Noise the intensity of which is so low that it can only be detected by means of a microphone and suitable amplifying equipment. How these subaudible noises, or microseisms, originate has not definitely been established, but it is believed that they are produced by incipient cracking or intermovement between fragments of crystalline aggregates in the rock. It has been found that almost invariably a period of increased microseismic activity precedes any large-scale ground movement or failure.