<p>I'm sure there are several possible reasons. </p>

<p>I'm not a seismologist, this is an amateur answer only.</p>

<h2>Tidal forces</h2>

<p>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.</p>

<h2>Real changes in local <em>g</em> due to displacement in the Earth</h2>

<p>Prompt gravity signals -- changes in local <em>g</em> 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!</p>

<p>See:</p>

<ul>
<li><a href="https://earthscience.stackexchange.com/q/9192/6031">Prompt gravity signal induced by the 2011 Tohoku-Oki earthquake - could this really be useful for warnings?</a></li>
<li><a href="https://earth-planets-space.springeropen.com/articles/10.1186/s40623-019-1006-x" rel="nofollow noreferrer">Earthquake-induced prompt gravity signals identified in dense array data in Japan</a></li>
<li><a href="https://phys.org/news/2016-11-gravity-sensors-earlier-earthquakes.html" rel="nofollow noreferrer">Gravity sensors might offer earlier warning of earthquakes</a></li>
</ul>

<h2>Instrumental effects</h2>

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

CC BY-SA 4.0
2019-07-13T10:25:59.653Z
2019-07-13T10:25:59.653Z
uhoh
<p>Recorded (or <em>raw</em>) and processed pre/post-stack seismic data (relative amplitudes) usually display a standard normal distribution, albeit with a very small amount of skewness. </p>

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

<seismic><statistics>
Seismic amplitude distribution
nate
# Seismic amplitude distribution

(Asked by: username_0 on 2019-07-11)

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)?

**Tags:** seismic, statistics


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## 1 answers

### Answer by username_1 on 2019-07-13. Score: 2

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?](https://earthscience.stackexchange.com/q/9192/6031)
* [Earthquake-induced prompt gravity signals identified in dense array data in Japan](https://earth-planets-space.springeropen.com/articles/10.1186/s40623-019-1006-x)
* [Gravity sensors might offer earlier warning of earthquakes](https://phys.org/news/2016-11-gravity-sensors-earlier-earthquakes.html)

## 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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