Volcano's Deadly Warning

PBS Airdate: November 12, 2002
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NARRATOR: An erupting volcano may be nature's most terrifying force. No one can predict exactly when it will strike. And its sudden fury can destroy thousands of lives. In Colombia, a single cataclysm wiped out an entire city.

But there may be a way to prevent such disasters. For volcanoes issue a warning, a cryptic signal, decoded after years of struggle, by one man.

BERNARD CHOUET (U.S. Geological Survey): Suddenly you realize the volcano is speaking to you, and you understand the language.

NARRATOR: Ignore the language at your peril, as one group of scientists tragically learned. Monitoring a volcano high in the Colombian Andes, the scientists climbed into the crater of the rumbling giant just as it awoke.

ANDREW MACFARLANE (Florida International University): I have this awful memory of a few moments kind of tottering around thinking, "Okay, you've got to get down. You've got to get out of here right now." People were being killed all around me. And I thought, "I'm not going to get out of this." I was sure I was going to die.

NARRATOR: Now another more horrific disaster may be brewing. A volcano near Mexico City is sending the same ominous signal.

CARLOS VALDES (National University of Mexico): It's like a red light flashing, "Something important is happening."

BERNARD CHOUET: The volcano is telling you, "Well, I'm under pressure here. I'm going to blow at the top."

NARRATOR: Will the message be ignored again?

Volcano's Deadly Warning up next on NOVA.

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NARRATOR: After 70 years of tranquil sleep, the Mexican volcano Popocatépetl is restless. A plume of gas pours ominously from its summit. And daily tremors hint at its power. Of all the volcanoes in Latin America, this mountain may be the most deadly. Two million people live in its shadow, their lives now under threat. But should they evacuate? And when?

These questions are up to scientists to answer.

DAVID HARLOW (U.S. Geological Survey, 1971-1994): Are we going to be heroes or going to be, you know, colossal goats in, in the end? And this is, this is the kind of stuff that gives you gray hair and makes this business very tough.

NARRATOR: It can be a life or death decision.

DAVID HARLOW: You're driven and motivated by saving lives, and at the same time you don't want to make people miserable for nothing.

NARRATOR: It's also a double-edged sword: force people to abandon their homes or leave them to risk the volcano's wrath. How can scientists make the right call?

This is the Holy Grail of volcano science, and one man thinks he has found it.

BERNARD CHOUET: When one looks at these kind of natural phenomena, volcanic eruption, you always think of the impact on human lives. The ultimate quest is to understand enough about the activity in that volcano to be in a position to make a prediction, predict the occurrence of an eruption.

NARRATOR: Bernard Chouet believes he has a radical new way to predict eruptions, something no one has tried before. If his idea works, it could prevent a catastrophe like the one that devastated the Colombian town of Armero.

The greatest volcanic disaster in recent history began quietly in the winter of 1984. High in the Colombian Andes, a monstrous volcano called Nevado del Ruiz awoke, smoking and rumbling. It alarmed the local people who lived on the flanks of the mountain. They sensed danger, but no one anticipated the volcano's destructive potential.

A small team of scientists ventured to the top of the volcano to investigate. Near the summit, they encountered thick clouds of sulfurous gas. Nevado del Ruiz had come alive for the first time in over 100 years.

But would it erupt? And what would be the consequences?

At the National Library, a young scientist named Marta Calvache made a frightening discovery. She tracked down manuscripts that described Ruiz's fury in the past. Centuries ago, volcanic blasts had melted the mountain's ice cap, sending torrents of mud down the valleys obliterating everything in their path.

MARTA CALVACHE (Colombian Institute of Geoscience): A lot of people died, and they describe how they died. Some of them they were in the mud. And people were not able to help them for several days, and they died there because of the lack of water and food. The details of the description were very frightening.

NARRATOR: Calvache's team drew up a hazard map showing the path of an eruption and where the melted ice and debris would surge down the mountain. The gravest threat was to the volcano's eastern flank. There, a single valley was fed by two rivers. Each of the rivers would pick up mud and rocks. Then they would meet, becoming one overwhelming deluge.

