Or, in other words - we know that all questions about earthquakes are also about seismology. Is it also true that all questions about seismology are about earthquakes? If so, then one of them should be synonym'd to the other, since one would want any question with one tag to also have the other.
I don't think that is a good idea. A question about earthquakes could be about more than seismology, for example rock mechanics, use of GPS (InSAR), tsunamis. Likewise, a question about tomography, seismic interferometry, or exploration seismology doesn't necessarily involve earthquakes.
It is certainly not the case that all questions about seismology are also about earthquakes. The entire field of reflection seismology has very little to do with earthquakes, so I think it would be a bad idea to synonymize the tags.
As an example, here's a diagram of a marine seismic survey: clearly seismology, equally clearly not an earthquake:
This is similar to the atmosphere -> meteorology synonym idea, which had detractors because questions regarding the atmosphere can be conceived that are not strictly the domain of meteorology (I don't agree 100% on this, but it is one of the viewpoints).
I'm not familiar enough with seismology to evaluate the merits of this synonym proposal. The best way to actually go about this is to just propose the synonym. You can do this by visiting https://earthscience.stackexchange.com/tags/seismology/synonyms and proposing the synonym and letting the community vote on it. To propose/vote you'll need 1250 reputation and at least 5 upvotes in questions/answers with the seismology tag.
Tangential to this, I'll be making a different meta post to encourage the use of at least one discipline tag in every question. While these tags may not end up synonymized, there is nothing stopping questions from being (re)tagged with both tags where appropriate.
I think the issue is not so much "do the two tags overlap perfectly?" but really "do we need both tags?". Is someone going to be interested in following the earthquake tag but not the seismology tag? Is someone interested in seismology but doesn't care about earthquakes?
Because in the end this is the purpose of tags: being able to find what interest you. Not labelling every small aspect of the site, just being able to single out your area of interest.
I'm one of those who thought meteorology and atmosphere shouldn't be merged. I'm of the same opinion here. I'll give some sample questions to illustrate:
- How is seismology used to detect minerals?
- How does seismology help predict volcanos?
- What manmade facilities should be relocated because of earthquake risks?