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Fault Displacement Markers (1. Coseismic Displacement (Method (Elevation…
Fault Displacement Markers
1. Coseismic Displacement
Timescale: few seconds i.e. info about fault displacement over a few seconds
Data: Interferograms
Interferograms represent displacement
Method
Elevation measured with satellite data
Difference btwn 2 images yields displacement
i.e. elevation change from before to after earthquake
Interferometry
Technique:
Superposition of waves causes interference
Information is extracted from the resulting interference pattern
Seismic interferometry
Utilises cross correlation of signal pairs
To reconstruct impulse response of a given media
Intensity
Pattern
= f ( phases difference between 2 waves which combine )
If waves are:
In Phase - undergo constructive interference
Out of phase - undergo destructive interference
Not in/out of phase - intermediate intensity pattern
Used to determine relative phase difference
L'Aquila Interferograms
Obserrved data
= interferograms measured by
differencing radar imagery from before & after
Modelled data
To recreate observed displacements
Seeks to understand seismicity / dynamics of fault
L'Aquila Earthquake 2009
SW dipping normal faults
Mw 6.3, slip ~0.6-0.8 m
Confirmed still cannot predict earthquakes
Scientists accused of being too optimistic / reassuring
Many daeths as people did not evacuate
2. Interseismic Displacement
Timescale: years to decades
Data: GPS-derived horizontal velocities i.e. interseismic displacement
Method
Detect displacement @ GPS stations over long period
Observe relative displacement of different pts across area of interest as f (time)
Appenines
Horiz Velocities
Moving South displacement increases & rotates to N
Reflects rotation of subduction zone @ rates of 5 mm/yr
2. East Appenines moving faster than West
Velocities higher on E Appenines
Evinces extension - Appenines are being stretched
3. Postglacial Fault Scarps
Timescale: kyr to tens kyr
Fault scarp = sharp contrast btwn bedrock & debris on hillside
e.g. Magnolia fault scarp, Central Appenines
Uplifted block (hanging wall) sits above
Pleistocene sediment (deposited colluvium)
Fault scarp formation
Glacial Max
Lotsa sediment produced
∴
Fault scarps not preserved
--> since degradation so rapid
V efficient hillslope processes
Postglacial
Less efficient hillslope processes
∴ Fault displacement = preserved & exposed
Scarp visible represents uplift since 18 kyr (LGM)
Measuring
fault scarps
Cosmogenic nuclide dating
of fault scarp exposure
confirm age ?
Extent of fault scarps
amount of displacement over 18 ka
offset ~ size of scarp
Findings
Offset msmts on fault scarps across Appenines
Agree with fault linkage theory
Displacement greatest in middle of faults
fault segments in array centre displaced > tips