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.

Iceland vs. Japan - the art of eruption forecasting

Finally I'm getting around to writing a new post, after I've taken my summer break since the end of the last term.

Work is in full swing again, undergrads are back, and campus is as busy as ever. After some intense work over the summer I managed to finally submit my manuscript about Hawai`i tremor. Fingers crossed that it gets accepted!

In the meantime, lots of volcano-y things have been happening, so an update is well overdue. Everybody has heard about the eruption of Bárdabunga, of course. We know that a dike (a vertical crack in the rocks, filled with magma) pushed its way through the Earth's crust for quite some time, before it reached the surface and started a stunning fissure eruption. How do we know that? Because lots of earthquakes happened underground where the dike was breaking its way up! But all this is, of course, yesterday's news - and I'm sure many of you have read tons about this eruption and seen some of the spectacular videos and photos.

Another big event was the eruption of Ontake-san last weekend. Pretty much out of the blue this volcano started to erupt explosively - and in the process sadly took many lives. Volcano disaster wise in Japan, this is about as bad as the 1991 eruption of Unzen, which killed over 40 people. After the Ontake eruption some people claimed that the disaster could have been avoided. But the truth is, from what I've seen in terms of data it was very difficult, or maybe even impossible, to see this coming. Why is that?

1. The eruption appears to have been a so-called "phreatic" eruption. That means that instead of magma pushing upwards through the crust, water was seeping into the volcano. This (cold) water probably reached a hotter region underground, where it immediately turned into steam. This steam wanted to rise and expand - it increased the pressure underground which then lead to the explosive eruption. A very similar thing happens in your kitchen: Have you ever heated up a pan or pot without anything in it, and then poured water onto the hot surface? You immediately get a big sizzle and lots of steam.

When scientists analyze the ash from this eruption, they will probably find mostly fragments from old rock that was broken into ash, and probably not many fresh magma pieces. Because no (or very little) fresh magma pushes upwards during these kinds of eruptions usually there aren't many precursors. No large numbers of earthquakes like we had in Iceland just a few weeks earlier, no big changes of the shape of the volcano like there was before the eruption of Mount St. Helens in 1980.

2. That "nothing" was happening on the volcano before the eruption is not 100% true. Since mid September there had been some more earthquakes than usual. However, the highest numbers were recorded on Sep 10 and 11, and they went down again afterwards. Furthermore, these "seismic crises" aren't unusual on volcanoes. Ontake had very similar periods with increased earthquake activity for example in the mid 90s, without eruptions following. Other volcanoes such as Long Valley caldera in California frequently have earthquake swarms - the latest one just a week ago, yet it hasn't erupted in the last 10,000 years or longer. Based on what we know about volcanoes, earthquake swarms CAN mean an eruption is coming, but they don't mean that an eruption HAS to happen. Often other warning signs accompany or follow earthquake swarms, in which cases eruptions become easier to forecast. These other warning signs could be a change on the volcano shape because of magma pushing rock out of the way, or more gases coming out of the volcano. Whereas in Iceland we had some idea what was gonna happen, in Japan we just couldn't see it coming. Despite all our research and efforts, unfortunately we aren't at a point where we can completely understand and forecast the processes happening below our feet in volcanically active areas.

In the case of Ontake, around 10 minutes before the eruption started another earthquake-like signal showed up on the instruments: Volcanic tremor. I've talked about tremor in one of my very early posts, but it might be time for a little update.

Volcanic tremor is a little bit like an earthquake, but with two main differences:

• Tremor ground oscillations are usually a little bit "slower" than earthquake ground oscillations: Whereas earthquake oscillations go back and forth anywhere between say 1 and 25 or more times per second, tremor oscillations only make it up to 5 or 10 times per second for one full cycle of back and forth.
• Tremor can go on for a really long time: Whereas earthquakes are usually over after a seconds, tremor can last for minutes, or hours, or days.

Luckily tremor usually only happens very close to the volcano, and the shaking is very small, so people don't usually feel it - otherwise shaking going on for several days or longer might be quite annoying. Yet, we can record these oscillations on our seismometers and usually when we see them we keep a good eye on the volcano to make sure we don't miss any eruption warning signs. Something like 2/3 of all tremor cases happen just before or during eruptions - but that also means that 1/3 of tremor cases don't appear to have anything to do with eruptions. That's why tremor isn't a very reliable warning sign - certainly worth to keep an eye out for but not a unique sign that something is about to happen. Lots of people have had ideas about what causes this tremor signal, but unfortunately many of these studies don't agree with each other, or only work for one specific volcano. In my research I study tremor from volcanoes in lots of different places: Hawai`i, Alaska, Latin America, ... I am trying to find out whether there are different tremor "types", that can tell us more about what causes tremor in different places. That way, maybe one day it will be easier for us to know whether the tremor that we record on our instruments is just harmless, or whether it tells us to get the hell out - and maybe disasters like the Ontake one can be avoided in the future!

