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Recorded (or raw) and processed pre/post-stack seismic data (relative amplitudes) usually display a standard normal distribution, albeit with a very small amount of skewness.

What would be possible causes and/or significance if amplitudes did not retain a mean value of zero (or very nearly zero)?

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I'm sure there are several possible reasons.

I'm not a seismologist, this is an amateur answer only.

Tidal forces

Local acceleration has very small long-term drifts due to tidal forces from the Sun and the Moon. These will have periods of roughly 1 day as the Earth rotates. Most seismometer data will have been run through a high-pass filter with a much higher cutoff frequency than this (of course along with other filtering). This might be implemented with some baseline restoring function other than a simple filter.

Real changes in local g due to displacement in the Earth

Prompt gravity signals -- changes in local g as mass distribution in the Earth changes -- have been detected, and this kind of measurement may become more widely used in the future. Of course a typical seismometer will not be sensitive to pick this up, unless of course your seismometer is sitting atop a substantial geological event!

See:

Instrumental effects

  • Drift due to thermal changes
  • Tilt due to geological changes (the z accelerometer is no longer pointing in the same direction)
  • Tilt due to improperly mounted seismometer that was shaken loose by the earthquake
  • others...
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