Faults: an introduction!
!Slip vs separation!
!Stratigraphic cutoffs!
!Fault growth, or scaling!
!Repetition or omission in boreholes!
!Fault linkage!
!
Reading: Chapter 8 in Fossen!
© Cambridge University Press 2011
Fault planes plotted on a stereonet
with slip vector.
Looking at the stereonet diagrams,
how can you tell is the slickenlines
lie on the fault surface?
Nojima Fault Rupture –
Kobe Earthquake
Kobe Earthquake Fault Rupture Museum!
Fault Components!
•  Footwall
•  Hanging wall
Hanging wall
•  Fault plane, fault surface
Footwall
Faulting Styles – Oblique slip!
Oblique-slip: a combination of dip-slip and strike-slip
Here displacement of a drainage ditch offset by the Kobe Earthquake
Offset of the ground surface (note how topography changes) from an
Earthquake in Baja in 2010. The slip along the fault is constant here,
so why does the amount of vertical displacement occur?
Offset of the ground surface (note how topography changes) from an
Earthquake in Baja in 2010. The slip along the fault is constant here,
so why does the amount of vertical displacement occur?
Oblique slip continued… Offset of the ground surface (note how topography
changes) from an earthquake in Baja in 2010. The slip along the fault is
constant here, so why does the amount of vertical displacement occur?
The key is the orientation of the ground relative to the rupture. Ground
surface perpendicular to rupture = tall scarp, parallel = lower scarp
Slip versus Separation!
Slip
Pre-faulted block
Faulted block
Separation
Eroded block
•  Slip - actual relative displacement between two points that occupied the
same location before faulting
•  Separation - apparent relative displacement between two points that may
have occupied the same location before faulting
© Cambridge University Press 2011
Types of Faults!
Note how the blocks
create apparent separation
Normal
Dip-slip
Thrust
Dip-slip
Strike-slip
Strike-slip
Determining slip is not a trivial exercise, note how you need a cross!
Section oriented such that it contains the slip vector and offset points
© Cambridge University Press 2011
Stratigraphic cutoffs allow you to match line of intersection of
hangingwall with fault plane with intersection of footwall with
fault plane. Both intersections were the same line before it was
separated across the fault
© Cambridge University Press 2011
Vertical boreholes appear to !
either omit or repeat strata!
!
For the former, this occurs where!
a normal fault cuts out or removes!
stratigraphic section!
!
Alternatively thrust faults repeat!
stratigraphic section.
© Cambridge University Press 2011
Displacement to length scaling – how faults grow in 3D
© Cambridge University Press 2011
Arguments for how faults grow vary, but recent data suggest most D:L!
profiles are triangular shaped and grow where slip rates are higher in the !
center. Plateau shaped displacement faults !
can be associated with reactivated faults
An important issue is how individual slip events (earthquakes)!
add up to make the displacement to length profile
Faults develop damage !
zones then localize slip !
onto a slip surface.
Fault arrays also link up to form simpler
single faults. These accommodate
most of the subsequent strain
Hard linked vs soft linked
A soft-linked !
relay ramp
Fault growth in a layer bounded by stronger rocks
The following exercise in Google Earth is designed to explore scaling
relationships in extensional fault networks.!
!
Scaling is a natural result of how faults grow. They accumulate
displacement, yet grow laterally with time. As shown earlier, these
displacement profiles can take different shapes, triangular, bell shaped
or plateau shaped. Triangular profiles seem to be more prevalent for
dip slip faults based to recent research!
!
Mmax : Fault length records the end result of the history of growth
for a fault. Earthquake geologists and seismologists are interested in
scaling because it tells us something about the distribution of slip that
might occur during a seismic event (which ultimately add up to make
the D:L profiles).!
!
Petroleum geologists are interested because many oil and gas fields
are formed in normal fault systems and faults can form pathways for
oil, or seals depending on fault geometry, the amount of slip and the
sediments they deform, especially clastic sediments.!
Let’s look at some spectacularly exposed normal fault systems in
Arches National Park. First, fire up GE.!
!
Start off by looking for normal faults in the Delicate Arch area, the
iconic landform of Utah. Type in the words Delicate Arch, then find
the Delicate Arch Parking area.!
!
Once you have found the Delicate Arch Parking area, follow the hiking
trail northeast through the red-colored Morrison Formation. The trail
will disappear as the trail starts to move up a ramp-like bedding
surface of the underlying Entrada Formation. !
