Geology Reference
In-Depth Information
exposed scarp. If the faulted strata are well
consolidated, they may maintain steep faces for
many centuries. Nonetheless, it is commonly
assumed that raveling of a steepened slope
occurs rather quickly (about 100 years in many
sediments), such that, not long after faulting, the
surface of a colluvial wedge is at or below the
angle of repose for unconsolidated debris (
∼
30
°
)
and the deposition rate is low enough for
pedogenic processes to begin to develop soil
zonations (A, B, K horizons). Recognition of such
soil horizons in trench walls pinpoints the upper
surface of colluvial wedges formed during
earlier earthquakes and delineates some of the
vertical extent of the wedges (Fig. 6.6A).
Particularly in colluvium that is poorly stratified
and contains few stratigraphic horizons with
which to judge offsets, even subtle soil horizons
provide markers for delineating displacements.
In addition, the degree of soil development can
be related to the interval between earthquakes,
that is, the time from first stabilization of the
colluvial wedge to its burial by the next youngest
wedge (Fig. 6.6B).
Many fault zones actually have more than one
fault strand at the surface. In order to account
for all of the displacement along a fault zone, all
of the strands or surface traces should be identi-
fied, and a paleoseismic record for each should
be developed. It is not uncommon for strike-slip
faults to splay into several branches as they
approach the surface. Sometimes these splays
all occur within a fairly compact zone, for which
a single trench or small set of trenches will
reveal the entire displacement record (Fig. 6.7A).
However, when strands are separated by tens or
hundreds of meters, each has to be examined
separately. In compact zones with multiple
splays (Fig. 6.7B), mapping the displacement of
a distinctive marker bed across each of the fault
strands can set useful limits on the total slip
(Lindvall
et al.
, 1989).
Dating of fault-disrupted strata in trenches
forms the basis for determining the timing of
individual ruptures, which in turn is the basis
for assessment of recurrence intervals and of
slip rates, if displacement is known. A goal in
many trenches is to generate as many reliable
dates as possible for a given stratigraphy,
SW
NE
Fault-Zone Cross Section
C
2
C
1
A
C
4
C
3
B
C
1 +
C
2
D
1
base of
trench
C
3
soil
horizons
C
4
0
0.5
1.0
meters
D
1
A
uns
ee
n
channel?
0123
meters
Excavation & Marker Bed
horizon containing channel
cut out by fill NW of here
excavation geometry
channel offset by
multiple faults
sandstone channel
view of
Fig.6.7A
B
minimum offset = 8.7 m
Fig. 6.7
Measuring slip on a fault with multiple
splays.
A. Sketch of a trench wall oriented approximately
perpendicular to a suite of splays along a strike-slip
fault. This outward branching geometry or “flower”
structure comprises 10 different splays across a
zone
∼
3 m wide. Not all splays have been active in
each earthquake. Note, for example, that two faults
on the right cut the upper B
t
soil horizon, whereas
the other splays do not. Modified after Lindvall
et al.
(1989). B. Map view of the displacement of a marker
horizon (a sandstone channel) across multiple splays
of a strike-slip fault. A network of shallow trenches
reveals the net offset of the marker channel. Across
any strand, the offset is typically
<
2 m, but the
cumulative offset is
>
8 m. Modified after Lindvall
et al.
(1989).
particularly if these dates will underpin the
reconstruction of the seismic record. Radiocarbon
dating of buried organics remains the most
commonly used dating technique in trenches.
Given analytical uncertainties in laboratory
analyses, the possibility of contamination
with young carbon, and the background
variations in atmospheric carbon in the past (see
Chapter 3), each radiocarbon date should be
carefully interpreted. Combining different dating