Calvache traced the path of the raging waters to a town of 28,000 people, Armero. Armero was a generally peaceful town, unaffected by the guerrilla warfare that disrupted much of Colombia. Now its townspeople were told that they risked annihilation from a volcano 40 miles away.

MARTA CALVACHE: They were saying, "That is not going to happen. It, it doesn't happen for more than 100 years. Why is it going to happen now?"

NARRATOR: It was easy to deny the mountain's violent potential. On the surface Ruiz seemed relatively calm. Only the scientists had seen, up close, the brewing inferno. And volcanoes can steam and rumble for years without exploding.

The authorities would agree to evacuate Armero only if they were told exactly when Ruiz would erupt and how serious the eruption would be. But the scientists couldn't answer either of those questions.

FERNANDO GIL (Colombian Institute of Geoscience): The volcano is incredibly complicated, and we were unable to give them a prediction. That is, we couldn't tell them how big the eruption would be, and we couldn't tell them when it would happen.

NARRATOR: The scientists feared that an evacuation, if it came at all, would come too late.

FERNANDO GIL: There was undoubtedly a great sense of doubt, a fear of not knowing what was going to happen at all. So yes, we felt impotent.

NARRATOR: In imminent danger, the town of thousands could only watch and wait. On November 13, 1985, the waiting ended.

MARTA CALVACHE: It was very, very dark. It was raining, raining very strongly. It was really especially bad weather.

NARRATOR: The storm concealed a greater threat. Forty miles away, Ruiz had erupted.

FERNANDO GIL: Around nine o'clock, when we were about to go to sleep, we began to hear sounds coming from the volcano. We went to the main door and looked towards the mountain. It was overcast, but explosions like flashes of lightening could be seen. The sound got louder, but all the time there was these muffled sounds.

NARRATOR: Hot volcanic gas was melting the ice cap, sending a torrential flood on its way down the valley. The next morning, the town of Armero was buried under six feet of mud and water, and many of its 28,000 inhabitants were missing.

MARTA CALVACHE: It was really shocking to see that. It's like, you don't realize it the first time, well, those are bodies. And then you see one and you start to distinguish, but many, many, many, many, many bodies there. I think it was—it was difficult.

NARRATOR: More than 25,000 people died. For the scientists who had foreseen the disaster, the reality was unbearable.

FERNANDO GIL: It was a very difficult thing to face. It was absolutely devastating that we had warned the people that this could happen, but we were unable to tell them exactly when.

NARRATOR: They were tormented by the thought that the tragic loss of life might have been prevented. There had to be a better way to predict volcanic eruptions.

The disaster sparked a new sense of urgency.

DAVID HARLOW: The scientists involved with that, including me, were very impacted by that eruption and that failure. And there was a, a group of us that were resolved not to let that happen again.

NARRATOR: What makes volcanoes like Ruiz erupt? All volcanic explosions are fuelled by one thing, magma. Magma is molten rock that flows into a volcano through the Earth's crust. Magma and hot gas then rise toward the surface. If the top of the volcano is sealed, they have nowhere to go. Pressure mounts, and when it hits a critical point, the volcano will blow.

BILL MCGUIRE (University College London): That magma has to get to the surface and so volcanologists, and those monitoring volcanoes, have the...one of their priorities is to identify that magma and to spot it as it moves up towards the surface.

NARRATOR: If it were possible to track the magma and tell how close it was to the surface, that might indicate when the volcano would erupt. But how can scientists look inside a volcano?

Seismographs create an indelible record of volcanic activity. When a volcano is on the brink of exploding, there may be hundreds, even thousands of small earthquakes each day.

BILL MCGUIRE: Seismology has been studied on volcanoes since the middle part of the 19^th century, and it's critical because you don't get a volcanic eruption without seismic activity, without earthquakes.

