What causes sounds that are lower than 0 decibels?

So 0 dB does not mean no sound, it means a sound level where the sound pressure is equal to that of the reference level. This is a small pressure, but not zero. It is also possible to have negative sound levels: - 20 dB would mean a sound with pressure 10 times smaller than the reference pressure, i.e. 2 μPa.

answer a fan shaped mass of sediment deposited at the base of a mountain front do to an abrupt change in stream gradient;