Rake (Pitch) of Fault

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Rake of a Fault: 

1. What is Rake in a Fault?

Rake (also called pitch) is the angle between the slip direction of the fault and the horizontal strike line of the fault plane. It describes the exact movement of one side of a fault relative to the other.

  • Measured within the fault plane (not in space).
  • It helps determine the type of fault movement (strike-slip, dip-slip, or oblique-slip).
  • It ranges from -180° to +180°.

2. Why is Rake Important?

Rake is used in:

  • Fault Classification – Helps identify fault types (strike-slip, normal, reverse, or oblique).
  • Seismology & Earthquake Studies – Used in focal mechanism solutions (beachball diagrams) to determine earthquake fault motion.
  • Tectonic Analysis – Understanding how Earth's plates move relative to each other.

3. How is Rake Measured?

To measure rake, we need two key references:

  1. Strike of the Fault: The horizontal direction of the fault plane (expressed as an angle from north).
  2. Slip Direction: The actual movement of the hanging wall relative to the footwall (measured within the fault plane).

Steps to Measure Rake:

  • Find the strike direction of the fault.
  • Identify the fault plane (surface along which movement occurs).
  • Determine the movement direction of the hanging wall relative to the footwall.
  • Measure the angle between the fault strike line (horizontal) and the movement direction within the fault plane.

4. Interpretation of Rake Values

5. Rake in Different Fault Types

A. Strike-Slip Fault (Rake = 0° or ±180°)

  • Movement is purely horizontal along the fault plane.
  • Right-lateral (dextral) or left-lateral (sinistral).
  • Example: San Andreas Fault (California, USA).

B. Normal Fault (Rake = -90°)

  • Hanging wall moves down relative to the footwall.
  • Caused by extension (pulling apart).
  • Example: East African Rift Valley.

C. Reverse Fault (Rake = 90°)

  • Hanging wall moves up relative to the footwall.
  • Caused by compression (pushing together).
  • Example: Himalayan Thrust Faults.

D. Oblique-Slip Fault (Rake = Between 0° and 90° or -90° and -180°)

  • Combination of horizontal and vertical movement.
  • Can be either strike-slip + normal or strike-slip + reverse.
  • Example: Denali Fault (Alaska, USA).

6. Example of Rake Calculation

Scenario:

  • A fault has a strike of 320° (measured clockwise from north).
  • The hanging wall moves 40° from the horizontal strike line in the fault plane.
  • The rake is measured as 40° within the fault plane.

If the movement was downward at -40°, it would indicate normal faulting with an oblique-slip component.

7. Rake in Earthquake Focal Mechanisms

  • Rake values help in drawing focal mechanism solutions (beachball diagrams).
  • These diagrams show the fault plane and slip direction during an earthquake.
  • Example: A reverse fault with minor strike-slip movement might have a rake of 110° instead of 90°.

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