Geology Reference
In-Depth Information
Zealand, storm-driven landslides were spread
rather uniformly across the landscape.
Why does this variability in landslide loca-
tion occur? Clustering of landslides near ridge
crests during earthquakes is expected because
of amplification of seismic shaking, which
may exceed 10-fold along ridge crests (Buech
et al.
, 2010). At Northridge, for example, mean
slope angles are quite uniform throughout the
range (see figure B), but landslides are pre-
dominant near the ridge crests. Whereas this
same phenomenon is seen in the Finisterre
Range, secondary landslide clusters occur
fairly low on the hillslopes. Topographic data
(see figure B) show that inner gorges and the
steeper slopes that are associated with such
gorges are prevalent in the Finisterre Range.
Modeling of seismic waves interacting with
topography suggests that the kinks in slopes
associated with the crest of an inner gorge also
serve to amplify shaking (Meunier
et al.
, 2008).
These same models predict that slopes facing
away from an epicenter should experience
higher accelerations. Hence, local topographic
site effects, such as the orientation of slopes,
gain in importance. In contrast, storm-driven
landslides either may be uniformly distributed
along slopes (as in the Southern Alps; see
figure A) or may tend to predominate closer to
channels. We could predict that higher pore
pressures, as well as local steepening of
hillslopes by river incision, would promote
landsliding proximal to channels (Gabet
et al.
,
2004b). Continued analysis of large spatial
data sets, particularly following large storms
and earthquakes, promises to shed more
insight on to when, where, and why landslides
occur within active landscapes.
mean slope
for landslides
landslide
cluster
B
50
40
30
33°
20
10
0
mean slope
Northridge
50
40
30
20
10
0
0
38°
landslide
cluster
Finisterre
1
Distance from Stream
seismic
shaking
750
550
350
150
inner
gorge
FIN
NR
0
1
2
3
Distance (km)
B. Slope distributions for Northridge (top)
and Finisterre (middle), with major landslide
clusters indicated. Mean slope for observed
landslides lies above the average for the entire
topography but, within its 1
σ
distribution
(containing 65% of all slides), it includes the
steeper hillslopes. (Bottom) Simplified topography
for Northridge (NR) and Finisterre (FIN), with
the latter showing an inner gorge and amplified
seismic shaking.
landslide size and frequency (Hovius
et al.
, 1997).
This approach has the advantage of recording a
direct time dependence. On the western slopes
of the Southern Alps of New Zealand, repeat
aerial photographs spanning 60 years have been
used to define the distribution and aerial extent
of more than 7000 landslides within an area of
about 5000 km
2
. These landslides range in size
from about 100 m
2
to 1 km
2
and define a power-
law magnitude-frequency distribution over two
orders of landslide size (Fig. 7.15). Stated in
cumulative form, the number of slides of
magnitude equal to or greater than area
A
c
, i.e.,
n
c
(
A
≥
A
c
), is
n
c
(
A
≥
A
c
)
=
k
(
A
c
/
A
r
)
−
b
A
r
(7.4)
where
A
r
is a reference area,
k
is the intercept
of the regression when
A
r
=
0, and
b
is the
