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Alexandra Allen
Neotectonics: ERTH 610
Neotectonics Final Study Guide
1.Fault Displacement:
(Results in crustal shortening) Dip-slip is reverse slip (also for thrusts) >45
Component of fault displacement which is parallel to the dip of fault plane or down
dip and has 2 components: Heave & Throw
(Results in crustal extension) Normal slip >30 dip high angle normal fault, 30 dip
low angle normal fault.
Strike-slip right or left lateral.
Net slip is the total amount if displacement measured parallel to the direction of
motion. 2 components are strike slip and dip-slip (legs of triangle)
Strike-slip is a strike-slip fault’s displacement is parallel to the fault strike
Scarp Morphology:
To analyze scarps, we measure:
-Strike of fault, the general direction in which the fault scarp is striking
-Minimum vertical separation, the height of the scarp from the ground
-Fan slope angle/footwall slope angle, the angle between the top of the FW and
bottom of the HW
-Hangingwall slope angle, angle of the top of the HW
-Scarp slope angle, the angle from the ground to the top of the scarp
-Lateral offset, any offset along the fault scarp itself
Normal fault scarps have a maximum height of 6 meters for a single event, so any
scarp higher than this has seen multiple events or
Compound scarps:
Fault scarp formed by multiple surface ruptures
2. Understand and be able to explain the various relations of fault scarp
morphology as related to the age of the scarp if provided with the plot we
discussed in class. What influence would scarp size have on the preservation
potential for the scarp?
3. What are colluvial wedges? How are they typically expressed in fault trench
exposures? And how is colluvial wedge stratigraphy used to help decipher the
paleoseismic history on a fault?
Colluvial wedges form from faulting as an event causes loose sediment to fall and
settle on top of the footwall in a normal fault. The sediment sitting on the footwall is
called scarp driven colluvium and is wedged between the top of the exposed
footwall and the side of the hanging wall.
Colluvial wedges are used in fault trench exposures to decipher paleoseismic history
of the fault, most significantly through dating. The sedimentation of the wedge is
instantaneous to the event so the “piles” of sediment are unchanged from their
source rock on both sides of the fault. This is preserved over time as more events
created more colluvial wedges that associate with specific earthquake events that
can be dated by sediments above and below each individual wedge and by any
organic matter found in the trench.
4. What is a graben? What geometric features did G.K. Gilbert, and others to
follow, attribute to the cause of the fault trace graben?
A graben is a down-dropped, depressed block bounded by faults on either side that
are normal, parallel faults dipping in, towards each other as a result of tension and
extension. Single or multiple grabens create a rift valley.
5. Compare and contrast interseismic elastic strain and coseismic deformation
along strike-slip faults versus mega thrusts.
In strike-slip faults, interseismic elastic strain is the strain that builds up between
earthquake events that rupture the same fault non-periodically over time. This
pattern of irregular events is the exact opposite of the coseismic strain
release/deformation on the same fault rupture event.
In strike-slip faults, coseismic deformation results during an earthquake, when the
highly strained rocks near the fault rupture, which remained lock at a given depth during
the inter-seismic period, are permitted to slide back to a less-strained state. The event
leaves displacement across the fault.
Mega thrust faults are a type of dip-slip fault with a dip of 45° or less.
Mega thrust earthquakes are earthquakes with a magnitude 9.0 or higher and they
result in large vertical displacement over a wide area, whereas a typical strike-slip fault
is localized.
6. What is slip partitioning? Provide two real world examples of fault systems where
slip partitioning occurs?
Slip partitioning is a form of crutch for slip rate along a fault. Instead of a fault/faults of a
certain length having one large slip rate, it/they are divvied up into multiple, smaller slip
rates, which together add up to the slip rate if it was one massive fault slip rate. Several
variables control the amount of slip partitioning and how including: anisotropy,
rheology, boundary conditions, and orientation of stress.
Slip partitioning occurs in Panamint Valley, the Walker Lane Belt, and Furnace Creek in
Death Valley, California along the Garlock Fault.
7. Secondary structures associated with strike slip faults.
Riedel Shears:
Form at 10°-20° angles clockwise to main fault strike in transtensional settings.
P-shears:
Form at 10°-20° angles clockwise to main fault strike in transpressional settings.
R’ Shears:
Antithetic shears form at 50°-60° angles clockwise to main fault strike.
Flower Structures:
Releasing Bend/Restraining Bend:
Step-over structures in right-lateral faults:
8. What is meant by the terms transpressional and transtensional?
Transpression and transtension are strike-slip deformations that deviate from simple
shear due to shortening or extension in the strike-slip fault zone.
Transpression is deformation resulting from shortening perpendicular to the fault plane
resulting in oblique shear and is confined by boundaries, sometimes plate boundaries if
the area is large enough, and by restraining bends in smaller regions. This deformation
results in structures including: foliations, lineations, folds, and faulting.
