I think we have to look at the theme of the shark's pursuit to find out how music is reflective of the shark's terror.  The alternating lower chords, first slow, then in an accelerating pattern reflect the shark's ability to hunt.  "Nature's finest killer" senses his prey and then moves towards it with steady and almost horrifyingly brutal precision.  This is reflected in the music's alternating chords as they stay consistent in tone, and move with an increasing cadence to reflect that shark getting closer and closer.  Remember that the fear of the movie was that anytime you step in the water, the shark can target you.  The music hits this in between the eyes by making the audience feel that they are not merely watching the target of the shark, but rather they are the target of the shark.  In doing this, pure terror is revealed and experienced.