Although the best seismologists are well aware of geochemistry, seismology is at heart physics: a highly abstract science informed by deep knowledge of how matter responds to energy. A seismologist usually wants to know how a particular mineral behaves under pressure, not how its isotopic balance, for instance, reflects its history.
But seismologists can bring insights to geochemical problems as well as geophysical ones—there's plenty of room for variety. You never know what problem will grab your interest. Remember that science is a team sport--any expertise that you lack, you can find in a collaborator. There is no one perfect seismologist.
What's important for your case is which subject you LOVE. You want to be your best at what you love the most, because if you're lucky, that's what you'll be doing your whole career.
I have been out of college for a long time, but I never regretted studying a wide variety of subjects rather than gaining a great depth in one subject. High school is too early to be so specialized. Even undergraduate study should be broad rather than deep.
That's one humble opinion.