Transtension is deformation from transtensive and extensive shearing from strike-slip
faulting and is bounded by strike-slip and dip-slip faults. This deformation results in
structures including: grabens, pull-apart basins and flower structures due to the
formation of releasing bends.
9. Riedel and P-shears are the most common secondary structure observed along
strike-slip faults and fault surface ruptures. Which of the two would you expect to
form preferentially along a a) transpressional strike-slip setting? And b) transtensional
strike-slip setting?
Riedel shears are more likely to be found in transtensional settings and are a result of
extension along a strike-slip fault
P-shears are more likely to be found in transpressional settings and are a result of
shortening perpendicular to the fault plane.
10. Some strike-slip faults are characterized by lots of steps and generally irregular
traces, while others are more smooth and linear. What are the factors/processes that
might be responsible for such relations and what are some implications that
complexities in fault traces might have for seismic hazards.
Some strike-slip faults are irregular and contain many step overs due to irregularities
that result from microplate interactions. This is directly related to fault displacement,
diminishing to zero at fault terminations, and must be accommodated or transferred to
other structures, laterally or vertically, away from the fault end-point, creating irregular
fault patterns.
Other strike-slip faults re smooth and straight because it only has one motion:
horizontal and does not experience step-overs due to releasing or transferring of
displacement.
11. What are event horizons and why are they important for paleoseismic
investigations? How might you be able to identify certain kinds of event horizons in a
trench or natural exposure.
An event horizon is a discontinuity in bedding that is a marker for a geological event like
an earthquake. It marks a seismic event which is important for dating of major
earthquakes and predicting a period for estimating the next rupture event. They can
also be used to see how many events a specific fault has experienced which is important
for predicting future earthquakes and analysis.
In a trench for a fault, disturbed bedding would be evidence of an event horizon such as
vertical offset between sedimentary units. It is essentially a manifestation of the seismic
data via bedding.
12. Compare and contrast the different kinds of information you would likely be able
to obtain from exploratory trenches excavated across surface traces of strike-slip
faults versus normal faults?
Trenches for normal faults can show the actual fault, amount of fault displacement,
material penetrated, thickness of bedding, age of most recent displacement, rotation of
pebbles, and presence or absence of fissures, gouge, slickensides, and breccia to make
calculations about seismicity and most recent events.
Because strike-slip faults have less visibility of physical characteristics of the fault in a
trench, displacement is more difficult to measure without event horizons or fluvial
terraces present.
13. How can geologists go about estimating the size of a paleoseismic surfacerupturing earthquake?
We can estimate paleoseismic rupture extent from measurements of displacement at a
single point on a fault. Using the variability of historic ruptures we find normalized slip,
then scale it to give the probability of finding a given displacement within a rupture for
any magnitude considered.
We use moment magnitude Mw and seismic moment Mo to make such calculations.
14. How does spreading rate (rapid vs slow) affect the expected sizes of earthquakes
at divergent plate margins and associated transform faults?
If the spreading rate relative to velocity is high (fast), magma is rising rapidly and the
lithosphere is hot. In fast spreading centers, the ridge stands at higher elevations than
for slow spreading centers. Rift valleys at fast spreading centers are narrower than at
slow spreading centers. As oceanic lithosphere moves away from the ridge, it cools
and sinks deeper into the asthenosphere. So, the depth to the sea floor increases with
increasing age away from the ridge.
Sea floor topography is controlled by the rate of spreading.
This graph shows that faster spreading rates result in more earthquakes compared to
slower spreading rates.
15. What is meant by fault segmentation? Why are faults segmented? How might you
go about demonstrating that a given fault is segmented?
Similar to slip partitioning, fault segmentation breaks up faults into smaller segments
and strands that result in limiting the distance an earthquake can rupture. These smaller
sections of faults will bear part of the slip that would occur along the main, larger fault,
resulting in smaller magnitude and shorter depth ruptures than would be seen along
one massive fault. Faults are segmented due to geographical and geological features
including river channels and mountain ranges deflecting the course of a fault as well as
bends in fault strike and possible merging or crossing of other small faults.
16. What are flexural slip faults? How can they be recognized?
Flexural slip faults are defined by fold-and-thrust belts that deform a sedimentary
wedge by low-angle thrusting and flexural-slip folding over rigid basement rock.
Flexural-slip faults form as beddings slip during flexural-slip folding and lay
unconformably where overlying deposits are deformed by renewed folding. The
bedding layers slip relative to each other with a single hinge axis. Flexural slip faults
result in parallel and parasitic folding.
17. Compare and contrast Chilean- versus Marianas-type convergent margins.
Chilean-type convergent margin:
Subduction zone characterized by strong coupling between two plates of oceanic and
continental rock where the upper plate moves outward over the lower plate where the
continental crust experiences compression. These subducting oceanic plate typically has
a shallow dip and develops thick accretionary prisms. High earthquake activity. Back-arc
regions develop foreland fold and thrust belts, in which seaward-dipping faults stack up
slices of crustal material. Weak coupling. Continental margins. High stress.
