Bernard Chouet is a good listener. A volcano seismologist with the U.S.
Geological Survey's Volcano Hazards Team in Menlo, Park, California, Chouet
spent years patiently lending an ear to strange seismic resonance coming from
volcanoes. In time he learned how these sounds could signal a dangerous rise in
pressure as magma welling up from deep within the Earth tried to find its way
out; if it didn't, the volcano eventually blew.
Using a new theory about these so-called long-period events, Chouet has
successfully predicted several eruptions, including that of Alaska's Redoubt
volcano in 1989. In this interview conducted for "Volcano's Deadly Warning,"
Chouet describes how he came about developing his novel theory, and how well
it's holding up to scrutiny both as a theory and as a useful tool in the
field.
NOVA: What's so extraordinary about volcanoes?
Chouet: Volcanoes are quite spectacular, especially at night—the scenery,
the mountains, the eruptions with all the magma coming out of the vent and the
gases flowing. The feeling of a mountain being alive is an extraordinary
discovery for someone who has always felt that the Earth was a solid piece of
rock. Then you see these explosions and realize that the Earth is an active
planet beyond anything you have ever dreamed about.
NOVA: Why was it so important to you to know how volcanoes work?
Chouet: It was a sense of exploration, a sense of discovery. My feeling after
studying engineering for many years was that pretty much everything had been
explored and discovered, and there was no room for great exploration around the
Earth. I felt that one of the last interesting frontiers to study was natural
phenomena. I realized that although volcanoes had been looked at for a long
time and people had always been fascinated by them, they were relatively poorly
understood, and this was a frontier that was worth exploring.
NOVA: Who were some of the early pioneers in trying to understand
volcanoes?
Chouet: The Japanese seismologist Takeshi Minakami was one of the early people
who got interested in making seismic measurements on volcanoes and finding out
if he could interpret anything. The instrumentation he had was rather limited,
so he was mostly interested in classifying events and establishing some kind of
order in the richness of the observations. He ended up classifying seismic
events based on the character of their signature as A-type and B-type
events.
NOVA: What's the difference between the two?
Chouet: A-type events have a very characteristic signature that starts with an
impulsive first arrival. These events occur when a volcano first comes alive
again and magma is moving at depth. To make its way to the surface magma must
create a plumbing system it can flow through. So the volcano is readjusting
itself with lots of earthquakes. The A-type earthquake is the sound of rock
breaking as the volcano readjusts itself to the magma movement.
B-type events include two different types of processes, one of which is just
like the A-type earthquake—it's rock breaking. The other type of process
that was buried somewhere in the definition of B-type events is the long-period
event. Unlike the A-type event, which reflects the brittle failure of rock, the
long-period event reflects the change in flow pattern of the fluid that is
being pushed through cracks.
Once the plumbing system of the volcano is unobstructed, magma can flow freely
through this plumbing, and A-type earthquakes cease to occur. In this
situation, you'd see almost exclusively long-period events. What the
long-period events are telling you then is how the magma is evolving as it
comes closer and closer to the surface. The long-period event has a distinct
signature marked by an emergent signal and then a slowly dying single dominant
tone. This is the sound of fluid under pressure. This long-period event gives
us the means to quantitatively measure that pressure and to track the
pressurization in the volcano.
NOVA: Can you give an analogy?
Chouet: Well, long-period signals in volcanoes and organ-pipe tones are very
similar, for example. They are both representative of resonance phenomena. In
an organ pipe, you have a column of air trapped between the walls of that
cylinder, which is made of metal. Air is blown across a sharp edge at the
bottom end of the pipe and sets up a standing wave in the pipe. There is a
pressure variation along the pipe associated with the resonance of the air
column in the pipe. You feel the pressure disturbance radiated from the open
top of the pipe through the atmosphere to your ear.
In a volcano, a change in the flow pattern of the fluid in a crack may in a
similar way trigger the acoustic resonance of the fluid, which applies a
pressure variation on the crack surface. The resulting vibration of the crack
wall is radiated into the solid in the form of seismic waves that propagate
through the ground to the surface, where seismometers can pick them up.
Imagine what would happen if you were to place a cork on an organ pipe and seal
all its openings and keep pumping air into a small hole at its base. If your
pumping is brisk enough, you will induce resonance of the air filling the pipe
with each pumping action, and each pumping action also raises the overall
pressure in the pipe. If you keep pumping vigorously, you keep inducing
resonance, and the pressure in the pipe keeps rising until eventually the cork
blows out.
