Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 158

randomly distributed, but occur as circular patterns interconnected by the linear
flow pattern. The shape of intrusions into the lower layer varies from circular to
elongate arcs, and from round to angular. Most of the load structures also show
arotational flow pattern that is distributed with a preferred local orientation.
This can be explained if the sediments of this unit migrated about 10 cm in
one particular direction during the event. In some cases the hydroplastic load
structures have been found in the next lower horizon that usually contains the
brittle deformation structures. This suggests that the rheological response of the
same sediment has changed during the seismic event.
Downward fissures and microfaults define the brittle deformation unit
(Fig. 6.3). A downward fissure is a fracture that is developed vertically or obliquely
and filled with the overlying sediment. The fractures frequently cross each other
or bifurcate. They normally occur directly below the load structures. If the micro-
faults are densely distributed (several per 10 cm), they tend to show horizontal
orientation, but adjacent microfaults tend to differ in orientation. Microfaults
are assumed to form by a differential distribution of both vertical and horizontal
forces.
There still remain uncertainties in using deformation features in muddy sed-
iments as evidence of prehistoric earthquakes. This is principally because too
fewstudies have been undertaken on this phenomenon. Their distribution glob-
ally, with respect to the major earthquake zones, along with insufficient knowl-
edge of other processes that could possibly cause such features remains to be
investigated. Their preservation in the recent geological record is also not well
understood. It is also likely that they may not occur in certain environments
and conditions. For example earthquakes that have occurred during the winter
months in permafrost areas of Canada have apparently not left muddy deforma-
tion features. In such situations any palaeoseismic reconstructions in these areas
would underestimate the frequency of such events. Past environmental and cli-
matic changes in a region would also need to be considered when undertaking
these studies.
Landform development (raised shorelines)
Coasts bordering subduction zones may rise or fall instantaneously dur-
ing an earthquake (coseismic uplift or subsidence) and/or more slowly during
interseismic periods (aseismic uplift or subsidence). Earthquakes with magni-
tudes above 8 that occur at the interface between subducting and overriding
plates generally cause uplift of the region closest to the subduction trench.
This process may also result in subsidence of the Earth's crust in a zone that
is parallel to the uplifted zone. Vertical earth movements shortly before or