Tuesday, March 22, 2011

Earthquakes - Exploring our website data - Part two.

Quake Drums : 101

The most common means of displaying seismic monitoring data is called a seismogram. Traditionally these were drawn by ink on paper, but today are usually created by computers using digital data.
You read a seismogram like you are reading these words; the earliest time is at the top left and then goes along the line to the end and then starts at the left of the next line. The top-left is therefore the oldest data and the bottom-right the most recent. All of our drums cover a 24-hour period.

The drums on the GeoNet website do not display 'real-time' data as the volume of data required to be sent across the internet is too great.  Instead, each seismogram is produced every 5 minutes and the web page updated as each new image becomes available.

An earthquake typically is a signal that is much larger then the normal background. A nearby earthquake usually has a very sudden onset and decays away relatively quickly (usually less than 5 minutes). With a more distant earthquake you may be able to see both the P- and S-waves and the earthquake decays away more slowly. Because of the limited resolution of the seismograms you cannot always see the both the P- and S-waves, especially for nearby earthquakes.

The traces are coloured in dark blue and purple so it is easier to see which data corresponds to each line. A seismogram is coloured red if is clipped, ie. the largest parts of the signal are not shown. If this was not done then a large earthquake would obscure much of the seismogram from view. In other words, if the signal is red the real size is larger than is shown on the seismogram.

Seismograms are not all at the same scale. This is as some sites are noisier than others and tend to be plotted at a scale that shows a smaller signal (these sites have a larger scale number). To plot these sites at the same scale as nearby, quieter sites would result in a seismogram completely blue (or red) and would show no useful information.

The seismometers that we use are very sensitive instruments and can sometimes be triggered by unrelated phenomena like weather, traffic or movement of livestock.

NOTE: Not all earthquakes that are widely recorded on these traces are felt by people.




  1. So the "P" wave is the rumble you can hear before the shaking starts. Does that mean that the more rumble you can hear the further away the epicentre is? (for trying to guess before geonet catches up)

  2. It varies with depth, size etc but it may give you an indication of it.