Skip to main content

Strike-dip relationship and apparent dip

Short answer

Strike and dip are perpendicular: the dip direction is always 90° to the strike. Apparent dip in a section making angle α with the strike is tan δ_a = tan δ × sin α, where δ is true dip. Example: true dip 30°, α=45° gives apparent dip 22.2°.

What is the relationship between strike and dip? How do you calculate apparent dip amount from the measured true dip amount?

Relationship between strike and dip

  • The strike is the bearing of the horizontal line on an inclined plane, and dip is the maximum inclination of the plane.
  • The dip direction is always at right angles (90°) to the strike. So when the strike is known, the dip direction is strike ± 90° (the side must be seen from the field), and when the dip direction is measured, strike = dip direction ± 90°.
  • Example: strike N30°E, dip 40° towards SE (azimuth 120°).
      N      strike
      |    /
      |  /
  ----+/------ E
     /|  \
        dip direction (90 deg to strike)

Apparent dip from the true dip

Apparent dip is the inclination of the bed measured along a vertical section that makes an angle α\alpha with the strike. It is calculated by:

tan⁡δa=tan⁡δ×sin⁡α\tan \delta_a = \tan \delta \times \sin \alpha

where δ\delta = true dip, δa\delta_a = apparent dip, α\alpha = angle between the strike and the section line.

Example: δ=30°\delta = 30°, α=45°\alpha = 45° gives tan⁡δa=0.5774×0.7071=0.4082\tan\delta_a = 0.5774 \times 0.7071 = 0.4082, so δa=22.2°\delta_a = 22.2°. The apparent dip is always less than the true dip, and is zero along the strike.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Engineering Geology I chapter 5 questions →