Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 156

In most cases muddy sediment deformed structures are composed of sandy
silts to silty clays and typically form in lacustrine sediments and settings. In a
study of these features on the Kawachi lowland plain, Osaka, Japan, Matsuda
(2000)foundahomogenised layer in the uppermost part of the deformation
zone, a plumose pattern in the middle to lower part of the zone, load struc-
tures below the plumose pattern, and downward fissures and microfaults in the
lowest part of the zone. It is likely that these deformation features form when,
during an earthquake, the sediment--water mix beneath a body of water (such
as occurs in lakes) behaves rheologically from top to bottom. So the uppermost
zone behaves as a liquid (liquidised), the zone below as a plastic (hydroplastic)
and the lowermost zone as a brittle solid.
Matsuda (2000)found a vague mixing pattern in the sediments of the upper-
most part of the deformation zone, especially towards the base of this zone.
Sediments up to pebble size within the otherwise fine-grained sediments were
probably derived via eddies from the underlying hydroplastic zone. The lower
boundary of this zone was observed to take on a waveform in localised areas
with a wavelength of 10--40 cm and an amplitude of 5--20 cm. There are two pos-
sibilities behind the formation of these waveforms. Either the wave movement of
more cohesive units below forms them, or they are a result of the gravitational
pressure of this unit and the shear force at the lower boundary layer. If the
lower boundary is difficult to distinguish, it is assumed to have been caused by
agentle velocity gradient in the deformation flow near the boundary between
the zones.
The plumose pattern found in the middle to lower section of the deformation
zone (the hydroplastic deformation structure) forms when the sediments from
the upper and lower zones mix (Fig. 6.3). Such features are recognisable because
they display a different colour and texture from surrounding sediments. The
plumose pattern consists of a series of slightly downward-convex flow lines and
flow lines that trend upwards with frequent directional changes. These flow
lines form where the sediment has been deformed in upward and downward
directions. Sediments in this zone experience repeated scraping or dragging over
the zone of lower sediments resulting in the latter being transported upwards.
The distance between flow lines is usually less than 20 cm and the upper flow
lines are slightly divergent or dispersed. Sometimes, in localised areas, the flow
lines spiral upward in a counter-clockwise direction. This plumose pattern is,
in most cases, easily distinguished and is assumed to form by instability at
the boundary between the upper and lower flowing sediment layers. The mass
transport responsible for creating the flow lines is assumed to occur by internal
motion within an irregular standing wave. The poorly developed eddies and
mixing of the sediments around the flow lines suggests that the mixing was