NARRATOR: As scientists pored over thousands of signals one stood out as the possible key to predicting volcanic eruptions. They called it the "A-type." It was instantly recognizable. It had a clear beginning and tailed off quickly. It was the sound of rock breaking.

DAVID HARLOW: A great hope was that these A-types would be, you know, the great predictor or forecaster of eruptions and that as this magma pushed itself closer to the surface we see an ever increasing number in A-type earthquakes or rock fracturing events.

NARRATOR: As magma forces its way up a volcano it breaks through rock. Perhaps the A-type signals could reveal the location of the magma and how fast it was rising to the surface.

In Colombia, scientists seized on the data from Ruiz, certain they would find a pattern of A-type seismic signals.

DAVID HARLOW: The idea was that—if we plotted those together—that the trends of those numbers would tell us something about the eruptive potential of a volcano.

NARRATOR: But this seismic blur revealed no consistent pattern to explain the eruption of Ruiz or any other volcano.

DAVID HARLOW: Every volcano created its own pattern and, and it was almost impossible to come up with, with a chart saying that this, this is the book that we're going to go by.

NARRATOR: It would take a scientist with an unusual eye to find the right signal hidden in the noise. He trained in aeronautics and physics, but then, intrigued by volcanoes, Bernard Chouet spent years trying to decipher their secret code.

BERNARD CHOUET: I realized that volcanoes were—although they had been looked at for a long, long time and people had always been fascinated by them—they were relatively poorly understood, and so this was a frontier that was worth exploring.

NARRATOR: When Chouet arrived in Colombia, he too went straight to the seismographs. He saw how other scientists had marked the A-types with red stickers, desperately searching for a pattern.

But something else caught his eye. Hidden among the A-types was another signal. It was called the B-type, but no one knew what it meant.

NORMAN BANKS (US Geological Survey, 1967-1999): It was sort of a mystical sort of thing. We didn't really understand fully what it was, nor did we understand that we could use it.

NARRATOR: The B-types were a mystery. They had no clear beginning and they tailed away slowly. Often they would merge with other signals, making them hard to detect at all.

DAVID HARLOW: It was really difficult to separate them and say definitively, "Well it was this type of event that would be helpful for forecasting." They were too messy.

NARRATOR: But Chouet saw something in them that everyone else overlooked.

BERNARD CHOUET: It stared you in the face. Wow, this is, this is obviously different. Embedded in, in the record among all these A-type earthquakes were classic looking quasi-monochromatic harmonic signature, beautiful textbook example.

NARRATOR: Or as he would put it more simply, "a long period event."

BERNARD CHOUET: Well the easiest way to visualize the difference between these two types of events is to draw them. And so the A-type event is characterized by a sharp onset. This sharp onset here is a signature, it's a sound of rock breaking. In other words, we, we're seeing the brittle failure of rock material. On the other hand, if you look at the long period event, it's characterized by this slow onset, gradual build-up of energy in the signal and then a slow decaying single tone which lasts for a while. What we're seeing here is resonance.

NARRATOR: All around us we hear sounds that are resonating. These tones are nothing more than air set into motion by a sudden pressure.

BERNARD CHOUET: We're here in this church because we have here a beautiful example of organ pipes. It's an instrument...musical instrument that is well known to everyone. And this is an excellent example of resonance and a resonator. In that case, you're talking about a pipe which is filled with air, and you're pumping air in that pipe, and the air vibrates, and you're hearing the sound of this vibration, that tone.

NARRATOR: As air is pumped across the base of an organ pipe it triggers a resonating sound wave. We hear this vibration as a single tone that gently fades away. On a graph its signature is unmistakable, a long period event.

BERNARD CHOUET: Each time I press the key, I pump more air in that pipe, and so you hear the result of pumping air in the pipe, which is its resonance.

NARRATOR: Each time air is pumped across the base of the pipe another resonating tone is heard, another long period event.

Chouet realized that a similar process was taking place in a volcano. Just like air in an organ pipe, magma and gas fill the cracks of volcanic rock. With each new injection of magma or gas, another wave will be triggered, another long period event.

