Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 198

Table 7.3 Criteria for differentiating different age landslides in Taranaki region, New
Zealand (after Crozier et al. , 1995 )

Group A (older) Group C (younger)

Surface texture Smooth Rough

Boundary definition Eroded; secondary slips Distinct

Surface drainage integration Well established None

Fluvial dissection of mass Up to 60 m None

Valley dammed lakes All drained Many

Crown scarp lakes Rare Common

Dammed tributaries None Common

Soil Volcanic ash mantle >15 cm

A horizon

Residual soils with <15 cm
A horizon

14 C dates (years BP)

31 500 ± 850 1380 50

1260 ± 50
1250

±

50
1470 ± 250

±

Group B landslides have characteristics intermediate between groups A and C.

techniques and approaches (synchronicity of events, comparisons with modern
analogues, spatial distribution and limiting equilibrium analysis), that the pre-
historic landslides had all been caused by seismic events as opposed to episodes
of heavy rainfall. Use of the limiting equilibrium analysis technique was critical
to this assessment of the cause. The technique determines whether the slope
geometry and the static shear strength conditions were susceptible to extreme
hydrological conditions prior to the landslide event. If a factor of safety of 1.0 or
less is found, the triggering is assumed to be hydrological. An analysis provid-
ing a factor of safety greater than 1.0 indicates that an added driving moment
from seismic activity may have been necessary to trigger the movement. The
technique is described by the following formula,

sum of resisting forces
sum of driving forces =

C + ( w ∗ cos β − u )tanø

w ∗ sin β

(7.3)

where: C = the cohesion of the rock, w = the weight on the shear surface,
= the angle of the shear surface, u = thepore water pressure on the shear
surface and ø = the angle of shearing resistance of the rock.
In all cases Crozier et al .(1995)found a factor of safety greater than 1.0
indicating that seismic activity triggered the landslides. They suggested that
thetriggering earthquakes must have had a minimum moment magnitude of
6.8--7.5.