Faults: picking the sticks, and what the scaling laws say about the map

With the survey loaded, step through inlines in the Section viewport and watch reflectors break. Where they break, pick. A fault on a section is a stick: a line of points down the discontinuity. A fault in three dimensions is a surface, and you build it the only way anyone ever has: stick by stick, line by line, correlating by eye as you step.

In the workstation

Open the Section viewport, choose PICK FAULT STICKS, and work a fault: click down the break, finish the stick, step two or three lines, pick it again. Neighbouring sticks draw dashed so you can see what you claimed last line. Then open the Map viewport: your sticks become a fault trace, and the survey starts to look compartmentalised, because it is.

A fault map is a set of assertions

Each stick says: there is a surface here, and the rocks either side of it have moved past each other. Most software will draw those assertions beautifully and check none of them. The check exists, and it comes from outside your survey entirely. Across published fault populations spanning seven orders of magnitude, maximum displacement is roughly proportional to length, @@D_{max} \approx \gamma L@@ with @@\gamma@@ near 0.03, and the scatter spans about an order of magnitude because faults grow by linking segments and no two histories are identical.

That width is exactly why the relationship is useful as a check rather than a prediction. A fault far BELOW the trend, carrying much less displacement than its length implies, is most often two separate faults picked as one continuous segment: merging doubles the length while the throw stays put. Far ABOVE the trend, either the fault continues past where you stopped picking, or the throw was measured across a relay zone and you summed two faults. And a single fault carries an elliptical displacement profile, maximum near the middle and dying to zero at the tips; a flat-topped profile means the surface was carried at constant offset rather than followed, and two peaks with a saddle means two faults.

The population is evidence too. Fault lengths follow a power law: many small, few large. If your map has only large faults, you have not mapped a population, you have mapped the faults that were easy to see, and every statement about seal or compartmentalisation inherits that bias. When you reach the Handover at the end of the lab, the workstation will tell you how many faults the reference interpretation carries. Most first passes find fewer than half.

Open the STATISTICS tab as the network grows. Every one of these checks runs live on your own picks: the displacement-length instrument with the published band, the throw profile of any fault you select, the population histogram and its log-log fit, and, when the Ogbon transmittal is loaded, recall and precision against the reference network. The 3D tab shows the planes themselves, lofted through your sticks against the seismic.