But there is one crucial difference between a volcano and an organ: air can escape from an organ. In a volcano, if the crater is closed at the top, the magma and gas have nowhere to go. Pressure builds and eventually the volcano will blow.

The signal that no one else had understood was a secret warning that now revealed itself to Bernard Chouet.

BERNARD CHOUET: It's a defining moment, because suddenly you realize the volcano is speaking to you, and you understand the language.

NARRATOR: The message hidden in the data from Ruiz was now clear. Scattered through the records were hundreds of long period events, more and more as Ruiz got closer to erupting. The long period events were a countdown to the eruption.

If this had been understood, 25,000 lives might have been saved. Chouet hoped these warning signs could prevent future tragedy. But he needed to prove that his method worked.

In December 1989, a volcano in Alaska put Chouet's idea to the test. The ice-covered colossus called Redoubt hid a fiery and quaking core. Seismic signals reached Chouet's colleagues in California.

BERNARD CHOUET: One of my colleagues approached me and they said, "We're seeing the very rapid appearance of these type of events. We don't know what they are." And it was obvious that all these events were long period events.

NARRATOR: There were 4,000 long period events in less than a day. To Chouet the threat was obvious. Pressure was mounting in the volcano.

But his colleagues were skeptical.

BERNARD CHOUET: 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, you know, thinking, "Well, maybe he's a little bit cocky, you know, maybe he's joking or something. We don't really know what he's doing."

NARRATOR: So Chouet went straight to scientists in Alaska.

TOM MILLER (Alaska Volcano Observatory, USGS): I received a phone call Bernard Chouet, who I did not know at the time, and Bernard said, introduced himself and said, "Uh, Tom I'd like to talk to you about Redoubt."

BERNARD CHOUET: And Tom said, "Well I can't speak now because Redoubt is erupting!"

TOM MILLER: And Bernard said, "Oh, I thought it would. Okay, I'll talk to you later." Well I, I had many duties to do right then, but I did not forget that.

NARRATOR: The eruption, in a sparsely populated region of wilderness, was relatively tame. But one more fiery blast could melt the massive icecap, and a devastating mudflow would cascade down the volcano to a depot holding millions of tons of oil.

The seismometers soon picked up another series of long period events, which Chouet interpreted as pressure rising sharply within the volcano.

BERNARD CHOUET: I told Tom, "I think we're going to have a major eruption on our hands."

NARRATOR: The scientists tried to convince the oil company that within 24 hours Redoubt would erupt again.

BERNARD CHOUET: They said, "Look, just a few hours ago we took a helicopter trip, we flew over that dome, and it's very quiet. Just a little, you know, wisp of steam coming out. It looks totally dead."

And we had to convince them that it wasn't as dead as they thought.

TOM MILLER: And it could be very hazardous.

BERNARD CHOUET: And I think the clincher was Tom faxing them that sheet of paper that showed the very rapid increase in seismicity. And for them, it was enough. They realized something was shooting up to the sky and maybe these people know what they're doing.

TOM MILLER: At that point they made a decision to evacuate the terminal, shut everything down and evacuate.

NARRATOR: The evacuation was completed by four o'clock in the afternoon. At 5:30 the volcano erupted, sending ash and debris seven miles into the sky. A torrent of mud and smoldering pumice flooded the valley. But no lives were lost. And despite being buried by three feet of mud, the carefully sealed tanks were secure.

BERNARD CHOUET: They called Tom back and told him, you know, that they thought he was walking on water. So it was a, it was a great success.

TOM MILLER: It worked. So we were quite happy, yes, very happy.

NARRATOR: Bernard Chouet had successfully predicted an eruption. But all volcanoes are unique. Would his method work on others?

BERNARD CHOUET: I thought that if, if Redoubt would behave in this way, then I would expect that other volcanoes would behave similarly. And then it was just a question of actually going after the next one and seeing what happens.

