Map exercise using the 3D fault model Using seismic data to position a dam

A map exercise using the 3D fault model A water company has decided to build a new reservoir to help counter water shortages experienced in the summer for several years. Three possible locations for the dam have been identified in the river valley and are marked on the map below as sites A, B and C. Use the seismic data from the 3D model to recommend geological reasons for which site should be chosen. A geologist or structural engineer will often undertake this sort of task. Hint: Mark on the map the CMP position of the fault plane at 0msec.

CMP Line S2-11 Line S km A B C N Map showing position of seismic lines and proposed location of the three dam sites Key Proposed site of dam Hint: Mark on the map the CMP position of the fault plane at 0msec Position of fault plane at 0 msec

Extension Taking a closer look at the fault plane and its movement

1. To investigate the movement along the fault plane plot on the map the position of the fault plane at 600msec.

CMP Line S2-11 Line S km A B C N Map showing position of seismic lines and proposed location of the three dam sites Key Proposed site of dam Position of fault plane at 0 msec Position of fault plane at 600 msec

If the fault had moved straight down in the dip-slip direction, then we would expect the plot of the fault plane lines to be parallel. But the two fault plane lines are not parallel, what other sort of movement must have occurred? A rotation.

We could try measuring the throw. 2. Can we find any other evidence to support the idea that a rotation has occurred?

Throw at yellow horizon 52msec Line S1-11 Throw at yellow horizon 45msec Line S2-11 The throw can be measured at the interpreted horizon (yellow dashed line). Why is the throw not the same amount on each line? Seismic data only shows a side-on 2D image of the subsurface along a line. This means it is not possible to actually see any strike-slip displacement. We have to use other evidence to deduce what has happened. It is very common to find rotated blocks. Which way has this block rotated? The seismic velocity is 2400m/s, calculate the throw in metres. 2400m/s = 125m 2400m/s = 108m ⇃ ⇂ ↾ ↿

CMP Line S2-11 Line S km A B C N Map showing position of seismic lines and proposed location of the three dam sites Key Proposed site of dam ● On line S2-11 throw at yellow horizon 108m ● On line S1-11 throw at yellow horizon 125m Displacement greater at south side of fault Position of fault plane at 0 msec Position of fault plane at 600 msec

The fault plane extends for several km to the north and south. The throw gets less and less until the fault dies out. Notice the shape of the fault plane. Regional view Fault plane derived from other seismic lines (not plotted on map)

The fault moved down in a dip-slip direction towards the east. By examining the seismic data more closely it can be seen that: The southern end of the fault has a greater downthrow than the north indicating a rotation has occurred. Conclusion Faults are not uniform, the throw is variable and they are often curved. Faults eventually die out.