Piercing point problems
A good example for piercing problem is in your Lab Manual, in the Problem on page 178.
In such problems, you are given a map, on which a fault cuts one or more planes (e.g., dike, bed, vein).
The attitude of the fault and the displaced planes are given on the map.
The map of course has scale and the direction of North.
Objective: Determine the amount and direction of the netslip (displacement) of the fault.
Here is the procedure to do such problems (see the solution on page 178):
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Identify the hanging wall (HW) and footwall (FW) on both sides of the fault on the map.
The HW is on the dip side of the fault! Label the blocks on either side of the fault as HW & FW
Identify the spots (points) along the fault where the planes, on either side of the fault, intersect
the hangingwall and footwall of the fault. Mark these points with letters, e.g., A, B on the FW side,
and A’, B’ for the corresponding points on the HW.
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On a graph paper, draw a horizontal line (in red), parallel to the trace of the fault on the map, to
represent the strike of the fault. Mark the ‘tick’ and write the dip value of the fault.
Plot and label the trace of the displaced planes in black (e.g., ‘dike’, ‘bed’, ‘vein’). Mark their dips.
Plot the fault (in red color), and the displaced planes (use different colors) on the stereonet.
Determine the pitch (rake) of the planes on the fault (where they intersect the fault). This is done
by reading the angle from the strike of the fault to the intersection point of each plane.
Identify the HW and FW on both sides of the trace of the fault on the graph paper
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On the graph paper, draw the pitch of the planes, on the FW side of the fault with solid lines,
where the planes intersect the fault on the footwall (e.g., points A, B).
On the graph paper, draw the pitch of the planes, on the HW side of the fault with dashed lines,
where the planes intersect the fault on the hangingwall (e.g., points A’, B’).
Determine where the two solid lines interest. Label it as N. This the piercing point on the FW.
Determine where the two dashted lines interest. Label it as S. This the piercing point on the HW.
Connect N to S. This gives you the vector NS (netslip), directed from N to S.
Put the arrowhead for the vector at S.
Determine the length of the NS using the scale on the map. This is the magnitude of the netslip.
Read the acute angle between the vector NS from the trace of the fault. This is the pitch of NS.
Plot the netslip (NS), using its pitch, on the fault plane (on stereonet).
Read the trend and plunge of the netslip.
Determine the sense of slip using your hand and a pencil.