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

Salt Creek Trench

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

TrenchingOnce 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

faultb) 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 materiale) injection dikes in subsurface, sand volcanoes

provide evidence of past earthquakesf) 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 extendedacross 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 planeand the offset is measured

Displaced Geomorphic FeaturesOffset 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.

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 + 1pOnce the organism dies this exchange stops and the 14C decays

14C -->14N + It’s half life is 5730 yrs, and present instrumentation can give ages back to

between 58-62 kyrs

Radiocarbon Dating

Cosmogenic Radionuclide DatingIn 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:N = # of CRN’s per unit volume rock

dN/dt = P - N t = timeP = production timedecay 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 DatingThis 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