4 September 2015

Faults aren’t lines

Posted by Callan Bentley

A new post on GeoSpace, a sister blog here at the AGU Blogosphere, calls attention to a new study by Simon Lamb and colleagues on the plate-boundary fault running through New Zealand’s South Island, the Alpine Fault. The post is apparently taken and lightly adapted from a press release on the Victoria University of Wellington website, where it was authored by a “communications adviser.”

I call your attention to it here today because the post uses the phrase “fault line” no less than five times to describe the Alpine Fault:


I have an issue with this. Faults are not lines. They are fractures, and thus more closely approximate the geometry of planes. Lines are one dimensional. Planes are two dimensional. In reality, faults are not perfect planes either – they have three-dimensional features such as grooves, asperities, jogs, and – the exact point of this research – dip angles that can twist, warp, bifurcate, merge, or otherwise vary.

The place where the “plane” of the fault intersects the “plane” of Earth’s surface should be called a fault trace. It is not a perfect line because it almost always wiggles a bit, due to variations either in the fault’s shape, or the shape of Earth’s surface, or both. If the phrase “fault line” is ever used, it should only be invoked in this strict sense – as a synonym for “fault trace.” But even then, it’s going to make me wince.

The phrase “fault line” is a particular pet peeve of mine, for precisely the reason identified by Dr. Lamb in this post – an individual could think “the fault line” (fault trace) is way over there, but in reality, the fault is a planar feature, which could dip underneath that individual’s location. Trading the language of planar 2D geometry for 1D lines masks this perception, and thus contributes to misunderstanding and risk.

So the Lamb, et al. (2015) paper demonstrates as its key point that the Alpine Fault is not a line. Yet the primary author is quoted three times as saying “fault line.” What’s up with that?

Surely the use of “fault line” in the piece was a goof-up by the “communications advisor” who wrote it, rather than Simon Lamb, the geoscientist who led the writing of the research paper? I worried because three of the five instances are within quotes attributed to Dr. Lamb. I wrote to him while I was composing this blog post to ask if he was misquoted. Or perhaps it was a case of him using “fault line” deliberately, figuring that more of the public would understand that phrase than “fault.” Or maybe “fault line” is synonymous with “fault” in New Zealand. Or –  worst case scenario –  perhaps Dr. Lamb doesn’t himself distinguish between the two. In that case, I’ll have to chalk this all up to a case of two professionals disagreeing over the proper terminology. I’ll update the post when I get new information. UPDATE: He has written back with the following response:

Dear Callan,

You are right, of course! Faults are 3-D features.

However, I’ve found that outside the geological community, the term ‘fault line’ has come to mean what we, as geologists, would call a fault. This is certainly the case in New Zealand. I think the reasons for this are that faults certainly look like lines on a map, but the term also distinguishes a geological fault from the word ‘fault’, as in ‘it is not my fault’ or an ‘electrical fault’, and finally it draws attention to the fact that we are talking about a spatial feature (albeit 3-D and not 1-D!).

In all my interviews, the journalists repeatedly used the term ‘fault line’ and I found it counterproductive to correct them as it only seemed to lead to more confusion. But if you listen to my Radio NZ interview (attached), you can see that they do appreciate the 3-D nature of it. In my first interview, I did start talking about planes and this just drew a blank, as I think in this world, where geometry is hardly taught at school any more, most people don’t know what a plane (or surface) is from a geometrical point of view!

The press release was written by the university’s media team, and we did discuss these points, but they decided to stick with ‘fault line’ in the hope of making sense to journalists, and not just drawing a blank, which would lead to a desert of interest.

But, good on you to plug away on these points!


I feel that “fault line” should never have appeared on an AGU blog. It’s my opinion that our team should have caught that before it was published and changed it to “fault.”

But wait; there’s more…

Shouldn’t a university press release about newly published research merit a link to the actual paper? This is a best practice in science blogging about new research – link to the research! The GeoSpace post added a link before publishing, and noted that the research was published in an AGU journal. But somehow I glossed over that – and went to Google instead.

I typed “g3 simon lamb alpine fault” into my browser’s Google-utilizing search bar, and got this piece by Sarah Jane O’Connor as my first hit: Stuff.co.nz Science: “Alpine Fault spreads across South Island, researchers say”

Though it too uses “fault line” terminology, this article is far superior to the UVW press release: (1) it includes a map graphic that makes the key point — not merely “rethinking” the fault but an attempt to describe the newfound aspects of its shape and structure, and (2) it links to the actual published research article.

Look at the map:


This map demonstrates in a nutshell the difference between a fault (planar, dipping, in this case steeply to the southeast in the northeast part of the island and more gently in the southwest) and a fault trace (the black line on the map). I am pleased to note the lack of the phrase “fault line” anywhere in the graphic.

However, even this gets the geometry wrong on two counts:

(a) “flattens” isn’t really as accurate as “dips more shallowly” – after all, “flat” means lacking relief. A wall can be just as flat as a floor, though one is vertical and one is horizontal.  I guess I’d let this one slide, since in popular parlance, many people equate “flat” with “horizontal.”

But then there’s (b) “a triangular shape” – not only are faults not lines, they aren’t triangles, either. You can project a fault’s presence at depth onto a map, and it may make a triangular area of the land, but the fault itself is a more-or-less planar fracture surface or collection of such surfaces. It’s not a hexagon; it’s not a square; it’s not a triangle. A perfectly developed fault in homogeneous rock that doesn’t hit the discontinuity of Earth’s surface will be more or less elliptical (an “oval”) with a 2:1 or 3:1 axial ratio, but a huge plate-boundary fault such as the Alpine Fault has a much more complicated geometry than even that ideal shape. So “triangle” is just confusing, unless you specify you’re referring to a portion of the projection of the fault’s position at depth onto the surface — and that projection abruptly stops southwest of Queenstown, something the real Alpine Fault doesn’t do in reality.

Please don’t use “fault line” when you mean “fault.”