Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 193

evidence along with probable tsunami deposits and evidence for coastal subsi-
dence all gave similar radiocarbon ages and led Jacoby et al .(1992)toconclude
that the landslides in this area were triggered simultaneously by an earthquake.
Although saturation from heavy rainfall and subsequent slope failure also had
to be considered as a triggering mechanism, this was deemed highly unlikely as
there are no historical records of such events occurring even during episodes of
extreme precipitation.
Dendrochronology does not appear to be a reliable technique for determining
the age of mass movement events where the failure surface of that movement
is approximately parallel to the natural slope of the area. This is the case with
planar slides where the mass movement occurs as a rigid block with no apparent
deformation. In these situations the trees are only slightly tilted and the degree
of tilting is not reliable evidence for the sliding process. Repeated landslide
activity in an area can also produce complex dendrochronological estimates. In
such cases, more thorough sampling and analysis methods are needed. These
can include specially adapted sampling techniques that take into account the
different stem bends, a systematic search for sudden changes in eccentricity or
in reaction wood sequences and sampling of trees of different generations (Lang
et al ., 1999).
Side-scan sonar
Side-scan sonar along with high-resolution seismic techniques have been
responsible for the identification of giant submarine landslides. These landslides
are instrumental in the evolution of many oceanic volcanic islands such as the
Hawaiian chain, the Marquesas Islands, La Reunion and the Canary Islands.
Krastel et al .(2001)mapped 12 relatively young (2 million year old) giant
landslides around the flanks of the Canary Islands (Fig. 7. 9 ). Recognition of
these landslides, their size and frequency are important for elucidating the risk
from correspondingly giant tsunamis, which can be generated when these slides
plunge into the ocean.
Interpretation of the pattern characteristics from side-scan radar images can
give information about the composition of the submerged deposits. For exam-
ple, speckled backscattering often depicts debris avalanches. Differences in the
amount of backscatter can also provide further information about the debris
avalanche as higher values of backscatter represent variations in the composi-
tion of the deposit. Side-scan sonar can also be used to determine the extent
of the submerged landslide and the size of individual blocks. At El Hierro in
theCanary Islands, side-scan sonar was used to study the morphology of the
island's submarine flanks (Krastel et al ., 2001). The speckled backscatter pattern