Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 155

While some authors have noted that the size and distribution of sand blows
can be used as a guide to the size and distribution of prehistoric earthquakes,
recent earthquakes have shown that this may not always be the case. The 1989
Loma Prieta, California and 1994 Northridge earthquakes resulted in an irregu-
lar distribution of liquefaction features that were not centred around the earth-
quake epicentre (Tuttle and Schweig, 1996). The irregular distribution of sand
blows possibly occurred due to several factors such as the characteristics of the
earthquakes themselves, the directivity, site conditions and Moho (boundary
between Earth's crust and mantle) reflections of seismic waves. Since these fac-
tors are often unknown for prehistoric liquefaction features, it should be noted
that there are large uncertainties associated with using the distribution of sand
blows as a guide to the epicentre of a prehistoric earthquake. Nonetheless, where
all other caveats have been checked, sand blows of some antiquity, either dated
through an absolute or relative dating technique, can provide important infor-
mation on the frequency of past earthquakes in a region.

Seismic deformation of muddy sediments

Fine-grained or muddy sediments that deform due to a range of factors
are referred to as hydroplastic sediments. Hydroplasticity refers to the signifi-
cant yield strength of a sediment--water mixture that is subject to cohesive or
frictional forces. Although deformation structures of sandy sediments are well
known and are characterised by liquefaction and water escape, the criteria used
to determine deformation in muddy sediments are relatively poorly known and
ambiguous. This is because hydroplastic sediments can be easily deformed by
anumber of factors including water flow or wave action on the sediment, the
weight of overlying sediment, bioturbation and non-seismic slope failure. How-
ever, if these factors can be ruled out it is possible that the deformation occurred
due to seismic activity. Matsuda (2000)notes that this is especially the case when
thefollowing occur:

(1) the siteisnearaseismic zone;

(2) there is a large lateral extent and synchronicity of deformation zones;

(3) liquefied and fluidised sediments are present;

(4) there exists a restricted deformation zone within a small vertical range

of strata;

(5) limited horizontal displacement occurs;

(6) there are features that indicate in situ initiation of deformation and a

cyclic deformation process;

(7) there are rare directional features in a structure; and

(8) there is a similarity to structures formed experimentally.