Seismic Monitoring of Volcanoes

Figure 1: A seismogram showing a typical earthquake

Seismic monitoring of volcanoes is so popular because:

• many volcanic eruptions are preceded by anomalous seismic activity;
• in its simplest form a seismic monitoring system is relatively cheap to set up and operate;
• there are numerous freely-available data analysis programs;
• a single monitoring station can record data from several volcanoes at once;
• it provides a continuous source of data, allowing rapid changes in a volcano to be monitored;
• there is a large volume of scientific literature published on seismic monitoring data that can help scientists in their interpretation of seismic data.

Figure 2: Earthquake signals are typically larger than background noise.

This does not mean that the method is without its problems:

• it is subject to potential interference from other sources of ground vibration such as wind and vehicles;
• a poorly-chosen site can sometimes, because of the interference noted above, be almost useless for seismic monitoring;
• because it provides continuous, high-resolution information the volume of data collected (measured by the amount of computer disk space needed to store it) is more than 100 times that of the next most data-hungry method (continuous GPS measurements).

Figure 3: Example of P- and S- waves, other earthquakes are circled.

How do volcanologists display seismic monitoring data?

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 the seismograms shown here cover a 24-hour period.

Figure 4: Typical wind noise on a seismic record.

Why can't I see the ground moving; why is the image static?

The seismograms shown on the GeoNet website do not display 'real-time' data. The volume of data required to be sent across the internet to show the ground as it moves is too great to display all the seismograms in 'real-time'. Instead, each seismogram is produced every 5 minutes and the web page updated as each new image becomes available. If you want to keep track of changes over a period of time remember to hit the 'refresh' icon on your browser every so often.

Figure 5: Traffic produces a lot of ground noise and makes volcano monitoring difficult.

What is the resolution of the seismograms?

GeoNet seismographs typically record the ground movement 100 times a second. To download a seismogram at this resolution from a web page would take a long time, even using fast broadband. To reduce the download time we draw a seismogram that displays only one data point each second. You can see the main features of a seismogram, but not all of the detail. GeoNet scientists use these seismograms for a 'quick look' at what is happening and other kinds of seismograms if they want to look in more detail.

What else do the seismograms show?

The label on the top-left of the seismogram in Figure 1 (i.e. OTVZ/10-EHZ/CH) says exactly which seismograph's data are shown (see the FAQ). On the top-right is the time of the most recent data (i.e. 2007/08/25 17:40:00 NZST). On the right side is the plotting scale (i.e. Scale: 200) and along the left and bottom are the time axes.

Figure 6: Noise from individual vehicles.

Are all seismograms plotted at the same scale?

No. 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.

What does an earthquake look like?

An earthquake typically is a signal that is much larger then the normal background (see Figure 2). 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 (see Figure 3) 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.

Figure 7: Noise originating from the Turoa skifield chair lifts.

What do the 'red bits' mean?

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.

Figure 8: An example of a earthquake swarm.

What about noise sources?

The main noise sources on seismograms are wind and vehicle traffic.

In Figure 4 wind noise is noticeable between about 13 and 8 hours ago, there is a short calmer period and then stronger wind starts (around 7 hours ago) and continues until the end of the seismogram (see Figure 4).

In some cases individual vehicles can be seen and in other cases there are so many vehicles that the ground vibrates continuously and looks like wind. In Figures 5 & 6, we can see individual vehicles going by Tukino on Mt Ruapehu, or from this site.

Figure 9: An eruption earthquake, recorded at Ruapehu.

Near Rotorua where daily traffic produces a lot of ground noise we see a continuous vibration signal, and this makes volcano monitoring difficult. Other sites sometimes show noise that is particular to that site. Examples include seismographs on Mt Ruapehu that record noise produced by skifield chair lifts (see Figure 7).

What is happening if there are lots of earthquakes?

In the central North Island volcanic region (Ruapehu to the Bay of Plenty coast) earthquakes frequently occur as swarms. These are sequences of many earthquakes, usually in a short time period (hours to days), without a single clear largest event. Swarms are often seen on seismograms from monitoring sites at or near the volcanoes.

Figure 10: Volcanic tremor can look very similar to wind noise.