NARRATOR: Chouet was nearing the ultimate goal of volcanology: with seismic signals, he could predict when a volcano would erupt. But other scientists remained unconvinced. They took another tack, hunting for clues in volcanic gas.

Stanley Williams' approach was radically different from Bernard Chouet's. He believed that to predict eruptions you had to get up close, climbing right into the heart of the crater itself.

LARRY MALINCONICO (Lafayette College): Stan's a very aggressive scientist. He's produced some tremendous science. He believes passionately in monitoring volcanoes. It, it's his love. He loves to do that.

NARRATOR: Williams monitored gases like sulfur dioxide. Every day, an active volcano can produce thousands of tons of it. And in this veil of gas lies vital information.

LARRY MALINCONICO: Developing an understanding about gases was sort of the key to sort of understanding volcanoes and perhaps even developing an ability to predict volcanic eruptions.

NARRATOR: As magma rises it releases more and more gas, which escapes through holes on the surface of the volcano called "fumaroles."

LARRY MALINCONICO: What we're looking at now is called a fumarole. As you can see, it's a fairly hostile environment. As the volcano starts to heat up and magma comes closer to the surface, we're going to get a component of gas coming out of here which is coming from the magma. It's going to have increased sulfur dioxide, increased HCL, the temperature's going to go up here, and all over here we're going to start to see a whole lot more gas come out. It's not an easy place to work.

NARRATOR: By measuring gas levels at these hotspots, Williams believed he could tell how close magma was to the surface. Williams was convinced that his method could be used to predict eruptions. He had witnessed rising gas levels before a number of volcanic explosions.

Back in Colombia, his ideas would be tested against Chouet's in a chain of events that would end in tragedy.

Deep in southern Colombia rises another volcano, Galeras. In the city of Pasto, 300,000 people live in its shadow, accustomed to its quiet rumblings. But in 1991, there were signs the volcano was growing more dangerous.

Scientists flocked to Colombia, among them Stanley Williams and his colleague John Stix. They descended into the crater of Galeras.

JOHN STIX (McGill University): There were huge amounts of sulfur dioxide being, being emitted, on the order of thousands of tons per day, which is a pretty good indicator that there was, there was magma underneath the volcano.

NARRATOR: The levels of toxic gas confirmed that Galeras was becoming more active. Then they discovered something even more alarming, a lava dome.

JOHN STIX: When you see lava domes, well, you think, "Hmm, this isn't so good," because pressure can build underneath lava domes and often times lava domes get blown out, blown out of craters.

NARRATOR: Lava domes are often the first stage in the build up to an eruption. Fresh magma breaks through the crater and cools to create a dome. While it stays open, gas can escape. But if the dome seals, the gas is trapped. And if pressure rises enough an explosion is guaranteed.

The vast amounts of gas at Galeras suggested that magma streaming up the volcano would seal the dome. Williams and Stix said an explosion was a virtual certainty. But they were not the only scientists in Pasto.

Bernard Chouet was there too. He agreed that Galeras would erupt when the dome sealed, but he went further. He told the Colombians they would know the dome had sealed by looking at the seismographs. When the volcano began to pressurize, a telltale sign would appear.

FERNANDO GIL: He had this idea that if the dome sealed, we would see a particular kind of seismic signal.

NARRATOR: The signal would be a long period event. For a while nothing out of the ordinary appeared on the seismographs. Life in Pasto continued as normal.

Then the scenario Chouet had envisioned began to unfold. Once, sometimes twice a day, the seismographs recorded an ominous trace. It was a long period event, and if Chouet was right, the countdown had begun.

BERNARD CHOUET: This was an indication that you were pressurizing the dome and that you're moving toward an explosion that would blow the dome apart.

NARRATOR: Four days later, Galeras erupted. It was a small blast, but powerful enough to blow open the lava dome and destroy a police station built on the crater rim. Both prediction methods seemed to work, Chouet's and Williams'.

But the next test would be tragically decisive, shattering reputations and lives. Six months after the eruption, Stanley Williams was back in Pasto. He was hosting an international conference on volcanoes. With Galeras still active it seemed like an ideal case study.

