf edge failure scars, what types of shaking mobilizes them? Ideal datasets may come from abundant available sediment cores in combination new contextual bathymetric and subsurface imagery data (see Hill et al., 2022). X-Ray computed tomography (CT) imagery, magnetic susceptibility (MS) data, and potentially chemical signatures may provide additional details of the sedimentology that link these newly recognized thick turbiditic mud caps to their triggers and depositional settings (e.g., whether ponded or not). Based on new contextual data, predictions may be made about where ponded sediments are expected to occur. Novel objective and quantitative approaches to correlation between deposits can be employed to assess correlation significance (e.g., Sylvester, 2023). Other avenues of relevant research may focus on analyzing new high-resolution bathymetry in Cascadia, which now makes it possible to explore key questions about sediment transport mechanisms; e.g., do observed counterintuitive drainage patterns require the occurrence of massive widespread sediment gravity flows that might result from great earthquake shaking?
Interested applicants are strongly encouraged to contact the Research Advisor(s) early in the application process to discuss project ideas.
References:
Goldfinger, C., Nelson, C.H., Morey, A.E., Johnson, J.E., Patton, J.R., et al., 2012, Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone, USGS Professional Paper 1661–F, doi.org/10.3133/pp1661F.
Hill, J.C., J.T. Watt, and D.S. Brothers, 2022, Mass wasting along the Cascadia subduction zone: Implications for abyssal turbidite sources and the earthquake record, Earth and Planetary Science Letters, 579, https://doi.org/10.1016/j.epsl.2022.117797
Kanamatsu, T., K. Ikehara, and K. Hsiung, 2023, Submarine paleoseismology in the Japan Trench of northeastern Japan: turbidite stratigraphy and sedimentology using paleomagnetic and rock magnetic analyses, Progress in Earth and Planetary Science (2023) 10:16 https://doi.org/10.1186/s40645-023-00545-3
McHugh et al., 2020, Isotopic and sedimentary signature of megathrust ruptures along the Japan subduction margin, Marine Geology, 428, 20 p., doi: 10.1016/j.margeo.2020.106283.
Nieminski, N., Z. Sylvester, J.A. Covault, J. Gomberg, and I.W. McBrearty, (2023), Evaluating turbidite correlations for paleoseismology, Geologic Society of America Bulletin, in preparation.
Sylvester, Z., 2023, Automated multi‐well stratigraphic correlation and model building using relative geologic time: Basin Research, bre.12787, doi:10.1111/bre.12787.
Proposed Duty Station(s)
Seattle, Washington
Areas of PhD
Sedimentology, geology, seismology, geophysics, marine geophysics, Earth science, mathematics, computer science, 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.)
