Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 157

PP

LD

LD

DZ

FS

HL

MF

PP DZ

LD

HL

FS

PP
LD

DZ

HL
PP
LD

FS
MF
DZ

Downward fissure
Microfault
Deformation zone

Homogenised layer
Plumose pattern
Load structure

Liquidised deformation unit
Hydroplastic deformation unit
Brittle deformation unit

Figure 6.3. Schematic diagram of deformation structures in superposed
deformation zones in muddy sediments (from Matsuda, 2000 ).

affected by the strong cohesive nature of the sediments; in this sense the zone
containing plumose patterns can be regarded as a hydroplastic deformation
unit. The morphological attributes of this zone are consistent with an intense
deformation force that overcomes the shear strength of the cohesive sediments
suggestive of deformation from seismic events.
Load structures may also be found at the lower reaches of the hydroplastic
deformation layer (Fig. 6.3). Load structures are shallow downward-convex defor-
mation features that penetrate into a lower stratigraphic unit or layer. They
tend to occur at the base of the plumose patterns where the flow of the upper
sediments is dragged over the lower sediments along a bedding plane. Their
formation is assumed to be a result of an uneven downward pressure on bed-
ding planes due to the oscillatory movement of the upper sediment during the
earthquake event. These intrusions into the underlying sediment layer are not