ANDREW MacFARLANE: It was a good conference. I mean, I have to say I was enjoying the conference very much. People were presenting interesting papers. It was lively. There's a sense of camaraderie, especially in a small field like that. There aren't that many volcanologists in the world.

NARRATOR: For Williams, the highlight of the conference was a field trip. He told local reporters that he planned to take a group of scientists deep into the crater. He was confident it was safe. Just before the conference, he had checked on the volcano, descending into the crater and measuring gas emissions. This time they were low, a reassuring sign that no eruption was imminent.

But Galeras was anything but quiet. Warning signals were inking their way across the seismograph, a series of long period events.

The two methods seemed to contradict each other. Gas measurements suggested Galeras was safe. The long period events spelled danger. The night before the field trip Williams met with other scientists at their hotel to decide whether to venture into the crater or not.

JOHN STIX: We were faced with a dilemma in a sense. Here was an active volcano, but a very quiet active volcano. Where were we going?

NARRATOR: And what were the seismic signals telling them? Some of the scientists were worried.

FERNANDO GIL: We were concerned by these long period events and what had happened when we'd seen them before.

NARRATOR: But the seismic method was still relatively untested.

JOHN STIX: There was a concern, but we didn't really understand what those events were, were, were telling us.

NARRATOR: One man who might have helped was missing. Bernard Chouet, like other U.S. government scientists, was not allowed to travel to war-torn Colombia.

Williams and Stix decided to rely on the method they knew. Low levels of gas suggested that there was no immediate threat.

JOHN STIX: There was a possibility that there could be explosive eruptions in the next weeks or months, and yet the activity compared to previous, previously, was extremely low.

NARRATOR: Volcano scientists are used to taking calculated risks.

JOHN STIX: Every volcanologist who, who works on monitoring active volcanoes puts himself or herself in that situation at some point. They have to. It's the nature of the beast.

NARRATOR: The field trip would go ahead as planned. Early the next morning, an international team of scientists from Russia, Great Britain, the U.S. and South America set off for the volcano. By 9:30 a.m., they were on the summit of Galeras.

MARTA CALVACHE: We made different groups going up to the summit, and most of the people were very happy.

ANDREW MacFARLANE: There was a lot of anticipation. At first you couldn't see anything, and in fact, when we hiked down to the crater, you couldn't see the crater.

NARRATOR: Through a thick cloud of smoke, 12 scientists descended into the mouth of the volcano. They were joined by three Colombian tourists.

ANDREW MACFARLANE: Most of the people in the group were there to sample those fumaroles, so there was really a fairly big crowd of people around the fumaroles.

NARRATOR: They could not have known that at 9:47 the seismograph came to life.

For four hours, the scientists worked quietly in the volcano. But then everything changed.

ANDREW MacFARLANE: I remembered at that time hearing three rock falls in about, in the space of about a minute. After a couple of them, I asked Stan, "You hear those rock slides?" And as I remember, it was right after that that the thing blew up. We heard "boom."

NARRATOR: The eruption hurled rocks the size of refrigerators more than a mile into the air.

ANDREW MacFARLANE: I was looking up. And you know, I could see the plume going up and it was gray. And the fog around it was gray and the blocks were gray. And I, I'm looking up trying to see something, and I hear like three or four heavy impacts around me, big blocks coming in that I'm not seeing. And I just think, "Well this isn't going to work. I'm just going to have to run for it." I remember out of the corner of my eye seeing one of the tourists get hit by some big piece and it, you know, I remember thinking at the moment he looked like, you know, a fence post getting driven into the ground. It was just horrendous. No way anybody could survive something like that.

NARRATOR: Some members of the group managed to escape, but others never had a chance.

MAN: You've got to send somebody down there.

ANDREW MacFARLANE: You know I was running and falling, getting up, running and falling. I ran. I passed Jose Arles. He was obviously dead. He was lying face down, not moving. And I fell down close to Stan.

