Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 197

development and assume that this complex continues to become unstable over
time due to episodic high-intensity rainfall events.
Aerial photography and field surveys
Examinations of aerial photographs combined with field studies are also
useful ways to identify prehistoric landslides. These approaches have been used
in a number of studies in the Himalayas (Hewitt 2001), the Canary Islands (Car-
racedo et al . 1999)andNewZealand (Crozier et al ., 1995). Carracedo et al .(1999)
were able to identify giant landslides of the order of 200 km 3 by observing
great landslide scars and deposits. Typical features identifiable from aerial pho-
tos include scarps, fans of landslide debris towards the base of slopes, dammed
valleys forming lakes, asymmetrically thickened deposits and systems of pres-
sure ridges or raised rims, the latter of which can sometimes be misinterpreted
as lateral and terminal glacial moraines (Hewitt, 2001). Other features include
independent streams or lobes of landslide deposits that have moved around
obstacles, and up and down valley and into other valleys at tributary junctions.
Hewitt (2001)also notes that sometimes a direct connection can be made or
observed between boulder deposits on the valley floor and a detachment zone
high on adjacent valley walls.
Crozier et al .(1995)usedfieldsurveysandaerial photographs to identify
over 100 deep-seated prehistoric landslide events in the Taranaki region of New
Zealand's North Island. They identified three separate groups of synchronously
occurring landslides based upon the morphological characteristics of the land-
slide deposits and resulting landforms.
Table 7. 3 lists the distinguishing features within two of these three groups.
They include the degree of dissection of the deposit, soil development within
thedeposit and suppression of relief from volcanic air fall deposits. The older
group of landslides show more advanced degrees of fluvial dissection and soil
development (Fig. 7. 12 ). The older group also have a more subdued relief because
they have been exposed to, and mantled by, volcanic ash fall deposits for a longer
period of time. The boundaries of the older slides were more eroded and it was
rare to see them still damming lakes because streams had had sufficient time
to breach the landslide barrier unlike the younger slides. Radiocarbon dates of
organic fragments within the slides confirmed these observations showing that
theyounger slides occurred approximately 1200--1400 years BP whereas the older
slides occurred around 30 000 years BP.
An interesting aspect of the Crozier et al .(1995)study was that they noted
that no comparable landslides, i.e. multiple deep-seated landslides, had occurred
during the 150 years of European settlement. They determined, using a variety of