What happened at Ontake is certainly worrying - after all there are lots of other volcanoes in the world and other "blue sky eruptions" (i.e. without clear warning signs) might happen elsewhere. Some people here in the Pacific Northwest started to worry a bit, and a radio station got in touch with Mark and me to check whether they could ask some questions in a radio interview. Of course I said yes, after all I love talking about volcanoes and I thought it could be fun. I expected that they would ask me some questions and then cut it and broadcast it at some later point in time. Instead, the whole thing was a 30 minute live interview - which I only realized as we started the interview! Whoops... That made it of course slightly terrifying, after all I hadn't ever given a radio interview. I also felt a little bit weird, sitting alone on the phone in one of our meeting rooms at work and yet talking to anybody who was listening to the radio station at the time. In my surprised state I probably sounded like a complete fool, and most likely made something like 80 out of "100 mistakes scientists make when talking to the media". But what the heck, everybody has to start somewhere, after all! If you're interested you can listen to or download the podcast here - don't judge me too harshly though! Thanks to Cfax 1070 and Terry Moore for hosting me - it was definitely a fun experience :)

Back with a BANG: Volcanoes 2014 and eruption forecasting?

Finally the silence is over. Happy New Year to everybody - we're just gonna ignore the fact that it's already Feb 6th.

I'm gonna start the year with an issue that has come up quite a bit lately when talking with friends and family... Eruption forecasting. Yep, I said it, the dreaded term. Sad events like 15 deaths due to the latest activity at Mt. Sinabung in Indonesia bring the forecasting topic into the focus of the public from time to time. 

Most recent eruption at Mt. Sinabung, Indonesia, Feb 1, 2014. Image from Twitter, @BBCBreaking.

So let's look into this a bit more. We're gonna learn about what signs of volcanic activity there are are at the surface, what we can do to monitor them, and what the difficulties with forecasting are.

To explore this topic in the detail it deserves, however, we need to start with something very basic: The difference between "forecast" and "prediction". If you look up the two words in a dictionary you will most likely find little difference between their meanings, often they're even listed as synonyms of each other. In science, however, things are a little different. In particular, in seismology (the study of earthquakes) the two terms have very distinct meanings: A "forecast" assesses the likelihood of an earthquake of a certain magnitude in a given area and time span, e.g. "there is a 1 in 10 probability that a magnitude 7 earthquake will occur in the Pacific Northwest in the next 100 years" (and of course I made this one up). A prediction, in contrast, is much more specific than that, e.g. "a magnitude 7 earthquake will occur within 100 km of Vancouver on Mar 15 at 10:45 AM" (again, obviously I'm making these things up. Yes, my imagination is just wild today.). In seismology, earthquake forecasting is done quite commonly, whereas the general scientific consensus is that earthquake prediction is currently (and might always be) impossible (despite some individuals or groups claiming otherwise...). 

So back to volcanoes. In volcano monitoring, people generally don't make "predictions" for when an eruption will occur. Instead, there are short-term forecasts (compared to the long-term forecasts that are usually given in seismology). These forecasts depend on how volcanic activity evolves over time. So what do we use to determine what our volcano is doing? Just like a patient in a hospital might be hooked up to a bunch of instruments measuring vital signs like heart rate, oxygen levels, and body temperature, our volcano is usually hooked up to a bunch of scientific instruments. The vital signs of a volcano are called "precursors", they are for example:

• Earthquakes - we usually look at how many there are say per day or hour, how big they are, at what depth they occur and whether that depth (and horizontal location) changes, and what "type" of earthquake they are. Types of earthquakes might be "regular" earthquakes with (relatively) high frequency waves, earthquakes with (relatively) low frequency waves, a mixture between the two (so-called "hybrids"), or volcanic tremor. These different types of earthquakes sometimes show how magma is moving from one place to another.
• Deformation - how the surface of the volcano changes its shape. We use instruments on the ground and satellites images to determine whether the surface is moving upwards and inflating like when you're blowing up a balloon, or deflating like when you let the balloon go. The deformation usually happens because of a change of pressure below the ground.
• Gases - volcanoes spit out gases in different places most of the time. The gases come - in one way or another - from the magma below the ground. The amount of gases, their temperature, and their type (e.g. sulfur dioxide or carbon dioxide) can help us to determine whether magma might be getting closer to the surface.
• Temperature - sometimes we see higher temperatures around volcanoes on satellite images.

