Search

Subducted sediment plays a key role in modulating pore pressure and seismic behavior at subduction zones. We investigated the seismic character of incoming sediments to test how sediment and basement variations relate to the along‐strike changes within the accretionary prism and plate boundary conditions at the Nankai Trough. High‐resolution seismic data reveal for the first time the presence of countourite mounded drifts in the Shikoku Basin. These features have probably introduced permeability heterogeneities into an otherwise homogenous mud‐dominant unit. Additionally, we found that normal faults in this unit are more extensive than previously documented, which probably enhances along‐strike fluid transport. The wedge taper is more correlative with the thickness of the mud‐dominant facies than the turbidite thickness. This may be due to the permeability heterogeneities associated with contourite deposits and normal faults, or due to the absence of thick turbidite deposits. Turbidite deposits can either aid the drainage of the margin where they are not confined by basement topography, or contribute to high pore fluid pressures where they are confined by less permeable mudstone or basement topography. When confined turbidite deposits and contourite mounded drifts are subducted, they may contribute to localized compartments of excess pore pressure which provide the necessary conditions for slow slip behavior. We determined that along‐strike variations in seismic behavior are likely related to the subducting basement topographic and sediment characteristics, which vary on a local (<10 km) scale. Plain Language Summary: Subduction zone faults, such as those off the coast of southwest Japan, have been shown to slip at a range of speeds. The ruptures range from typical earthquakes, which occur within seconds or minutes, to slow earthquakes, which can rupture over the course of days or weeks. The sediment characteristics of the incoming plate play a key role in determining the type of slip behavior along a margin. When sediments are subducted, they are compressed and expel the fluid trapped in the sediment. If the fluid is unable to escape, the fluid pressure builds up, and can reduce the strength of the plate boundary fault. The type of sediment, as well as whether it has been faulted or modified by the ocean currents, can control how well fluid can escape. The sediment characteristics and corresponding plate boundary strength vary greatly on a sub‐10 km scale. Slow earthquakes in our study area are co‐located with subducted seamounts. We suggest that this is because the sediments commonly deposited on the sides of large seamounts are conducive to forming high pore fluid pressures upon subduction. This result in a locally weaker plate boundary fault, which may be a necessary condition for slow earthquakes. Key Points: High‐resolution seismic reflection data reveal for the first time asymmetrical contourite mounded drifts in the Shikoku BasinBasement topography and paleocurrents influenced the location of turbidite and contourite deposits, as well as how well sealed the deposits areContourites, turbidites, and normal faults result in localized heterogeneities in the hydrogeologic properties of the subducting sediment [ABSTRACT FROM AUTHOR]

Copyright of Island Arc is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)