The Santa Maria volcano is an active volcano in Guatemala and is part
of a mountain range known as the Central American Volcanic Arc.
This extends from southern Mexico to Panama and has its origin in the
subduction of the Cocos Plate under the Caribbean Plate.
In 1902, an extremely strong eruption of Santa Maria occurred.
Within two days, 5.5 cubic kilometres of magma were ejected.
This corresponds to about 2,500 times the size of the Olympic Stadium
in Berlin or about one million gymnasiums.
This made the eruption of 1902 the third-strongest in the 20th century
worldwide.
Twenty years later, a new volcano began to form on the southwest flank
of Santa Maria.
This is called Santiago.
After a strong eruption in 1929, Santiago is still very active.
Several times a day, there are usually small explosive eruptions in
which mainly gases, water vapour and small rocks are ejected.
Hello and welcome.
In order to better understand its volcanic activity, Santiago has been
monitored by multidisciplinary teams for many years now.
A combination of various methods can help to provide an early warning
system against any strong eruptions.
In this video, I shall introduce you to a number of these measurement
methods as well as their evaluation.
Seismic networks, for example, can detect shocks from earthquakes or
explosive eruptions.
These events are then localized so that the spatial and temporal
earthquake distribution in a subsurface can be understood.
The comparison of seismic data with infrasound measurements and
thermal images serves to characterize the repeated eruptions and map
systematic processes.
To monitor the volcanic activity on Santiago, the Geophysical
Institute of the Karlsruhe Institute of Technology, KIT, installed
four seismometers within 8 km of Santiago's craters in 2018.
Three instruments had broadband sensitivity, while one was a short
-period seismometer.
Using this network, both tectonic and volcanic earthquakes in the
subsurface as well as eruptions of Santiago were to be observed.
Here, you can see the recording of a tectonic, that means a non
-volcanic earthquake about 100 km away by the vertical component of
station STG 8.
Under the seismogram, the corresponding frequency spectrum is shown.
Here, you can clearly see the onset of the P-wave.
It can be recognized as very abrupt with frequencies between 2 and 10
Hz.
About 15 seconds later, the higher amplitude S-waves follow with a
similar frequency range.
For comparison, here is a record of an explosive eruption of Santiago.
First, note that both the amplitude and timescales have changed.
The eruption signal is significantly shorter and weaker here than the
earthquake I just showed you.
In the spectrogram, it can be seen that a seismic signal of an
explosive eruption builds up more slowly than the abrupt P-wave onset
of a quake.
As a result, the initial onset is less clear.
The excited frequency range of 1 to 5 Hz is considerably narrower.
Characteristics such as the excited frequency range as well as the
waveform itself are used to categorize recorded seismic events.
By using travel-time differences between individual measuring stations
and wave types, the seismic events can then be localized.
By also using spectrograms, it can be determined whether one is
dealing with a distant tectonic or a close, possibly volcanic
earthquake or even eruption.
Further distinguishing features can be measured using infrasound
waves.
These are used, for example, to characterize eruption processes and to
monitor rock falls or impacts.
Infrasound refers to sound at frequencies below the human hearing
threshold of about 20 Hz.
At high acoustic levels, however, they are also perceptible to humans.
Infrasound has the property that it hardly decays even over long
distances, since the waves are barely scattered due to their low
frequency and therefore long wavelength.
Large volcanic eruptions can even be measured globally within the
infrasound range.
Infrasonic sensors can be constructed like microphones so that they
are sensitive to low oscillation frequencies.
Oscillating membranes generate electrical currents from which the
sound pressure can be calculated.
In order to protect infrasonic measurements from wind, tube systems
are also installed, which can pick up sound from different directions.
The tubes open into a common chamber, where tiny pressure fluctuations
are measured using a microbarometer.
This setup ensures that the influence of wind is reduced.
This illustration shows a comparison of infrasound and seismic
observations for an explosive eruption of Santiago in 2014.
In black, you can see a two-minute recording of the vertical component
of a seismometer that was installed about 500 meters north of the
active crater.
The three coloured traces are infrasound recordings from about 700
meters to the east.
At that site, an array of three instruments was built to locate the
direction of the sound signal.
Here you can see a clear signal on all the recordings.
The bottom diagram shows cross-correlations between the three
infrasound transducers.
In a cross-correlation, the time delay between two signals is
determined.
To do this, the infrasound signal of one station is shifted piece by
piece versus another and both traces are then multiplied and
integrated.
Within the diagram, it is now evaluated how similar traces differ in
time.
At stations 2 and 3, the signals arrive almost simultaneously, where
there are differences of about 0.05 seconds between the other two
station combinations.
With a sound velocity of 343 m per second, this results in a
difference of about 17 m.
With several such measurements, the position of the sound source can
be located using infrasound arrays.
In this case, Santiago's eruption crater was identified as the source
of the infrasound.
This is how seismic signals and infrasound could be used to localize
the volcanic activity.
In addition, infrasound also allows the path of the ejected rocks to
be analyzed.
You can see this here.
In this six-minute sequence, you will again recognize the seismogram
and infrasound measurements.
But the cross-correlations now show a different picture.
The time delays are no longer constant as they were before, but now
show systematic trends.
This means that the distances to the measuring instruments change.
This observation is therefore interpreted as erupted and impacting
rocks with diameters of up to 10 m.
When impacting on the flank of the volcano, the boulders repeatedly
produce sound signals that can be localized by the measuring array.
In addition, infrasound measurements and seismic recordings can be
combined with geological and geochemical surveys to assign ejected
rock, lava and ash to individual subsurface reservoirs.
Also, deformations of the volcano can be recorded using special
cameras.
These, for example, show uplift of the crater of just a few decimetres
just before Santiago erupts.
These observations can in turn be combined with thermal images showing
the development of the volcano's external temperature.
Thermal cameras work in a similar way to other optical cameras.
The only differences are the electronic sensors, which are not
sensitive to visible light, but rather infrared light.
Infrared covers wavelengths from 770 nm to 1 mm.
With decreasing wavelengths, thermal radiation increases, which can be
measured as follows.
The shorter the wavelength, the warmer is the radiation source.
Here on the left, you can see such a thermal image, which was recorded
about 16 km south of Santiago.
The colours indicate the temperature measured from a distance, which
here in Santiago's two crater openings is 60°C and more.
During strong eruptions, temperatures of up to 500°C could be measured
in the crater.
In the background, you can see the outline of the Santa Maria volcano,
which gave birth to Santiago about 100 years ago.
In this video, I have introduced you to the physical measurement
techniques from the field of volcanology and how they are used to
study Santiago in Guatemala.
You have seen how frequency spectra can be used to characterise
seismic events.
Seismic travel time analyses can be used to localise the events.
Infrasound signals supplement these measurements and provide
information about moving objects such as erupted and blasted rock
material.
In addition, thermal images can provide information about temperatures
at the crater rim and erupted gases.
The combination of different methods makes it possible to understand
the eruptive processes more and more, so that future eruptions can be
predicted better.