Usually, when we see more earthquakes per hour, a lot of deformation, a lot of gases, and high temperatures, we become worried that magma might be getting close to the surface and ready to cause an eruption. This is what we call "unrest". Volcano observatories use alert or hazard levels to put a number on the state of volcano unrest. Below are examples of two different alert/hazard level systems from two different volcano observatories (GeoNet, New Zealand; and Montserrat Volcano Observatory, Lesser Antilles):

Alert levels for frequently active volcanoes in New Zealand (courtesy of GeoNet)

Hazard levels for Soufrière Hills Volcano, Montserrat (courtesy of Montserrat Volcano Observatory)

You can see that Montserrat has zones in addition to the hazard levels, and access to the zones is controlled based on what the hazard level is. The way the alert/hazard level is determined depends on the observatory and the specific volcano. The assessment is based on what is known from previous eruptions, scientific studies, and sometimes from other volcanoes.

So far so good. So we now know that a volcano has vital signs like a person, and that we might be able to use them to tell us whether an eruption might be happening soon or now. But of course, things aren't that simple. Unfortunately, volcanoes are like people in another sense (not just in terms of the vital signs analogy): Sometimes they have their own mind, behave in ways that can't be anticipated, and surprise us all. Also, many volcano may look similar but have quite different behaviours from one to another. For example, on some volcanoes precursors build up over weeks or months, whereas on other volcanoes we get only short or no warning at all. Whereas many volcanoes have MORE earthquakes just before an eruption, Telica Volcano in Nicaragua, for example, sometimes goes quiet and has no more earthquakes within an hour or so before explosions (listen to Mel Rodger's recent podcast on this). Similarly, whereas many volcanoes inflate before eruptions, Uturuncu Volcano in Bolivia has been inflating quite a lot for over 10 years without an eruption (read James Hickey's blogpost on this).

And just like we have good days and bad days, even one volcano can change its behaviour from one eruption to the next. Obviously in that case we're gonna have a hard time making a good forecast. 

Furthermore, the situation is complicated by people. One would think that it's always better to be safe than sorry, so ideally we would move everybody who lives close to a volcano to a safe place? Obviously that's quite unrealistic. Some countries have so many volcanoes that there simply would be no space at all to put people: On the website of the Global Volcanism Program, a search for volcanoes in Indonesia returns 1182 matches. Granted, some of them might be individual cones on one bigger volcano, or synonyms for different craters and cones, but the number is still really really large if we were to take those duplicates out. Where would we move all the people living close to those volcanoes? We also can't just take them away from their homes, the places where they grew up, away from their property, their fields, their places of income. Even evacuating an area can have significant economic losses the longer it lasts (ignoring the obvious potential loss of life and damage to the economy through the eruption itself). To make things even more complicated, there's the famous "cry wolf" phenomenon. People tend to become less responsive to evacuation orders or instruction for precaution if they have experienced several scenarios in which no eruption occurred in the end. In other words, if you cry wolf too often nobody will believe you anymore.

We can see now that it's quite difficult to give good eruption forecasts. The volcanoes can give us hints, but ultimately we might never know for sure what's going to happen. As scientists, in many cases, we are advising decision makers from a purely scientific perspective with what we know about a volcano and its state. Ideally, there is a dialogue between scientists and decision makers, who will then have to take into account economic, psychological, and other considerations to make a call for evacuation or against it. In Indonesia at Sinabung, on Friday authorities decided to let people back into the area (but with a certain distance to the volcano) after 10s of thousands had been evacuated following eruptions in the previous weeks. Clearly they did not anticipate the eruption that happened just one day later. A fairly large eruption at Tungurahua Volcano, Ecuador, which also happened on Saturday, thankfully appears to have had a less fatal outcome than the one in Indonesia. In the end, the outcomes of an eruption depend on many factors. As scientists, we are doing our best to study the processes happening on volcanoes. We might not make huge leaps, but every project is a little step towards understanding our volcanic neighbours a little bit better, and maybe make forecasting a tiny bit more reliable.