Impact of horst and graben subduction on sediment flux and shallow décollement geometry in the Japan Trench

Subduction zones, where two tectonic plates converge, generate the world’s largest earthquakes and tsunamis, and present significant hazards to coastal communities. While it is generally understood where these types of events can occur, little is known about the geologic processes that dictate when they will occur and what their maximum magnitude will be.

The Northeastern Japan margin (the Tohoku margin of the Japan Trench), which experienced a devastating M 9.1 earthquake and tsunami in 2011, is an ideal location to evaluate the geologic properties of the plate interface and surrounding rocks that host large earthquakes and tsunamis. The magnitudes of the 2011 earthquake and tsunami were much larger than had been predicted in hazard models, in part because the fault rupture produced an unprecedented large displacement (up to 50-60m) in the shallow part of the plate boundary.

This large shallow slip directly led to intense ground shaking and seawater displacement that produced the large tsunami. This event highlighted significant gaps in the understanding of the physical processes that control shallow seismogenic slip in subduction zones. In this project, new high-resolution sub-seafloor geophysical imaging and seafloor bathymetry data, recently collected by the Japan Agency for Marine Science and Technology, will be used to evaluate the geometry and composition of the rocks that host large earthquakes in the shallow portion of the plate interface in the Japan trench.

The role that horst-and-graben-topography on the incoming plate has on dictating the geometry and physical properties of the plate interface and surrounding rocks will be evaluated. Results from this work will help build a theoretical framework for understanding how seafloor topography, fault geometry, and fault rock composition impact the seismogenic and tsunamigenic potential of subduction systems.

Such a framework can help improve seismic hazard and risk assessment models not only in Japan, but also in other analogous margins, such as portions of the western Alaska, north Cascadia, Chilean, and Central American subduction zones. This project will support education, training, and networking opportunities for a PhD student and undergraduate student who will participate in data analysis and in an education and outreach initiatives.

In addition, a slide set featuring a diverse range of geoscientists in marine geology and geophysics will be developed to use in undergraduate major and general-education classrooms. These slides will provide examples of how course topics apply to real world problems, will amplify the work of geoscientists in underrepresented groups, and will provide exposure to a diverse set of scientists with whom students may identify.

Décollement geometry, composition, and the fluxes on input sediments are fundamental parameters that control shallow megathrust mechanics and the long-term mass balances at subduction zones. Although the subduction of horst and graben is common, there exist few direct constraints on how normal fault subduction affects the geometry, mechanics, and structural evolution of the plate boundary interface and surrounding wall rock, parameters proposed to control the potential for shallow seismogenic and tsunamigenic slip.

How incoming plate relief affects the geometry of the shallow décollement and the relative volumes and composition of accreted versus tectonically eroded material in a portion of the Japan trench known to host shallow seismogenic slip will be tested. Preliminary observations from high-resolution bathymetric and multichannel seismic data in the Japan trench suggest incoming plate relief, a function of outer rise fault throw and incoming plate sediment thickness, is a fundamentally important parameter governing the geometry and composition of the décollement by controlling where material is accreted, subducted, or tectonically eroded.

It is proposed that sediment accretion and décollements that step over horst and graben are favored when lower plate relief is below a threshold value, and that tectonic erosion and planar décollements that smooth seafloor topography only form when relief is above a threshold. This idea will be tested in the Japan trench by integrating submarine tectono-geomorphic and structural mapping of high-resolution bathymetry and seismic reflection data to measure incoming pate relief, to quantify the upper plate response to deformation along the décollement, to map where imbrication, extension, and slumping occur in the frontal prism, and to evaluate the geometry of the shallow décollement.

This project will produce a subsurface structural model of the shallow décollement and frontal prism and a map of regions experiencing sediment accretion versus tectonic erosion that, when compared to the known seismic and paleoseismic history of the Japan trench, can be used to evaluate the impact of incoming plate relief on sediment fluxes, shallow décollement geometry, and the mode of shallow slip. Results from this study are applicable for understanding the seismogenic potential of other margins with thin incoming sediments and horst and graben topography, such as Alsaska, Tonga, Chile, and Central America.

In addition, a slide set featuring a diverse range of geoscientists in marine geology and geophysics will be developed to use in undergraduate major and general-education classrooms. These slides will provide examples of how course topics apply to real world problems, will amplify the work of geoscientists in underrepresented groups, and will provide exposure to a diverse set of scientists with whom students may identify.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.