Seismic amplitude distribution
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:


Prompt gravity signal induced by the 2011 Tohoku-Oki earthquake - could this really be useful for warnings?
Earthquake-induced prompt gravity signals identified in dense array data in Japan
Gravity sensors might offer earlier warning of earthquakes


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...