Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 195

1.50

1.50

1.25

1.25

1.00

1.00

0.75

0.75

0.50

0.50

0.25

0.25

0.00

0.00

East Canary
Ridge

Tenerife

El Hierro La Palma

Figure 7.10. Ages of giant landslides around the Canary Islands (from Krastel et al. ,
2001 ).

by thelandslides would very likely cause serious inundation on neighbouring
islands and may also travel thousands of kilometres across the Atlantic Ocean
and possibly affect the Americas.

Stratigraphy

Repeated landslide movements will often result in an accumulation of
landslide deposits towards the base of a slope. This pile is composed of individ-
ual landslide sediments which can be identified by their degree of weathering
of component materials, the presence of soils, organic matter and other mate-
rials at the top of individual event layers, and also their relative position in
thestratigraphy. Younger landslides normally overlie older ones so the sequence
usually gets older with depth. Stratigraphic sequences can be identified in expo-
sures or by drilling or boring cores from several sites within a study area (Elliott
and Worsley, 1999).
Chihara et al . 1994)usedstratigraphic techniques to determine the geo-
historical development of the Tochiyama landslide complex in north-central
Japan. They identified debris-avalanche and other landslide deposits. Three land-
slide layers are evident in the complex with the lower or older two composed
of debris-avalanche deposits and the uppermost sequence composed of talus.
Twodistinct tephra layers separating these deposits allowed a chronology of
these events to be developed. The oldest landslide layer was deposited between
46 500 and 25 000 years BP and the next upper layer deposited since AD 1361.
These landslides were caused by local undercutting by streams, initially during