Paleoseismology
Methods:
Trenching
Displaced Geomorphic Features
Historical Records
Radiocarbon Dating
Cosmogenic Radionuclide Dating
Reference:
Burbank, D.W., and Anderson R.S., Tectonic Geomorphology, 2001, Blackwell Science
Trenching
• Practical Objectives:
– Identify and date layers within a stratigraphic
succession that contain information about the
faulting history
– Document the amount of displacement from
faulting activity
Trenching
Trenches should contain:
• abundant datable material
• provide structural and
stratigraphic markers
• preferentially thinly bedded
deposits - better at illustrating
discrete measurable offset
- relic shorelines
- small scale channels
Trench orientation/scale:
Salt Creek Trench
• 1 perpendicular to fault trace
• 2 parallel to fault trace,
located on either side of trace
• depth of the trench should be
appropriate for scale of fault
• length of the trench should be
long enough to cover the
deformation zone
Trenching
Once the trench has been excavated:
• stratigraphic horizons are meticulously mapped
• material for dating various horizons is removed
 Fault displacement history constructed
Stratigraphic and structural relationships:
a) increasing offset with depth, growth on the
fault
b) incomplete erosion can give the appearance
deformation from topography of underlying
surface
c) erosion of the upthrown block can create
colluvial wedge
d) fissures opening along fault trace fill with
colluvial material
e) injection dikes in subsurface, sand volcanoes
provide evidence of past earthquakes
f) Liquefaction can cause folding of surface
sediments - lower limit on age of earthquake
The trench support structure and some sediment packages
The edge of a channel and corresponding channel fill
Offset bedding:
characteristic of fault
deformation
Desiccation cracks in cross section, indicative of a dry lake bed
Displaced Geomorphic Features
Geomorphic features that can be offset:
• rivers, streams, channels, terraces
• debris flows & raised levees
• alluvial fans
• ridges & gullies
• beach ridges, coral platforms, delta plains, wave cut notches
• Anthropogenic features:
• roads, orchards, fences, telephone poles, drainage channels, etc
 Anything that has an easily identifiable shape/outline that can be offset
A key feature to identify is the piercing point - unique rock types that formerly extended
across the fault and can be used to determine displacement.
Displaced Geomorphic Features
Landforms can be altered over time through erosional processes and may not directly
intersect the fault plane, but detailed topographic and geologic mapping can reveal these
relationships
 horizontal offset:
once fault plane is specified, linear features are projected onto fault plane and
the offset measured
 vertical offset:
subhorizontal features (e.g. channel bottoms) are projected onto the fault plane
and the offset is measured
Displaced Geomorphic Features
Offset features can illustrate both the processes that initially
displaced them and also processes that can modify them
Fluvial incision/erosion creates:
• channel walls, terraces, gullies
Aggradational/depositional phases leave:
• broad wide alluviated surfaces with few distinctive features
• can bury previously existing features obscuring previously recorded
seismic events.
Earthquakes occurring during incision events are better preserved
in the geomorphic record.
Historical Records
• Records from towns/cities near fault zones
locally Missions have good records.
• Travelers/settlers journals, observations they made of the
landscape and perhaps events.
• Less exact but still useful are myths and legends of local
cultures.
Radiocarbon Dating
The most commonly used dating method to date geomorphic
features
14C
is formed in the atmosphere through the interaction of cosmic
radiation and nitrogen, and every living thing exchanges 12C and 14C
throughout their life.
1n + 14N --> 14C + 1p
Once the organism dies this exchange stops and the 14C decays
14C -->14N + b
It’s half life is 5730 yrs, and present instrumentation can give ages back to
between 58-62 kyrs
Cosmogenic Radionuclide Dating
In the last few decades we have been able to date the exposure
time of surfaces through the exposure to cosmic radiation.
Characteristics of Cosmic radiation:
• charged particles are directed into Earths atmosphere by the magnetic field
• stronger beam of particles at higher latitudes
• atmospheric attenuation reduces the atmospheric production of radionuclides with
a 1/e length scale of roughly 1.5 km within the lower atmosphere
• cosmic radiation impacting the surface produce cosmogenic radionuclides (CRN),
decaying with a 1/e scale of 60-70 cm
Corrections need to be made for latitude and longitude, because of differential exposure rates
Commonly used CRN and their production rates
(atoms/gram of quartz/year at sea level):
14C
- 21.0, (1/2 life 5730 yrs)
10Be - 5.81, (1/2 life 1.5 million yrs)
26Al - 34.9, (1/2 life 720 kyrs)
36Cl - 4 - 9, (1/2 life 308 kyrs)
Cosmogenic Radionuclide Dating
CRN’s have been used in two distinct settings:
— Bare Bedrock
— Depositional Surfaces
to identify either exposure rate and/or erosion rate of that surface.
The concentration in a rock parcel is determined by:
dN/dt = P - lN
N = # of CRN’s per unit volume rock
t = time
P = production time
l = decay constant
The complexity of this method lies in the history of the production rate. Depositional
surfaces have a significant problem in that they likely consist of material that has an
“inheritance”, i.e. prior exposure
e.g. Fluvial terrace inheritance derived from:
• exhumation through the CRN production boundary layer as hill slope is lowered
• transport within the hill slope or fluvial system
• final deposition and exposure
Cosmogenic Radionuclide Dating
This is the 10Be record from
Lake Bonneville
Samples taken from a sand bar that is associated with
the last lake highstand at 14.5 ka.
The grey area is the inherited age from prior exposure
of the quartz grains.
If the age line had not been shifted to account for
inheritance the age would have been calculated at
~26 ka, 11 ka too old.
One way to limit the effect of inheritance:
• collect samples from a range of depths
• > 2 m age due entirely from inheritance