Geology Reference
In-Depth Information
Box 7.3
Landslides in landscapes.
Whereas the role of landslides as major agents
of erosion is unequivocal in many rapidly
eroding mountain ranges, the controls on
where landslides occur within the landscape
is less well known. Given the critical role
played by basal shear stress in triggering land-
slides, it is not surprising that the preponder-
ance of slides occur on the steepest hillslopes.
The combination of digital topography with
extensive GIS databases of landslides under-
pins new insights on the location of major
landslides within a landscape and on differ-
ences in their hillslope positions as a function
of (i) whether the slides were triggered by earth-
quakes or by storms (Densmore and Hovius,
2000) and (ii) the “facing direction” of the
topography with respect to the position of the
epicenter (Meunier
et al.
, 2008). Given a GIS
database of landslides, flow lines can be
calculated that pass through each landslide
and represent the theoretical path of water flow-
ing across the landscape from the ridge crest
above the slide to the river channel below it. The
position of each slide can then be expressed as a
function of its proximity to the crest and channel.
Compilations of data from several active
ranges reveal intriguing contrasts in the posi-
tions of landslides on hillslopes (see figure A).
For example, landslides caused by the 1994
M
w
=
6.7 Northridge, California, earthquake
cluster along the ridge crests, a result consist-
ent with the amplification of seismic shaking
along ridges. For the Finisterre Range, Papua
New Guinea, the 1922
M
w
=
6.7 and the 1994
M
w
=
6.9 earthquakes on the Ramu-Markham
Fault prompted landslides clustered near both
the crests and the channels. In contrast, for
the rapidly eroding Southern Alps, New
far from channel
1
A
ridge-crest
slides
0.1 km
2
ridge-top
medium slide
0.01 km
2
Northridge
Earthquake
slope-
clearing
large
slide
valley-
bottom
small
slide
slides cover
increasing %
of slope
in the channel
0
1
Distance from ridge
1
1
ridge-crest
slides
1 km
2
1 km
2
0.1 km
2
0.1 km
2
Finisterre
Earthquake
New Zealand
Rainstorms
valley-
bottom
slides
0
0
0
1
0
1
Distance from ridge
Distance from ridge
A. Distributions of landslides as a function of distance from a ridge or channel. (Upper left) Interpretational key
for other panels. Note that slides spanning an increasing fraction of a hillslope plot closer to the origin. (Upper
right) Northridge earthquake landslides occur mostly near ridge crests. (Lower left) Finisterre landslides occur
both near crests and near channels. (Lower right) Landslides in the Southern Alps are uniformly distributed.