He had blood down the side of his face and sort of lifted his leg at me and said, "My leg is broken, it's broken, it's severed."

I tried...I thought, "Well there's only one thing I can do for him that's going to be meaningful and that's to pick him up."

I tried to reach him and my legs weren't responding very well. And you know, I was too weak. And I just thought, you know, if I try and do this we're both going to get killed. And so I had to leave him there. And that was really awful because I thought, at the time I thought, you know, "I'm probably leaving him to die because I don't think, I don't think I'm going to make it and he can't move."

NARRATOR: Rescue workers found Andrew MacFarlane at the bottom of the crater, deep in shock, with a fractured skull. Stanley Williams was also pulled out alive, but it would be two years before he would walk again.

Both physically and emotionally shattered, Williams was unable to talk to NOVA about the tragedy.

Nine people died that day, lives that would have been spared if the team of experts had interpreted the volcano's warning signs differently.

In hindsight, there were two explanations for the low levels of volcanic gas that Stanley Williams measured before the conference. One was that the magma had dropped to a safe distance from the surface. The other was that the volcano's lava dome had again sealed.

With gas trapped inside, Galeras had become a ticking bomb. The clearest signs of danger, long period events silently making their way across the seismograph, were disregarded by the scientists.

LARRY MALINCONICO: I think we can be a little myopic at times where we focus in on the information that, that we feel, or well, that we're experts on. We don't feel it's the most important area, but that we're experts on it, and in a science like volcanology you have to be able to look at things collectively.

BERNARD CHOUET: It's always easy in hindsight. If I'd been there, of course, I'd have looked at the records and I, I would have seen immediately the parallel between July and, and, and December, and I would have started sounding bells, alarm bells in there. I know I would have done that. Whether I would have been successful in, in preventing people from taking a trip to the crater on that day would have depended on how, how they would react to what I was saying.

NARRATOR: Not surprisingly, scientists have become more responsive to this new method of volcano prediction. Bernard Chouet's idea is being adopted throughout the world.

Recently it faced its most critical test yet. It happened at Popocatépetl, right outside of Mexico City. This giant volcano has been active since 1993. For the 2,000,000 people living nearby, its rumblings became part of daily life.

Then in the year 2,000, Carlos Valdes and other scientists monitoring the volcano saw a change. Their seismographs recorded a new signal, a long period event. They had heard about Bernard Chouet's ideas and asked him to come and take a look.

BERNARD CHOUET: I told them about the significance of long period events, which they weren't aware of at the time.

NARRATOR: From then on they carefully tracked these warning signs.

CARLOS VALDES: It's like a red light flashing. When you see these signals, something important is happening.

NARRATOR: The signals began to occur more frequently. Pressure was building in the volcano.

BERNARD CHOUET: The volcano was singing its song. I mean actually, this is a little like chirping if you want, with these, these sustained waves from the long period event.

NARRATOR: But would the seismographs tell them what they desperately needed to know? When should people be evacuated? Valdes knew the consequences of a false alarm. If people abandoned their homes and nothing happened they would be wary of leaving the next time.

CARLOS VALDES: We feel so bad asking people to leave their homes. And you have to keep your mind in the scientific work and say, "Look, other volcanoes have done this. The potential of this volcano is that these particular villages could be in danger."

NARRATOR: The scientists had to forecast exactly when the volcano would erupt. On December 16th, the number of long period events multiplied dramatically. The volcano's message was clear.

BERNARD CHOUET: This is a siren song, so to speak, because it's telling you, "Well, okay, I, I'm under pressure here. I'm going to blow at the top."

NARRATOR: Valdes had to make a decision.

CARLOS VALDES: We could clearly see that it would be in the afternoon of the 18th.

NARRATOR: The order was given. Two thousand soldiers raced to the most vulnerable areas to help get people out in time. Thirty thousand people had to be evacuated in 24 hours. It was a monumental effort with an uncertain outcome.

