One eye, two eyes, red-ite, blue-ite
Wandering the poster halls at AGU one could be excused for thinking that poster printers had a discount for use of red and blue ink, so common were the two colors on seismological posters. Just like political maps now use red for Republicans and blue for Democrats to the point where we talk of red and blue states, in seismic tomography red is stuff that is seismically slower and blue seismically faster; this wasn’t always the case and it is mildly amusing to look back to see where this arose.
Of course part of this is the development of color in scientific publications. While color was present in presentations long ago, it wasn’t common because figures were handdrawn. Making the same figure twice—once in color and once in black and white—was not a good use of funds or time.
When seismic tomography was first developing in the late 1970s and 1980s, most publications were still black and white or had hefty charges for inserting color figures. So, for instance, Kei Aki’s foray into making a tomographic map of California (Kei wrote one of the base papers behind seismic tomography) used black and white contours as did Sue Raikes’s results for southern California, though Raikes added patterns to the highs and lows. When Gene Humphreys made his early tomographic maps while working with Rob Clayton, they used black and white patterns with whiter areas having lower velocities and blacker areas were higher velocities.
Because much of the tomography being done was imaging the mantle, where the presumption was that most of the seismic velocity variations were from temperature, putting red at the slow end and blue at the fast end of a color bar made a lot of sense as red converted warmth and blue cold. GG can’t say when this started, but it may well be back in oral presentations in the 1970s (poster presentations were rare back then). The convention was strong enough that Gene Humphreys and Ken Dueker, in discussing their work on imaging the mantle in the western U.S. in the early 1990s, often posed the question of “what was the green mantle”? (Green in their maps was intermediate from red to blue).
Anyways, move forward to today and that use of colors is deeply entrenched, but the implications suggested might be too simple. Slow material (“red-ite”) might be wet, it might have a different orientation of seismic anisotropy, it might be more fertile mantle, in addition to possibly being warmer. “Blue-ite” could be the opposite of those things. The problem in some sense is that with the colors so deeply entrenched and the interpretation so well known that many non-seismologists (and, to be fair, a lot of seismologists) take these colors as direct proxies of temperature, which they are not (we should discuss another day the issues with the lateral variations in velocity we usually show and how challenging those can be to interpret in a cross section).
Anyways, just a thought. If you see red and blue seismic anomalies, “red-ite” and “blue-ite” might be better terms than warm and cool…