Geology Reference
In-Depth Information
Normal Fault
D α L 1.4
Thrust Fault
Strike-Slip Fault
D α L 0.8
Maximum
Displacement
versus Length
Fig. 4.11 Length-displacement ratios on faults.
Compilation of maximum displacement as a function of fault length for lengths spanning eight orders of magnitude.
Maximum displacement ranges from 0.3% to 30% of fault length. Regressions through these data indicate that, for
faults longer than 100 m, maximum displacements scale at approximately the 1.4 power of fault length.
Displacements on smaller faults may scale at approximately the 0.8 power of fault length. The dashed lines are
lines of equal strain ( D / L ). Note that the displacement in earthquake ruptures is 2-4 orders of magnitude less than
the accumulated slip on faults of the same length. Modified from Scholz (1990), Schlische et al. (1996), and Davis
et al. (2005).
of a single fault? How can we distinguish in the
modern record between different modes of fault
growth? If faults do not link up, how is regional
deformation accommodated among multiple
faults?
Simple models (Cartwright et al. , 1995; Walsh
et  al. , 2002) suggest several ways in which
elongate faults may develop. During fault
growth by radial propagation, an individual
fault simply lengthens through time and
accumulates more displacement, following the
idealized bow-shaped displacement gradient
(Fig. 4.12A). Displacement increases steadily as
the fault grows. Alternatively, fault segments
that ultimately link up may begin as smaller,
individual faults. But, at the moment their tips
link up, the total length of the fault suddenly
increases, whereas the total displacement does
not substantially change (Fig. 4.12B). This
linkage causes a departure from any previously
established displacement-length relationship.
Over time, the composite fault may smooth out
the slip deficit near the former segment bound-
aries, such that only small departures from
the  expected displacement may mark these
boundaries. Eventually, the displacement-length
geometry for a fault resulting from segment
linkage may be indistinguishable from that of a
similarly long fault that grew from a single rup-
ture. In such situations, patterns of subsidence
recorded by syntectonic sediments are probably
the most reliable discriminator among different
modes of fault growth (Anders and Schlische,
1994; Cowie et  al. , 2000). In a third mode of
fault growth (Fig. 4.12C), the fault lengthens
rapidly early in its history, but then simply accu-
mulates displacement with little to no fault
lengthening (Walsh et al. , 2002). This last model
supports the concept of strong, persistent barri-
ers to fault propagation (Fig. 4.10C and E).
 
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