2:30 PM - 2:45 PM
[HCG21-04] Reconstruction of Mass Transport Process Behavior
using mesoscale fault analysis and numerical analysis :
Example from the Kazusa Group, Kiwada Formation
Keywords:Landslide, Mass-transport
The mass-transport process (MTP) refers to large sediment transport phenomena driven by
self-weight, such as submarine landslides, and deposits formed by the process are called mass-
transport deposits (MTD). Understanding the behavior of MTP is important for estimating
the risk of damaging offshore coastal infrastructure and for understanding the recurrence inter-
vals of massive submarine earthquakes. Previous studies have estimated that intense horizontal
compressive stresses parallel to the flow direction during the cessation of MTP. These infer-
ences are based on features such as the surface structure of modern MTD and deformation
structures in sedimentary blocks within MTD in the geological record. Additionally, numerical
experiments have suggested that highly viscosity MTPs experience rapid deceleration of their
leading edge upon reaching depositional basins from slopes. The collision of the leading edge
and the main body of the flow results in compression within the MTD. However, most studies
on compressive forces during MTP flow were qualitative, and only one study has quantitatively
estimated the stress field by analyzing of mesoscale faults within MTD. Thus, it remains un-
clear whether horizontal compressive stress during flow cessation is common. Furthermore, the
existing numerical experimental result was based on the one-dimensional model; thus the flow
area ’s lateral expansion effects were ignored. This study aimed to address these problems by
1. Conducting numerical experiments using a two-dimensional plain model of MTP to investi-
gate whether horizontal compression at flow cessation is a broadly observed phenomenon under
various conditions. 2. Investigating MTD in the Pleistocene Kiwada Formation distributed in
the Boso Peninsula to verify the hypothesis of horizontal compression during MTP deposition.
The numerical experiments revealed that the driving distance of the MTP front decreased as
viscosity and yield stress increased, resulting in a collision between the leading edge and the
main flow body. When the viscosity coefficient was set at 20,000 Pa s and yield stress exceeded
3,000 Pa, horizontal compression was observed during cessation. In addition, principal axes of
compressive stress were radially distributed in the central part of the MTD, while they aligned
parallel to the outer edges in peripheral regions. The chaotic bed containing the sedimentary
blocks in the Kiwada Formation, the Kazusa Group, was investigated as a field example of MTD.
Multiple inverse analysis was conducted on mesoscale faults in sedimentary blocks within MTD.
The results revealed that the MTD experienced horizontal compressive stress during deposi-
tion. Additionally, the direction of the maximum compressive stress axis varied across regions.
This diversity in horizontal compression directions may correspond to the variability in com-
pression axis orientations observed in numerical experiments. In conclusion, the results of this
study ’s numerical experiment and field investigation support the hypothesis that strong hori-
zontal compressive stresses generally occur during MTP cessation. The paleostress analysis of
MTD conducted in this study represents an effective method for reconstructing MTP behavior,
potentially constraining flow rheology and direction.
self-weight, such as submarine landslides, and deposits formed by the process are called mass-
transport deposits (MTD). Understanding the behavior of MTP is important for estimating
the risk of damaging offshore coastal infrastructure and for understanding the recurrence inter-
vals of massive submarine earthquakes. Previous studies have estimated that intense horizontal
compressive stresses parallel to the flow direction during the cessation of MTP. These infer-
ences are based on features such as the surface structure of modern MTD and deformation
structures in sedimentary blocks within MTD in the geological record. Additionally, numerical
experiments have suggested that highly viscosity MTPs experience rapid deceleration of their
leading edge upon reaching depositional basins from slopes. The collision of the leading edge
and the main body of the flow results in compression within the MTD. However, most studies
on compressive forces during MTP flow were qualitative, and only one study has quantitatively
estimated the stress field by analyzing of mesoscale faults within MTD. Thus, it remains un-
clear whether horizontal compressive stress during flow cessation is common. Furthermore, the
existing numerical experimental result was based on the one-dimensional model; thus the flow
area ’s lateral expansion effects were ignored. This study aimed to address these problems by
1. Conducting numerical experiments using a two-dimensional plain model of MTP to investi-
gate whether horizontal compression at flow cessation is a broadly observed phenomenon under
various conditions. 2. Investigating MTD in the Pleistocene Kiwada Formation distributed in
the Boso Peninsula to verify the hypothesis of horizontal compression during MTP deposition.
The numerical experiments revealed that the driving distance of the MTP front decreased as
viscosity and yield stress increased, resulting in a collision between the leading edge and the
main flow body. When the viscosity coefficient was set at 20,000 Pa s and yield stress exceeded
3,000 Pa, horizontal compression was observed during cessation. In addition, principal axes of
compressive stress were radially distributed in the central part of the MTD, while they aligned
parallel to the outer edges in peripheral regions. The chaotic bed containing the sedimentary
blocks in the Kiwada Formation, the Kazusa Group, was investigated as a field example of MTD.
Multiple inverse analysis was conducted on mesoscale faults in sedimentary blocks within MTD.
The results revealed that the MTD experienced horizontal compressive stress during deposi-
tion. Additionally, the direction of the maximum compressive stress axis varied across regions.
This diversity in horizontal compression directions may correspond to the variability in com-
pression axis orientations observed in numerical experiments. In conclusion, the results of this
study ’s numerical experiment and field investigation support the hypothesis that strong hori-
zontal compressive stresses generally occur during MTP cessation. The paleostress analysis of
MTD conducted in this study represents an effective method for reconstructing MTP behavior,
potentially constraining flow rheology and direction.