But a day later, the volcano erupted exactly as predicted, on the 18th of December. Although moderate in size, it was the biggest eruption of this volcano in a thousand years.

Finally, penetrating the mysteries of the volcano and understanding its hidden language, scientists were able to warn people with confidence and save lives.

DAVID HARLOW: It takes somebody to say, "Look, we can really do this. We can go after this and understand what's going on in a volcano."

NORMAN BANKS: Science goes in steps and, and this is, this is a big one. In volcanology, this is a biggie.

NARRATOR: It is too early to claim that there is now a foolproof method to predict all volcanic eruptions. But the work begun by Bernard Chouet offers new hope of avoiding tragedies when nature unleashes its most terrifying force.

BERNARD CHOUET: Science works through a painstaking process of research and testing and research and testing. You have to show that process is applicable to other volcanoes as well. Because there's infinitely more richness in nature than one can imagine.

NARRATOR: A volcano threatens to erupt. Who makes the call about whether and when to evacuate? Hear from the leader of a U.S.-based volcano SWAT team, ready to assist at hot spots around the world, at PBS.org or American Online, keyword PBS.

To order this show or any other NOVA program, for $19.95 plus shipping and handling, call WGBH Boston Video at 1-800-255-9424.

Next time on NOVA: Venice. The tidal waters that flow through its famous canals now threaten to submerge it. Scientists race to stop a catastrophe, but can they save the Sinking City of Venice?

NOVA is a production of WGBH Boston.

Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.

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Science: it's given us the framework to help make wireless communications clear. Sprint is proud to support NOVA.

And by the Corporation for Public Broadcasting, and by contributions to your PBS stations from viewers like you. Thank you.

I'm PBS.

PRODUCTION CREDITS

Volcano's Deadly Warning

Produced And Directed by
David Belton

Produced for NOVA by
Susan K. Lewis

Edited by
Stephanie Munroe
Martin Sage

Narrated by
Gene Galusha

Camera
Tim Cragg

Sound Recordist
Adam Prescod

Animation
422

Online Editor
Ed Ham

Colorist
Mark Kueper

Audio Mix
Richard Bock

Production Assistant
Christian Rodriguez

For Horizon

Research
Tristan Marshall

Archival Research
Jenny Foster

Production Coordinator
Laura Burrows

Production Manager
Anna Mishcon

Commissioning Editor
Bettina Lerner

Series Producer
Matthew Barrett

Executive Editor
John Lynch

Archival Material
ABC
CMI, Colombia
David Lea, Montserrat
Graphic Films
National Geographic Television
NBC News Archives
NHK
PHIVOLCS
RUV, Iceland
Smithsonian Institution
TV Azteca, Mexico
USGS

Special Thanks
CENAPRED, Mexico
Ed Klimesauskas, USGS
National Institute of Geophysics and Volcanology, Sicily
James Plunkett, Drift River Terminal
University College, London

NOVA Series Graphics
National Ministry of Design

NOVA Theme
Mason Daring
Martin Brody
Michael Whalen

Post Production Online Editor
Mark Steele

Closed Captioning
The Caption Center

Production Secretary
Queene Coyne

Publicity
Jonathan Renes
Diane Buxton

Senior Researcher
Ethan Herberman

Production Coordinator
Linda Callahan

Unit Managers
Holly Archibald
Denise Drago

Paralegal
Nancy Marshall

Legal Counsel
Susan Rosen Shishko

Post Production Assistant
Patrick Carey

Associate Producer, Post Production
Nathan Gunner

Post Production Supervisor
Regina O'Toole

Post Production Editor
Rebecca Nieto

Coordinating Producer
Laurie Cahalane

Supervising Producer
Lisa D'Angelo

Senior Science Editor
Evan Hadingham

Senior Series Producer
Melanie Wallace

Managing Director
Alan Ritsko

Executive Producer
Paula S. Apsell

A BBC/WGBH Boston Co-Production
© 2002 BBC
Additional Program Material © 2002 WGBH Educational Foundation
All rights reserved