The seismogram in Figure 8 is from Handcock Road (a site between Taupo and Rotorua) shows a typical earthquake swarm, starting at about the 12 hour mark and continuing until the end of the plot. The larger of the earthquakes would be locatable, and some of those probably felt nearby.

What do volcanic eruptions look like?

Explosive volcanic eruptions usually produce a distinct earthquake signal that can be seen on seismograms. These signals usually have several peaks and without a sharp onset. The size of the signal will usually be quite large so some parts of these events are often coloured red (clipped).

Figure 11: An example of RSAM from Ngauruhoe. The large peak on October 16 is caused by a magnitude 6.7 earthquake in Fiordland.

In Figure 9 the signal is from the eruption of Ruapehu at about 8.30 pm on September 25 2007. The seismogram is from a sensor at Whakapapa Village about 9 km from the crater.

Do all signals that look like an eruption actually mean that an eruption has occurred?

No. Volcanoes like Ruapehu and White Island frequently have earthquakes that are almost identical to those produced during an eruption, but no eruption occurs. If an eruption has occurred it will always be confirmed on the GeoNet home page.

Figure 12: RSAM plot for Ruapehu with volcanic tremor.

Are there other signals generated by volcanoes that are not earthquakes?

Yes. Some volcanoes produce a signal called 'volcanic tremor'. This is a continuous or semi-continuous ground vibration produced by degassing, underground boiling, magma movement, ash eruptions and a range of other sources (see Figure 10). Scientists often do not know the exact cause of volcanic tremor.

Figure 13: SSAM plot for Ngauruhoe, showing wind noise.

What does volcanic tremor look like?

On the seismograms volcanic tremor can look very like wind noise, and it is difficult to distinguish the two signals using just the seismograms (see Figure 10).

Figure 14: SSAM plot for Ruapehu with volcanic tremor.

Are there any other means of displaying seismic data?

Yes, volcanologists frequently use two other display techniques. These are called RSAM and SSAM.

Figure 15: RSAM and SSAM plots showing affect of wind.

What is RSAM and what does it show?

RSAM stands for Real-time Seismic-Amplitude Measurement. It represents the overall signal size over periods of 10 minutes. In situations when the number of earthquakes is so high that individual earthquakes can't be seen, or the level of volcanic tremor is such that seismograms no longer show a change in signal level, then RSAM is an excellent way of showing changes with time (see Figures 11 & 12).

A problem with RSAM is that it measures the overall signal size regardless of what produces the signal. If the signal is caused by volcanic tremor then RSAM is very useful. But when the wind blows strongly the RSAM value will still go up and scientists don't learn anything about the volcano, only the weather!

Figure 16: RSAM and SSAM plots showing volcanic tremor.

An example of RSAM from Ngauruhoe in October and November 2007 is shown in Figure 11. The large peak on October 16 is caused by a magnitude 6.7 earthquake in Fiordland. The other smaller and wider peaks are all due to ground shaking caused by strong wind.

In Figure 12, peaks in RSAM caused by volcanic tremor at Ruapehu in July 2006 are shown. On its own there is nothing about the RSAM peak that tells you if it is caused by volcanic tremor or say wind or some other non-volcanic source.

What is SSAM and what does it show?

SSAM stands for Seismic Spectral-Amplitude Measurement. It shows the relative signal size in different frequency bands (see Figures 13 & 14). Different seismic signals have energy at different frequencies. To use a musical analogy, different seismic signals produce different notes or tones; SSAM shows the strength of each tone in the overall sound. The stronger the signal the 'warmer' the colour. Purple is the coolest colour, while red is the warmest.

Using SSAM it is possible to get an idea of whether a signal is produced by earthquakes, by wind or traffic noise, or by volcanic tremor. Earthquakes, wind and traffic noise all tend to have energy at a wide range of frequencies (a wide-band signal), while volcanic tremor tends to have energy in a more limited range of frequencies (a narrow-band signal).

Which is best, RSAM or SSAM?

Neither. The key is to look at RSAM and SSAM plots together. RSAM will give a measure of the signal size and SSAM more information on the likely source of the signal. See how the wind differs from the volcanic tremor in Figures 15 & 16. Sometimes the wind is more prominent than the volcanic tremor, hence some of the difficulties volcanologists face understanding volcanic activity.