The earth is so big, and as I understand, constantly moving and roiling away inside – why isn’t there a constant low level rumble?
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There is a detectable rumbling, and instruments built specifically to measure it can record it. But since everyday tectonic movement doesn’t really make a difference to how we live, there’s no reason for our bodies to have adapted to sense that part of our environment. And when seismic events do affect our lives, it’s often at frequencies that we don’t really consider to be sound. The seismic waves in earthquake events are primarily between 10 hertz and 1/100th of a hertz, which are significantly lower than the lowest frequencies we perceive as sound. Below about 20 hertz we stop ‘hearing’ pressure waves and experience them more as bodily thuds.
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