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



The high-pass filter used to unmask the tremor was selected to avoid the 'attenuation' of the tremor signal.  What exactly is meant by 'attenuation' in this context?
Attenuation in this context refers to a reduction in amplitude of some signal component as it passes through a system (in your case, the system being the highpass filter. Linear filters are designed to pass certain frequency components while stopping (or attenuating) others. As the name implies, a highpass filter is designed to pass high-frequency content while attenuating low-frequency content.

As you noted, the tremor features that you're interested in are high in frequency and small in amplitude. Thus, they may be masked by low-frequency components in the original seismogram. A highpass filter can be used to accentuate the high-frequency tremor waves by attenuating any low-frequency waves that might also be present.