Well, I started thinking about volcanoes in 1969, when I was 24. I spent time in Sicily when Mt. Etna was very active. I found that truly awesome and started to wonder what was going on in the Earth and if there was any way to fathom it out.

“...instead of being an isolated kind of thing, we now see these long-period events as part of a much bigger picture.”

The concept of long-period events was originally used by people at the Hawaiian Volcano Observatory to describe an event that has characteristics distinct from a so-called tectonic earthquake. If you’re looking at a seismogram of a tectonic earthquake, its spectrum is very broad. In contrast, a long-period event has a very, very sharp spectrum, what we call a resonant spectrum. If you compare seismic traces, it’s immediately apparent that the long-period event looks like a ringing bell and the other looks like a mess of all different frequencies piled up on each other. That very, very narrow spectrum is the telltale sign of a resonator. Think of a bell sound. An organ pipe. They’re all different types of resonators in nature.

It goes all the way back to the early 1980s; I started developing a seismic source model to investigate the source of volcanic tremors and these long-period events. What became pretty clear from the model was that long-period events are a manifestation of some resonance occurring within a volcano. In order to have a resonance, you need some kind of resonator, a cavity of some type. Think of an organ pipe, but instead of air in the pipe you have volcanic gasses or fluids and instead of a pipe, you have a fissure. So you have these volcanic fluids moving to the surface from somewhere deep in the Earth, and during this transit they’re going through all kinds of transformations and pressure perturbations. When the pressure perturbations are sharp enough, they can trigger the resonance of the fluid-filled fissure.

These long-period events provide a direct window into the activity of the fluid, because that’s where they occur. In a volcano, that’s what you want. So developing a model for this is very nice because this model can be applied to observations, and from that you can determine all kinds of parameters of the source: what the pressure perturbation is, how big it is, how it evolves with time, what the geometry of the conduit is, where it is—all these different things. You can’t do that with earthquakes, which involve the breaking of rock and illuminate points of weakness in an edifice, points that are here, there, and everywhere.

Earthquakes tell you about the mechanical properties of the edifice, while long-period events tell you about flow processes in conduits.

That still goes back to the mid-1980s. There was an eruption in Colombia of a volcano called Nevado del Ruiz. When it erupted it unleashed a mudflow that wiped out a town and killed 24,000 people. What a disaster. And so I was invited to go down and look at the volcano and see what happened. As I sat down with the local seismologists to look at the records, I found out that these records were peppered with long-period events. There were earthquakes, as well, in those records, and they were all marked by red dots, but not these long-period events. I asked the local seismologists why not, and they told me that was because they didn’t know what these long-period events were. I explained what they were, and we applied my model to those events, and we were able to infer that these were a manifestation of pressurization. They were a precursory activity telling us that the volcano was pressurizing and getting ready to vent. So I started using this methodology with some success. This then brings us to Mt. Redoubt in Alaska in late-December 1989. To make a long story short, we saw these long-period events appear in the seismic records. They started to ramp up very, very slowly and then suddenly the activity started rapidly accelerating. That told us the volcano was going to erupt and we managed to convince people at a local oil storage terminal to evacuate, despite the fact that they would have to stop operating and would be in danger of having oil freeze in the pipeline and losing millions of dollars per day in lost revenue. Anyway, it took them two hours to shut down the operation and get the last person out, and two hours after that the volcano blew up and sent a huge mud flow down the valley that basically submerged the plant in three feet of mud. But everything was secure and no one was hurt.

Well, these people were pretty impressed with this, and it made some noise around the community. I got a call from Nature, from the editor Laura Garwin, and she said what we were doing sounded really interesting, but nobody knew about it, and asked if I could write a review article about these long-period events, and I said sure.

As you know in life somehow it’s not always the most direct path that we take. By the time I was working on this, there was another volcano in Colombia that became active, Galeras. This was another interesting case, and by the time I had written the review article for Nature, another group of researchers had written a paper just on Galeras. Laura asked me to reshape my review and remove what I had written on Galeras, since it was already covered in this other paper. Since I didn’t really know how to reshape it and still keep true to myself, I put the paper on the shelf and there it remained for three years.

Eventually, my colleagues convinced me that this kind of information was too important not to be published. So I contacted Laura again, she said, sure, send in what you have, and we reshaped it with essentially no mention of Galeras and the paper came out in 1996.

From my perspective, it didn’t seem to get much response, with the exception of my colleagues here at USGS, who really liked it. About a year later, there was an international meeting of volcanologists in Puerto Vallarta in Mexico, and while I was attending some sessions at this meeting, people started telling me that everyone at the other sessions was talking about these long-period events, and that I had started a revolution. Everyone was talking about the Nature paper. That was my first inkling that people were actually reading this paper.

It has built up rather than changed. A lot has been confirmed, but instead of being an isolated kind of thing, we now see these long-period events as part of a much bigger picture. To understand this, you have to take into account the fact that technology is moving ahead. In the mid-90s, for instance, we started using broadband seismometers and strainmeters. With these, instead of looking at events in the seismic band—between 1 and 20 hertz—we’re now looking at events in a wider band extending between 0.001 and 20 hertz. We also have satellite observations that provide measurements of deformations occurring over hours, days, and even months. Volcanoes can deform very, very slowly. For example, we may be dealing with very viscous magma, which is oozing inside the Earth and may move from point A to point B over a period of minutes or hours, and now we can see these processes. To use a metaphor, the long-period events we were looking at were like the hair on the elephant’s back. Now the whole elephant is coming into focus. The more complete picture of volcanic processes we are getting with modern broadband seismometers, strainmeters, and satellite sensors allows us to make a synthesis of what’s actually happening inside a volcano.

We’re conducting laboratory experiments and developing mathematical models to explain the lab results. Once these models have been thoroughly tested we’ll try to extrapolate them to seismic results. We’re also looking at a few of what we would call laboratory volcanoes. These are volcanoes that are typical in activity and are keys to understanding some of the fundamental processes we’re after. For example, in volcanology, we talk about Hawaiian processes: these involve very fluid magmas in non-explosive, more effusive-type volcanoes. Then there are mild explosive-type volcanoes. Those are the so-called Strombolian processes associated with magma that is a little more viscous, making an eruption more explosive. Then there are Vulcanian systems. These are explosive volcanoes. An example is Popocatepetl in Mexico. Those three types constitute the fundamental types of processes that volcanologists have been describing. If we can quantify the processes occurring inside these volcanoes, we’re in good shape. All of these include long-period, very-long-period, and ultra-long-period events. There’s a whole range of frequencies in each of these volcanoes, and so there’s a lot we can study.