Faults
Note
Coming in Spatial Integration version 3.
Requires the Professional edition. In the Community edition, geological models in a project can be viewed read-only: shown, hidden and faded, but not created or changed.
A fault is a surface. On its own it is modelled like any other surface, from points on it and, where you have them, its orientation. What makes it a fault is what it does to the rest of the model: it can cut and offset older surfaces, stop against an older fault, or splay from one.
Add a fault on the Features tab: type a name and click Add fault. Then give it data on the Constraints tab (see Constraints) and build it.
The Fault Surface
The fault's field is positive on its hanging wall and negative on its footwall, and the surface is where it is zero. Its data is usually:
- Points along the fault, from a digitised trace, drill intervals logged as fault, or picks on sections.
- Orientations, from dip and dip azimuth columns or structural measurements.
Faults from points alone
You do not need orientations. With Derive normals from points when there are none ticked in the fault's Fit parameters (the default), a fault with no normals takes its orientation from its own points: one plane through them when there are fewer than 20, a local estimate at each point when there are more. The points must not all lie on one straight line - a trace digitised along flat ground only defines a line, and needs either picks at depth or an orientation.
The orientation found this way has no preferred side. By default the positive (hanging wall) side faces up. Set Positive side faces azimuth for a vertical or overturned fault, so the hanging wall is on the side you expect.
Fault Geometry
A fault that offsets other features needs a geometry, open under Fault geometry when the fault is selected. It describes the volume the fault acts on as an ellipsoid around it, and how far it moves things.
| Field | Meaning |
|---|---|
| Centre | The middle of the fault, where the displacement is largest |
| Semi-axes: along strike, down dip, across | The half lengths of the ellipsoid. Along strike and down dip are the fault's extent in its own plane. Across is how far from the fault surface its movement reaches into the rock either side |
| Dip, Dip azimuth | The fault plane's average orientation, which sets the slip direction |
| Displacement | The total movement at the centre, in model units, shared equally between the two walls |
| Type | Strike-slip moves the walls along strike. Every other type moves them down dip |
| Displacement also tapers down dip | Off: the displacement dies away towards the fault's tips along strike only. On: towards its upper and lower edges too |
The easiest way to fill this in is Derive from data: build the fault first, then click it, and the centre, semi-axes and orientation are taken from the built fault and its points. Adjust the displacement and anything else, then click Apply geometry.
The sign of the displacement follows the label hanging wall down dip +:
- For a dip-slip fault, a positive value moves the hanging wall down: a normal fault. Enter a negative value for a reverse fault.
- For a strike-slip fault, a positive value is right-lateral (dextral): standing on one side and looking across, the other side has moved to your right. Enter a negative value for left-lateral.
The displacement is greatest at the centre and dies away smoothly to nothing at the edges of the ellipsoid, as a real fault's throw does towards its tips.
Cutting Other Features
Select the older feature - a surface, a series or another fault - and open Cut by. Choose the fault and click Add cutting fault. The fault must be younger: its Age number must be higher.
When the older feature is built, its data is first moved back across the fault to where it was before the fault moved, the feature is fitted there as one continuous surface, and the result is moved forward again. The surface is built a fault block at a time, each side of the fault separately, and cut exactly at the fault, so the offset shows as a clean break rather than a bend.
Build the fault, with its geometry, before what it cuts. Build all does this for you.
Tip
If a surface bends across a fault instead of breaking, check that the fault has a geometry and a non-zero displacement, and that the surface lists it under Cut by.
Faults Ending at Other Faults
A younger fault often stops against an older one. Select the younger fault and open Stops against. Choose the older fault and which side of it the younger fault lies on - Its side: from this fault's picks works it out from where the younger fault's data is - and click Add.
The younger fault's surface is then cut where it meets the older fault, and its displacement stops there too: features it cuts are offset only on its own side of the older fault.
Splays
A splay branches off an older fault and curves into it. Tick Splays from it (merges into it) when adding the relationship. Points from the older fault's surface near the splay's own data are added to the splay's fit, so the two meet smoothly instead of at an angle.
Merge distance is how far from the splay's data those points are taken. Leave it at 0 = auto: the distance is worked out from the gap between the splay's data and the older fault, plus half the spacing of the splay's points. Set it by hand when the junction looks too abrupt (increase it) or the splay is pulled out of shape (decrease it). Build the older fault first.
Stopping a Fault Without Ending Its Movement
Truncated by also cuts a fault where it meets another feature, but only trims the mesh. Use Stops against for one fault ending at another, since that stops the displacement as well, and keep Truncated by for a fault cut off by an erosion surface. See Surfaces and Stratigraphy.