That's basically the process that goes on in volcanoes. You're seeing the
pressure disturbances that are related to each pumping action, and then each
long-period event is the result of one pumping action. The fluid filling the
crack resonates and is trying to escape, but there's nowhere to go. The more
pumping you do, the more pressurizing you have. Eventually you have a blow
up.
NOVA: Apparently Minakami decided not to pursue the B-type signal. Why not?
Chouet: I think the answer is complexity. The B-type event looked very complex
compared to the A-type. The A-type had a very short signature with a very sharp
first arrival, so it looked like you could actually locate these things. The
B-type, on the other hand, had this slowly emerging signal, which made it
impossible to locate. In many cases it also displayed a very complex,
long-lasting signature. In essence, Minakami concluded that this was something
that we couldn't really address. At the time it was true—you couldn't really
unravel all that complexity.
I also was trying to put things together in my head, and I thought trying to
classify signatures was a little bit like classifying flowers. There are many
types of flowers so you classify all these things, and then you have a feeling
for the complexity and richness of nature. And that's fine, but if you do that
on many different volcanoes—and there are quite a few volcanoes that are
active at any one time—you're going to find that there's a very rich variety
of seismic signatures on volcanoes. So you end up writing pages and pages and
pages of signatures and classification, and people look at this and say, "Well,
this is hopeless, because you've just got too much richness to deal
with."
Chouet's solution
NOVA: So how did you approach this problem?
Chouet: What I wanted to know was, what specific events may occur in a volcano
that are a telltale sign that you are actually proceeding with pressurization
and toward a possible eruption. To do that you have to understand what is going
on. You have to be able to interpret that process, that evolution of the
volcano. You have to interpret the signature.
“Suddenly you realize the volcano is speaking to you, and you
understand the language.”
NOVA: You began trying to interpret those signals on Mt. St. Helens,
right?
Chouet: Yes. Mt. St. Helens erupted in 1980. It blew up and carved a huge
crater and dispersed a lot of material all over the countryside, wiped out
forests, killed people. During the summer of 1981, a colleague and I deployed a
seismometer inside the crater, right next to the base of the lava dome that was
growing in the crater. While we were there we recorded a lot of B-type events.
Being so close to the lava dome, the events were easier to locate, and we saw
that we had a collection of A-type events and B-type events, and among the
B-type events we had these peculiar signatures.
NOVA: What was peculiar about them?
Chouet: It stared you in the face. Anyone seeing these wiggles on paper would
say, "Wow, this is obviously different." I remembered from when I was still in
engineering school what happens in hydroelectric plants in the pipelines that
carry the water when you suddenly shut the valve controlling the jet of water
hitting the turbine blades. If you stop the water flow very quickly it
generates a very high pressure right there at the valve. That pressure pulse
reverberates in the pipeline, going back and forth between both ends of the
pipeline. Each time the pressure pulse arrives back at the valve, it hits the
surface of the valve with a huge hammer blow. This is called the water hammer.
The water hammer effect is well known in that field as a resonance effect in
the pipeline.
NOVA: So these long-period events are like water hammers, telling you what's
going on with the magma, the liquid rock?
Chouet: Yes. The long-period events are important because they reflect a
process that involves the fluid. You care about the fluid. You want to know
where the fluid is, what it's doing, how much pressure it's under, whether the
pressure is going up or down. By looking at these long-period events, we have
this direct window into the fluid.
NOVA: How so?
Chouet: The key principle is pressure, and how fast you're pressurizing the
volcanic edifice. This is essentially a pressure-cooker situation. The evidence
of this pressurization comes through the long-period events, which are a
manifestation of pressure accumulating and magmatic or hydrothermal fluids—mostly in the form of gases—trying to move in response to this excess
pressure and trying to shoot through the available fractures and cracks that
permeate the edifice.
A somewhat analagous situation is what happens when you boil water in a
teakettle. When the water starts to boil, you have this singing steam coming
out of the teakettle. In a way the volcano is also singing its song. Individual
long-period events are little chirping sounds the volcano makes while
pressurizing. When the long-period events occur in rapid succession, a
sustained signal results. The volcano then is literally singing its tune. This
is a siren song because the volcano is telling you, "I'm under pressure here.
I'm going to blow at the top."
NOVA: It must be quite exciting to see this line on a paper and be able to
infer so much from it.