Chilean style subduction zones result in more earthquakes because the subducting plate
is subducting at a low angle so essentially more area of the plate is scraping underneath
the upper crust, creating a larger area for earthquakes to rupture. Strong coupling also
allows for more earthquakes as blocks of rock are strongly bounded.
Marianas-type convergent margin:
Sunduction zone characterized by steeply dipping slabs resulting from vertical sinking of
the slabs. Old oceanic crust is denser than the asthenosphere on which it sits andand
when it subducts it will sink vertically in addition to moving down-dip. This results in the
slab rolling back and the hinge at which it bends moves away from the volcanic arc, and
the upper plate is extended, developing a small mid-ocean ridge spreading within the
volcanic arc- known as back-arc basins. Successive waxing and waning of back-arc sea
floor spreading results in extinct volcanic chains and small oceanic basins by some
subduction zones. Island arcs and deep trenches. Low stress.
18. For a strike-slip environment, there sure are a lot of folds and thrusts in central
California. In particular there are numerous active folds involving Pliocene and older
strata along the west side of the central valley east of the San Andreas fault. Many of
these folds are targeted as traps for hydrocarbons such as those in the Kettleman Hills,
Lost Hills, and Coalinga areas.
What is the orientation of most of the folds in this region relative to the San Andreas
Fault?
What tectonic significance (and interpretations) can be made of the fold orientations?
These are rotating crustal blocks.
19. How can fluvial terraces be used to provide information useful for seismic hazard
assessments associated with blind thrusts? How might you go about determining rates
of activity on blind thrusts using fluvial terraces?
Fluvial terraces, when uplifted or are at the surface can show displacement of a surface
fold scarp that deforms shallow marine and fluvial terrace deposits. Because blind
thrusts do not show any disturbance or structures at the earth’s surface, they go
unnoticed and urbanization is at risk to earthquakes when communities settle in areas
with blind thrusts. So fluvial terraces are evidence that there is seismic activity in the
region, though no surface traces are observable. Great displacement across blind thrusts
is hidden at depth so measuring uplift rates can be difficult, but can be done using fluvial
terraces. Terraces that have been correlated can be used to measure uplift rates using
soil and gravel markers and whether they are deformed by seismic activity or not.
20. How might geologic map relations in areas of active blind thrusts and fault
propagation folds be used to estimate earthquake sizes (mw) associated with these
types of structures?
Blind thrusts do not have surface ruptures but do have near-surface expressions in folds
that grow or are uplifted during earthquake events that can be used to measure
earthquake size through fault slip associated with band migration and deformation
along active axial surfaces, bounded by fault bends in folds.
21. What are fault asperities? How are large-scale asperities thought to form at
convergent boundaries and how do asperities relate to potential earthquake sizes
along certain types of subduction zones?
Fault asperities are areas of irregular friction along the fault that will lock up and then
fail when the stress along the boundary exceeds the strength of the asperity. When
stress builds up along these asperities, massive earthquakes will occur, especially at
convergent boundaries where mountains form.
22. How have observations of coastal deformation associated with the 1960 Chilean
and 1964 Alaskan earthquakes aided in assessing seismic potential along the Cascadia
subduction zone?
These subduction margins indicated landward motion of coastal sites but seaward
motion of inland sites. The deformation experienced in these margins are results of
accretion, subduction, deep earthquakes, and active volcanism. Faulting, including mega
thrust faults that were found in Alaska and Chile are believed to be similar with the
faulting in Cascadia, meaning it is prone to the large rupture events seen in these
regions due to younger, less dense plates with less slab pull and a very large area
coming into contact with the overriding plate.
23. Be able to briefly describe basis and methods for the following kinds of data:
a) Triangulation- determining something's location using the locations of other things. It
is commonly used by geologists to find the locations of Earthquakes by using multiple
stations that record P and S wave arrival.
b) Leveling- done with a brunton to measure things such as height of scarps,
embankments, terraces etc.
c) GPS- Uses satellites to pinpoint location based on latitude and longitude and can
measure elevation. A GPS receiver monitors multiple satellites and solves equations to
determine the precise position of the receiver and its deviation from true time
d) VLBI- used to measure things such as crustal movement. Geometric technique to
measures the time difference in the arrival of a radio wave emitted by a distant
astronomical radio source between grounded radio telescopes. Knowing the time
difference (τ), and the angular separation (β) between the viewpoint and the baseline
between the antennas of the telescopes, the distance between the telescopes can be
determined, and likewise distance between continents.
e) InSAR- technique for mapping ground deformation using radar images of the Earth's
surface that are collected from orbiting satellites, generally used to see underground
deformation from faults, fault scarps, and buried structures.