Geology Reference
In-Depth Information
includes features such as faulted beds, offset
stream channels, or uplifted shorelines (Plate
1B). Indirect indicators of earthquakes require a
conceptual linkage between the observable data
and the earthquake that caused it. In some
instances, the evidence reflects the coseismic
offset itself, whereas in others it reflects other
parts of the seismic cycle. For example, drowned
forests, stratigraphic evidence of tsunamis, or the
chronology of rockfall deposits require an
interpretation of their genesis in order to tie
them into specific faulting events. We discuss the
types of information that can be generated with
direct observations first, and later we examine
several kinds of indirect paleoseismological
observations.
Overall, the basic practical objectives in trenches
are: (i) to identify and date layers within a
stratigraphic succession that either have been
disrupted by faulting or overlie fault traces
without disruption; and (ii) to document the
amount of displacement in past faulting events.
Sites for trenches have to be carefully chosen in
order to maximize the useful information that
they may generate. To the extent possible, trench
sites should contain abundant datable material,
and they should provide stratigraphic or
structural markers that can be used to measure
offsets. Typically, it is impossible to know what is
likely to be found in the subsurface as a trench is
excavated, so good judgment and good luck
combine to create a data-rich excavation. Because
radiocarbon dating is still the most frequently
applied dating technique in trench analyses,
trenches are commonly sited in swampy areas
where fault displacements have dammed local
streams or have ponded the groundwater table.
In such circumstances, datable organic matter is
more likely to be preserved within the young
strata associated with the fault. Similarly, thinly
bedded and/or channelized deposits are more
likely to reveal discrete, measurable offsets than
will massive deposits, such as debris flows, which
are rather homogeneous in all directions. Thus,
the likelihood of having a rich stratigraphic
record can be enhanced by choosing sites of
low-energy deposition and/or sites where linear
stratigraphic markers with which to measure
offsets are likely to be preserved. For example, if
relict lake shorelines or small-scale channels are
oriented approximately perpendicular to a fault,
they can offer particularly good stratigraphic
markers or piercing points across a fault.
In trenches, the analysis of the stratigraphic
record of earthquakes is typically a time-
consuming process. Commonly a week or more
is spent analyzing a single trench. Thus, the
choice of sites for trenches is not a trivial
exercise: you want the maximum information
for the time invested. Trade-offs have to be made
between competing objectives: obtaining more
detailed information on a single segment of a
fault, comparing rupture histories in different
areas, obtaining maximum information on the
magnitude of offsets, and developing the most
Surface rupturing versus buried faults
Faults that rupture the Earth's terrestrial surface
are relatively easy to discern if they cut well-
preserved geomorphic markers, are not signifi-
cantly eroded, and are not masked by too much
vegetation. Such faults provide ready targets for
paleoseismological studies. On the other hand,
buried faults tend to cause less obvious and
commonly more diffuse deformation of the
ground surface. Many thrust faults, for example,
have both upward splays that rupture the
ground surface, as well as buried ramps and
faults that accommodate slip beyond the splay.
Slip on buried ramps will tend to drive regional
rock uplift that can be confidently identified
only at its margins (where differential uplift
occurs) or in the presence of robust markers
with known positions with respect to sea level
(Fig. 6.2 and Plate 2). Where such markers occur,
they provide an opportunity to define a slip his-
tory for buried faults that lie far beneath the sur-
face. More commonly, however, the presence of
buried faults and the seismic hazard they repre-
sent is poorly known until these faults rupture
in a unpredicted earthquake.
Trenching
In order to provide a detailed record of past
displacements and their timing, excavated
trenches across faults often provide key insights.
Search WWH ::




Custom Search