<p>When in the swamp, Toph can sense things from around the world. Unless we're talking earth quakes, that goes pretty far beyond what ennhancing vibrations should be able to do, especially since the spirit vines aren't even at the locations she's sensing.
</p><p>While it's true that airbending is considered "the most spiritual element," firebending also has a significant spiritual component. It's not unreasonable that spirituality would make an earthbender more powerful.
</p><p>Obligatory plug: In <i>Blood & Steel</i>, I had a similar plot point, about how meditation, seismic sense, & spiritual awareness could form a positive feedback loop, since they're all focused on intense awareness of one's surroundings.
</p><p>Though, technically Aang's issue wasn't with earthbending's lack of spirituality, it was with its "stand your ground" mentality.
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