Geology Reference
In-Depth Information
main channel
Offset Channels
Due to 1 or 2 Events
A
Offset Channels
Due to 8 to 20 Events
B
Fig. 6.18
Channel offsets along the Kunlun Fault, Tibetan Plateau.
Offset stream channels on the Kunlun Fault in northern Tibet provide evidence for slip in one to more than a dozen
earthquakes. A. Sketch of channel offsets that record one or two earthquakes. Displacement in a single earthquake is
∼
4 m, as shown by the three left-hand channels, whereas the channel on the right records two earthquakes. B. IKONOS
image of channels with offsets of 30-70 m. If the average coseismic slip is 4 m (as in top panel), these offsets appear
to record up to 20 earthquakes. The mix of channel offsets (30, 50, 60, 65, 70 m) suggests that (i) this region is
characterized by punctuated channel-forming processes, and (ii) the most recent resetting occurred when the channels
with 30 m offsets were initially formed as straight channels. Modified after Fu
et al.
(2005).
but the past record of earthquakes can be
discerned through trenching.
Climatically induced unsteadiness in the
production of geomorphic markers may create
landscapes with different aged features that
show differential offset along a single fault. For
example, along the Kunlun Fault in the northern
margin of the Tibetan Plateau (Chen
et al.
, 2004;
Fu
et al.
, 2005), certain segments of the fault
displace channels that record only one or two
earthquakes, whereas other segments preserve
channel offsets that result from up to 20 faul-
ting events (Fig. 6.18). The offsets due to mul-
tiple earthquakes provide an opportunity to
determine long-term slip rates, but they require
reliable dating of offset features.
How is a catalog of displaced features
along a fault transformed into a useful estimate
of paleoseismicity? First, the magnitudes of
individual displacements can be plotted as a
function of distance along the fault (Fig. 6.19).
Such a plot should indicate whether consistent
patterns of offset exist along the fault, whether
the displacements fall into discrete groups,
and whether evidence exists for decreases in
displacement toward the terminus of the rup-
ture (Figs 4.9 and 4.29). On strike-slip faults,
displacements can be measured directly in the
field or from remotely sensed data. For example,
both lidar topographic data and Quickbird
imagery (each with
∼
1 m spatial resolution) have
been used to assess strike-slip displacements
(Frankel
et al.
, 2007; Zielke
et al.
, 2010; Klinger
et al.
, 2011). The power of these high-resolution
data is that offset features can be sequentially
restored such that multiple slip events can be
extracted from an image even if the ages of each
event remain unknown.
