Paleoseismology

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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
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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

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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
• 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

Salt Creek Trench
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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

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The trench support structure and some sediment
packages
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The edge of a channel and corresponding channel
fill
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Offset bedding characteristic of fault
deformation
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Desiccation cracks in cross section, indicative
of a dry lake bed
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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.

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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

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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.

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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.

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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 --gt 14C 1p
• Once the organism dies this exchange stops and
  the 14C decays
• 14C --gt14N b
• Its half life is 5730 yrs, and present
  instrumentation can give ages back to between
  58-62 kyrs

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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)

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Cosmogenic Radionuclide Dating

• CRNs 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
• N of CRNs per unit volume rock
• dN/dt P - lN 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

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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
• gt 2 m age due entirely from inheritance