!
Use the 3D perspective tool to identify faults in this area and
distinguish them from stratigraphic contacts. The light pink rocks are
Entrada Sandstone, the red, grey and green rocks are younger
overlying sediments (Morrison, Dakota, etc). Strata in this region
generally dip southward.!
!
Determine what type of contact the trail crosses. (offset fault
contact, stratigraphic contacts, or ???).!
Now look in nearby areas for faults. These can be identified by
contacts that cut across stratigraphic contacts.!
!
When you find nearby faults, determine in which direction the
displacement on them dies out.!
!
You can use the File, Save, Save image to obtain a 3D image, then
use the resulting jpg as a graphic tool to annotate features of
interest (not now, but this is a handy tool for making
presentations of geology).!
!
You can make a topographic profile by making a path, make sure
the path is highlighted in the menu on the left, then go to Edit in
the menu bar and Show elevation profile.!
!
Now lets go to a spectaular array of normal faults in the Needles
District of Canyonlands. You should be looking at a set of horsts
and grabens aligned on the east side of the Colorado River Canyon,
south of its confluence with the Green River.!
Look for faults that don’t have sand infilling them. Can you find
any where the displacement profiles are obviously tapered? It may
help to vertically exaggerate the scene, say to 1.5. !
!
Do they always taper down to no displacement, or do some have
displacement profiles that go up, then down and then up again
along their lengths.!
!
Why do you think this is? !
!
Can you identify faults that have become linked together? What
evidence tells you that this might have occurred?!
!
Note that the bigger, deeper grabens are partly filled with eolian
sand, so displacement profiles may not represent the actual
displacement along the fault. !
!
Relay ramp on trail to Delicate Arch!
!
The following exercise in Google Earth is designed to explore scaling
relationships in extensional fault networks.!
!
Scaling is a natural result of how faults grow. They accumulate
displacement, yet grow laterally with time. As shown earlier, these
displacement profiles can take different shapes, triangular, bell shaped
or plateau shaped.!
!
These represent the end result of the history of growth for a fault.
Earthquake geologists and seismologists are interested in scaling
because it tells us something about the distribution of slip during a
seismic event (which add up to make the D:L profiles).!
!
Petroleum geologists are interested because many oil and gas fields
are formed in normal fault systems and faults can form pathways for
oil, or seals depending on fault geometry, the amount of slip and the
sediments they deform, especially clastic sediments.!
Lets look at some spectacularly exposed normal fault systems in
Arches National Park.!
!
Start off by looking for normal faults in the Delicate Arch area, the
iconic landform of Utah. Type in the words Delicate Arch, then find
the Delicate Arch Parking area.!
!
Once you have found the Delicate Arch Parking area, follow the hiking
trail northeast through the red-colored Morrison Formation. The trail
will disappear as the trail starts to move up a ramp-like bedding
surface of the underlying Entrada Formation. !
!
Use the 3D perspective tool to identify faults in this area and
distinguish them from stratigraphic contacts. The light pink rocks are
Entrada Sandstone, the red, grey and green rocks are younger
overlying sediments (Morrison, Dakota, etc). Strata in this region
generally dip southward.!
!
Determine what type of contact this is. (offset fault contact,
stratigraphic contacts, or ???).!
Now look in nearby areas for faults. These can be identified by
contacts that cut across stratigraphic contacts.!
!
When you find nearby faults, determine in which direction they dip
and the direction displacement on them dies out.!
!
You can use the File, save, save image function to obtain a 3D
image, then use the resulting jpg as a graphic tool to annotate
features of interest (not now, but this is a handy tool for making
presentations later on in your career).!
!
Now lets go to a spectaular array of normal faults in the Needles
District of Canyonlands. You should be looking at a set of horsts
and grabens aligned on the east side of the Colorado River Canyon,
south of its confluence with the Green River.!
Look for faults that don’t have sand infilling them. Can you find
any where the displacement profiles are obviously tapered? It may
help to vertically exaggerate the scene, say to 1.5. !
!
Do they always taper down to no displacement, or do some have
displacement profiles that go up, then down and then up again
along their lengths.!
!
Why do you think this is? !
!
Can you identify faults that have become linked together? What
evidence tells you that this might have occurred?!
!
Note that the bigger, deeper grabens are partly filled with eolian
sand, so displacement profiles may not represent the actual
displacement along the fault. !
!