Tuesday, 10 March 2015

Volcanic whistles and more

We've been talking about science and being a scientist and girl power for a while, so I reckon it's time to get back to volcanoes. In August 2012 I went to Hawai`i for the AGU Chapman conference on Hawaiian volcanism. The lovely people at the Hawaiian Volcano Observatory (HVO) were nice enough to let me stay for a bit to get familiar with the volcano and to get some of their data to play with. It was also a good opportunity to catch up and collaborate with my friend and colleague Jess Johnson again. My supervisor Mark and I had decided that Kilauea would be a good volcano to study earthquake recordings.
What were we trying to achieve by studying Kilauea Volcano? The main question we had was 

"Do the continuous earthquake recordings look different for different types of eruptions?"

Intuitively one might say, of course there are differences (or at least that's what I would have said). But we wanted some real evidence. Kilauea tends to erupt most of the time, and the style of activity varies, so this was an ideal place for us to go and test our hypothesis. 
I got data from the continuous recordings of earthquake activity around Kilauea volcano for an eruption in the eastern section of the volcano (called the East Rift Zone, close to a crater called Pu'u `O`o) in 2007, and a similar eruption in 2011. These eruptions were related to magma breaking and pushing open a big crack a few kilometres underground. This is called a dike intrusion.
I also got data from a series of more explosive eruptions in the western part of the volcano, close to a crater called Halema`uma`u, in 2008. During the year 2008 these explosive eruptions slowly formed a connection between the magma supply at Kilauea and the surface. Now there is a lava lake that's something like 200 m across, and visitors can no longer access the crater like they used to. The explosions probably only moved magma around that was a few 100 metres below the surface, not kilometres like in 2007 and 2011.
So we had data for two types of eruptions:

fissure eruptions in the east (i.e., lava fountains)

vs. explosive eruptions in the west

We used a technique called Fourier Transforms to find out what frequencies the earthquake waves were composed of at the different points in time (if you want an analogue explanation for what Fourier Transforms do check out this old post). That way we can make what's called a "spectrogram". You can learn a lot of things about the earthquake waves by doing that. For example, you can get an idea if the movement of the ground is from an earthquake that happened close by, or from an earthquake that was hundreds of kilometres away. The graphic below shows the seismic ground movement (the black wiggly thing) and the spectrogram (the rainbow coloured rectangle) for a few days in 2011. I've added some labels to explain in a bit more detail what we can see on it.

Seismic ground movement and "spectrogram" from Kilauea Volcano during an intrusion and fissure eruption. The coloured spectrogram shows how much of each frequency we have in the seismic wave at each point in time. Red means a lot of that frequency, blue means not a so much of that frequency. The red diagonal streaks across the graphic are the volcanic whistles, described below, where the frequency goes up (or down) over time. You can just hear the first one when you listen to the audio from the link in the text below.

So by doing that we learned three main things about eruptions at Kilauea:

1) Fissure eruptions and dike intrusions at Kilauea really do generate ground movement that is different from explosive eruptions.

2) These fissure eruptions and dike intrusions show two phases of ground movement:
Phase I (purple in the graphic above): The first phase is made up of lots of short earthquakes, close to the dike intrusion and the eruption. These earthquakes are probably related to breaking the rocks when the magma pushes open the crack. 
Phase II (blue in the graphic above): The second phase starts a few hours after the first phase. It doesn't have as many short earthquakes, but instead shows continuous (small) movement of the ground for a few days. This continuous movement is what we call "volcanic tremor". Phase II happens quite far away from the eruption and the dike. 

3) The second phase has something called "frequency gliding" (the diagonal streaks in the graphic above). It means that the frequencies of the waves slowly change over time, a bit like a kettle on the stove that starts whistling at a higher and higher tone when the water is boiling.

To give you an idea what I'm talking about I've taken some of the ground movement and sped it up by a lot. You can play the movement that happened over roughly 1 day in just over 1 minute. That way we can actually HEAR the ground move. Click here to listen to the earthquake activity during the 2011 eruption at Kilauea. At first you just hear some noise like the wind. That's before anything is happening. All of a sudden (around 7 seconds in) you start hearing a lot of clicking sounds, maybe like gun shots or like rain drops on a metal roof. Those are the little earthquakes during Phase I. Then it gets a bit quieter again, and then you start hearing something continuous, like a boiling kettle (around 45 seconds). That's the volcanic tremor from Phase II. If you listen really carefully you can even imagine that you're hearing the frequency gliding, i.e., the whistling getting higher and higher.
You may remember a study in 2013 from Redoubt Volcano up in Alaska. Redoubt was also whistling, for around 1-3 minutes before some of the explosions that happened there in 2009. They called it "screams". It turned out that the screams where actually little earthquakes getting closer and closer together in time, until you can't distinguish them anymore and they're just one continuous scream. 
The screaming or whistling at Kilauea is quite different: It's really slow and lasts for many hours. Nobody has seen gliding that lasts this long anywhere before. Also, the little earthquakes that you can hear in the beginning actually SLOW DOWN before the whistling starts, so the explanation from Redoubt doesn't work here. Many other models that explain this type of behaviour can't produce whistling that would last for several hours, so we spent some time exploring what could generate a signal like that. In the end we decided that the Kilauea tremor and whistling may be related to bubbles in the magma: We think that it's possible that gas bubbles in the magma reservoir beneath the western crater Halema`uma`u can form "bubble clouds", or areas where lots of bubbles collect in one place. These bubble clouds can start swinging, or oscillating, if there is magma flow or something else that can start the oscillation. This swinging is transferred into the ground. When the magma flow changes (for example when a crack breaks open somewhere else in the system, like the dikes in 2007 and 2011) the frequency of the bubble cloud tone can change, and produce the whistling that we observe. 
To know whether that is actually what was happening at Kilauea we would need some more info, for example a detailed study of where exactly the whistling was coming from on the volcano. However, it was still really interesting to see how by comparing the earthquake recordings from several different eruptions we were able to identify similarities and differences, and how that - in combination with other observations during those eruptions - made it quite tricky to come up with possible explanations for what we observed. Many studies focus on just one eruption, but we showed that we can learn a lot by looking at the bigger picture.
If you're still reading this you really must have a lot of spare time, so feel free to check out the journal article that we wrote about all this.

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