Faulty Lines

In light of a newly published pop-economics book, Fault Lines: How Hidden Fractures Still Threaten the World Economy, here is a redacted (***) and edited (...) message I sent to a well-known blogger a couple of years ago:

I'm a working geophysicist (for close to 40 years in industry and academia), among other things (and faithful reader of *** -- good work!). Please allow me a nerdy quibble. 

There is a "geological" phrase -- fault line -- that began to appear in the popular press (including the Internet) a decade or more ago (maybe a Lexis-Nexis search is in order to pin when down); you use the phrase in this morning's post (http://***).

Invocation of the phrase is, I take it, to also imply that earthquakes may or actually do occur along the line's extent. So, its usage is apparently clever and cosmopolitan.  However, fault line, is not, in fact, a commonly utilized term among geoscientists (if you go to the US Geological Survey website and search for it, you'll find very few hits).

Geologists study the rocks up close and personally (... to invoke a media term...); geophysicists study at distance rocks and processes (such as earthquakes) that affect rocks. A fault is a surface that separates bodies of rock on either side and along which there has been relative displacement of the two bodies of rock. (A fracture is also a surface which may or may not have had relative displacement of the separated rock bodies. Thus, faults are subsets of fractures.)

Faults (and fractures) are in effect two dimensional features in three dimensional space. The San Andreas fault stretches from north of San Francisco to almost the Mexican border. In three dimensions, it is like a curtain (another two-dimensional object) separating rocks to the west (the California Coast Ranges plus the Los Angeles Basin) from the rest of California to the east. Those rocks on the west are more-or-less part of the huge Pacific plate, and are moving continuously to the north-northwest along the fault and relative to the rest of North America.

The emergence of a fault at the earth's surface is preferably referred to as, not a line, but a trace. Earthquakes along any fault are centered at depth (the hypocenter). A point on the earth's surface directly above the hypocenter is termed the epicenter, which may or may not correspond with a point on the fault trace (depending on whether the fault's surface is vertical or inclined).

I'd like propose an alternative usage of this metaphor... "one of the primary fault surfaces separating... "... instead of "one of the fault lines separating..."

By using surface instead of line, additional (vertical) dimensionality is added, rather than a surficial and superficial meaning. Further, the fault surface could be crinkled and bent. When I've seen this meme (Dawkins' word...), I often had the impression that the author meant to imply greater depths of meaning than the one-dimensional term, line, implies.

Sorry to waste your time, but you're the first journalist I've ever written about this -- mainly because I like your work so much. (I suppose you could run this by ***a more technical blogger***; he might then get into Euclid's Elements and who know what else, though.)