Faults

What are they and what should I know about them?

Nathan Carey

First of all, what exactly is a fault?

A fault is a planar fracture or discontinuity across which there has been displacement. In other words, a plane of rock which has undergone movement parallel to that plane. This may be difficult to visualize with just an explanation. Below are some examples of faults.

 I mage from Miller (2015).

Personal Image (2018)

 I mage from Fossen (2013).

Image from Miller (2016).

What are the different types of faults?

  • Normal Faults
  • Reverse Faults (Including Thrust Faults)
  • Strike-Slip Faults

Before getting into the different types of faults, you must understand the difference between a HANGING WALL and a FOOTWALL.

The hanging wall is the block of rock above the fault line. You can hang something from the hanging wall as if it were a ceiling.

The footwall is the block of rock below the fault line. You can walk on it as if it were the floor below you.

Image from Pearson Prentice Hall, Inc. (2012).

If you're having trouble understanding the difference between the hanging wall and the footwall, click on the link below for a video explanation.

Now that the difference between the hanging wall and footwall is clear...

How can faults be exposed at the surface?

  • Fault Trace - the visual or topographic expression of the intersection between a fault and the earth's surface (Wilkerson, 2019).
  • Fault Scarp - a topographic step on the earth's surface that represents the exposure of a fault plane.
  • Fault-Line-Scarp - a topographic step caused by a fault but not representative of the fault plane itself (Wilkerson, 2019). This is more of an erosional response to faulting.
  • Blind Fault - when a fault does not cause any interruption at the earth's surface.

What are some other indicators of fault displacement?

How is fault displacement described?

  • Fault Slip - the direction, magnitude, and sense of displacement (Wilkerson, 2019).
  • Fault Separation - the apparent relative displacement measured in a given direction as observed from an outcrop, map, or cross section (Wilkerson, 2019).
  • Vertical Separation - the distance that a bed has been vertically displaced (Wilkerson, 2019)
  • Fault Throw - the vertical offset due to dip-slip movement (Wilkerson, 2019).
  • Fault Heave - the horizontal offset due to dip-slip movement (Wilkerson, 2019).

References

Normal Fault Image

Miller, Marli, 2015, Big Quakes Can Trigger Other Shakes Thousands of Miles Away:  https://www.smithsonianmag.com/science-nature/big-quakes-can-trigger-other-shakes-thousands-miles-away-180956985/  (accessed November 2019).

Reverse Fault Image

Fossen, Haakon, 2013, Reverse Faults:  https://structuralgeo.wordpress.com/2013/07/17/reverse-faults/   (accessed November 2019).

Strike-Slip Fault Image

Miller, Marli, 2016, The 1976 Tangshan Earthquake:  marlimillerphoto.com  (accessed November 2019).

Strike-Slip Fault Diagram

Bentley, Angela, 2012, Fault Types:  https://www.slideshare.net/angelabentley/faults-types  (accessed November 2019).

Hanging Wall Image

Pearson Prentice Hall, Inc., 2012, Hanging Wall vs Foot Wall:  https://classconnection.s3.amazonaws.com/790/flashcards/520790/png/screen_shot_2012-04-19_at_125858_pm1334858352757.png  (accessed November 2019).

Displaced Marker Image

GeoNet, 2010, M 7.2 Darfield (Canterbury) Sat, Sep 4 2010:  https://www.geonet.org.nz/earthquake/story/3366146 

Background Information and Slickenlines Image

Wilkerson, M.S., 2019, Faults Presentation.

 I mage from Fossen (2013).

 I mage from Miller (2015).

Personal Image (2018)

Image from Miller (2016).

Image from Pearson Prentice Hall, Inc. (2012).