Orientation of planes with respect to the geographical coordinate system

Imagine any plane (such as the plane shown in Fig. 5.5 cutting horizontal sedimentary strata). The strike is the line which results from the intersection of such plane and a horizontal plane (in the figure above the water surface). The magnitude of the strike is the angle between the strike line and the north. The angle of dip is the angle between a horizontal plane and the plane under consideration. A layer is said to dip in a given direction when it gets deeper at the fastest rate into such direction. One can think of the $dip$ as the direction of a droplet of water moving down on such plane.

Figure 5.5: Definition of strike and dip.

There are two conventions for reporting the magnitude of strike.

• Quadrant convention: it reports strike with angles ranging from 0 to 45$^{\circ }$. The quadrant convention is useful in the field. For example:
  • “N45$^{\circ }$E” means 45$^{\circ }$ clockwise from the North towards the East
  • “S45$^{\circ }$W” means 45$^{\circ }$ clockwise from the South towards the West
  • “N45$^{\circ }$E” is the same as “S45$^{\circ }$W” (Figure 5.6)
• Azimuth convention: it reports strike with angles ranging from 0 to 360$^{\circ }$ measured from the North in clockwise direction. The azimuth convention is most useful for data analytics and mathematical implementation. For example:
  • “045$^{\circ }$” = “225$^{\circ }$” (three digits always)

Figure 5.6: Compass and azimuth terminology for strike.

The dip is the angle between an horizontal plane and the line of maximum slope in the measured plane. It is reported with angles between 0 and 90$^{\circ }$. The maximum dip is 90$^{\circ }$ (vertical plane). If the layer/fault is tipped even further it is said to be overturned.