I'm told that the signal in the graph below was was obtained was obtained by passing a seismogram through a highpass filter to obtain the high frequency-small amplitude waves common in tremors far from the epicenter of an earthquake.

![High pass filtered seismogram][1]

The high pass filter used to unmask the tremor was selected to avoid avoid the 'attenuation' of the tremor signal.  What exactly is meant by 'attenuation' in this context?

  [1]: https://i.sstatic.net/uajxw.png