Chouet: It's a defining moment, because suddenly you realize the volcano is
speaking to you, and you understand the language. It's a little bit like
learning a foreign language. At first, you're sort of floating there, and you
don't understand, you're lost. And suddenly the language is understood with
clarity, and it's a completely new perspective. The idea that the volcano is
sending information and that you are able to interpret this information and
characterize what the volcano is doing, that's like learning a new
language.
Calling Redoubt
NOVA: Alaska's Redoubt volcano "spoke" to you just before it blew in December
1989. Can you tell me that story?
Chouet: One of my colleagues asked if I would pass by his lab. He and his
colleagues had some interesting activity from a volcano in Alaska that they
wanted me to look at. They said, "We're seeing an increasingly rapid occurrence
of these types of events. We don't know what they are. They seem to be very
similar from event to event."
I looked at their records, and I could see that it had started just a short
while ago. By the time we were looking, this activity was building up to one
event per minute. And it was obvious that all these events were long-period
events. I said, "I think you have an eruption on your hands."
This came out the blue for them, so they were a little taken aback, thinking
he's a little bit cocky, maybe he's joking or something. They went back to
their business, and I went back to my office. The next morning I poked my head
into the lab and asked them, "Well, has this volcano erupted yet?" And they
said, "For all we know, no, but if you're so sure, why don't you call the
scientist in charge?"
I went to my office and called Tom Miller, who was the scientist in charge at
the Alaska Volcano Observatory in Anchorage. And Tom said, "I can't speak now
because Redoubt is erupting!" I put the phone down and went to tell the guys in
the lab, "Yeah, it's erupting right now." Suddenly I became part of the team. I
had something to say that was of interest. They put me in the loop, and we
started looking at the activity from there on.
Then on January 2, 1990, seismic activity changed from a linear increasing
trend to very rapid acceleration. I told Tom Miller, "I think we're in a
similar situation to where we were on December 14th, and we're going
to have a major eruption on our hands within 24 hours or maybe two days from
now." We went back and forth, because calling an eruption meant we would have
to evacuate people. There was an oil terminal roughly 40 kilometers from
Redoubt, and closing down the plant and evacuating people meant shutting down
the operation, with the risk that the oil might freeze in the pipeline, so we
needed to be sure of what we were saying.
Tom called these people from Anchorage, and they said, "Look, just a few hours
ago we took a helicopter trip over that dome, and it's very quiet. Just a
little wisp of steam coming out, it looks totally dead." We had to convince
them that it wasn't as dead as they thought; underneath it was pressurizing.
I think the clincher was Tom faxing them a sheet of paper that showed the very
rapid increase in long-period events. They realized something was shooting up
to the sky and that maybe these people know what they're doing. So they
evacuated the plant around 5 p.m. and at 7 p.m. the volcano blew up. They
called Tom back and told him that they thought he was walking on water.
Making it universal
NOVA: How did that make you feel after so many years of working, monk-like, on
developing this model?
Chouet: It's a wonderful feeling, because you feel that you understand the
language now—that the volcano is talking to you and you understand what the
volcano is saying. You can actually track the whole thing. Of course, we don't
know everything about volcanoes. It's going to take a long time to get to the
point where we're in a position to make very accurate forecasts of the state of
a volcano. But this was a very nice first step and a great feeling.
NOVA: Why just a first step? Having made a successful prediction at Redoubt,
couldn't you just apply that to all volcanoes?
Chouet: You've seen it for one volcano and that's it. That's one volcano. You
have to show that there's some universality and that this process is applicable
to other volcanoes as well. So you move to a different type of volcano and try
to understand this different type of volcano using the same kind of model and
see if it works. And you discover that you understand its language. But then
you realize by looking at other volcanoes that there are still aspects of the
language that you don't get, so they don't quite fit within the model.
NOVA: Why not?
Chouet: Because there's infinitely more richness in nature than one can
imagine. You always try to break it down to the components and simplify. Then
you realize that maybe you've simplified too much, so you add a bit of
complexity to the model. You don't want to add too much because if your model
has too many parameters and too much complexity then it becomes as complicated
to understand as it is to understand nature to start with.
So you try to keep your model as simple as possible and see how much you can
explain. But you have to keep modifying it to see if you can explain these
other volcanoes that fall outside the range. It's a process of continuous
feedback between observation and theory and modeling. I find it quite exciting
when volcanoes cannot be explained by the model, because it means there's
additional information that is buried in there from which one can learn more.
