Can amplitudes be used to differentiate one signal from another? I have use case, where there are lot of sensors placed under ground.The sensors give amplitudes each second.So my aim is to detect if some anomalous activity takes place at that point.

IF an activity such as digging takes place, there will be variation in amplitudes, so can that data be fed to a machine learning model and find the anomalous activity taking place?

I have signatures of activities like drilling,digging,augering etc.But my question is amplitude a unique feature here or do I need to consider things like frequency?
Taking FFT and studying the frequency of each activity would be helpful. Each activity should have its own unique frequency. You can use combination of both if any two activities have the same frequency or amplitude.