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.