If all the volcanoes follow the behavior predicted by the model, then I'm out
of a job. But fortunately, nature's rich enough so we can always keep adding to
this whole thing.
“Within 20 years we should be able to make forecasts of volcanic
activity that are at least as accurate as weather forecasts.”
NOVA: Yet you have had success at other volcanoes, right?
Chouet: It's a very happy circumstance when you can see that you have another
volcano coming on line, so to speak, that produces the kind of behavior that
you'd expect based on the model you have developed. Popocatepetl in Mexico is
very good in that respect. It's working just like Redoubt and other volcanoes
such as Galeras and Pinatubo were working. So it's not one volcano, one
particular case you're talking about. It means you're talking about a kind of
universal mechanism at play. The more volcanoes that produce this kind of
behavior, the more you feel reinforced in your use of such a model.
NOVA: How close do you think you are to a universal mechanism?
Chouet: How close we are away depends on how many people work in the field. So
far this is a relatively small field, so the work is done by a few individuals.
I would imagine that if one received adequate financial support for the
carefully designed, large-scale experiments that are required, within 20 years
one should be able to resolve a lot of questions and perhaps be in a situation
where one could start making forecasts of volcanic activity that could be at
least as accurate as weather forecasts.
Convincing colleagues
NOVA: You spent years on your own working on this. What was that like?
Chouet: I was working pretty much alone but I was also standing on the
shoulders of giants. I was borrowing from different fields and putting this
into the context of my own idea of what was going on. So I was benefiting from
the work of all these other people, and that's usually the case in science.
Even though it took overall perhaps five or six years developing the model,
more and more during that time I could actually recreate signatures out of the
model that looked familiar and similar to what was observed in nature. That's
when I thought, This ought to be right. It's so similar, and I can explain the
richness, I can explain the duration, I can explain all these different
frequencies.
You still have the work of convincing your peers. And that's hard, because
people are set in their ways. Scientists are very conservative by nature. They
have to be, because it takes a long time to develop theories. Once a theory has
been accepted, it's been tested and tested over and over again. Then someone
comes along with a new observation that doesn't fit the theory. This rocks the
boat so people have to decide whether to throw away the theory or to modify it
so that they don't have to re-invent the wheel.
NOVA: Why are scientists so reluctant to accept a new theory?
Chouet: For the same reason that people with different religions fight each
other. Each one believes that they have a corner on the truth, and actually we
don't have a corner on the truth. You have to take all these bits of
information coming from many different disciplines and reconstruct something
that makes sense.
Sometimes people are too narrowly focused on their discipline. You talk to some
of the people making gas measurements, and they say, "Well, the only way you
can find out about the volcano and where it's going is by measuring gases." In
some cases the volcano is just sealed enough to allow this gas to accumulate at
depth. So if you try to interpret that volcano on the basis of quantity of gas,
you'd say, "Well, the volcano is muy tranquilo—very quiet, and this
is a good day to go in the crater." But it would just be the opposite.
We're not working in a vacuum where we suddenly get plopped on this planet and
say, "Nobody has thought about this before." You can be sure that almost any
idea you have, people have thought about it before. Maybe they didn't write
about it, maybe they didn't pursue it. It's very humbling, because in a sense
there's nothing really to invent. There are only things to be perceived and
interpreted. It's a question of awareness and saying, "Am I getting all the
messages there? Am I putting all these pieces together in the proper way?" If
you're not, you're not making progress.
NOVA: You've been quite modest in this interview. According to many people that
we've spoken to, you have taken a bigger step forward than most, if not all.
What do you feel about that?
Chouet: I feel that everyone comes around in this life blessed with some kind
of gift: a gift of imagination, say, or a gift of artistic design. And you look
at these people and say, "Wow, this is incredible what they did." Mozart made
fantastic music, Rodin did fantastic sculptures, Einstein developed fantastic
theories. What was great about these people is that they used this gift, and
they left it for people to enjoy. I don't think they had to pretend that they
were better than others. They were just excited by what they were doing, and
they shared it with others.
I view what I do in a similar vein. I think that I have a gift which makes me a
natural person to look at volcanoes and wonder about their inner workings. I'm
blessed in that I've met people along the way who were very helpful in terms of
providing the tools. I'm blessed with patience and persistence, because this is
a long-term endeavor. I consider these to be gifts, and I would be remiss not
to use them.
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