Vandal Science

On Monday of this past week there was a little flurry of activity here after lunch. I didn’t pay much attention, since things are always going on at the Aso Volcanological Laboratory (AVL) in Kyushu, Japan, where I am currently taking my sabbatical leave. In any case, I knew Kagiyama-sensei and Utsugi-sensei were leaving for Taiwan that afternoon, to do a geothermal/geomagnetic survey. Preparations for a survey are always a little hectic, and since I wasn’t going I wasn’t all that interested. Later, though, when I went to the break room for the afternoon “ko-hii” (coffee), Komori-san looked a little worried. “There have been harmonic tremors this afternoon.” I was surprised. “At Aso?” I asked. “Hai (yes).” Well, now…

Harmonic tremors are one of the signs of impending volcanic eruption. Ordinarily I wouldn’t worry too much about it, but when you’re sitting on top of one of the largest volcanic calderas in the world it puts a different perspective on things. For someone that has watched “Supervolcano” perhaps one too many times, it was a little disturbing. At the time there was no one else I could ask about it; all the staff were out in the field except for Komori-san and myself, so I didn’t know what to think. After ko-hii I went upstairs to my office and found an email from Kagiyama-sensei that read, in part:

“From this morning, the activity of Aso Volcano increased. The amplitude of
volcanic tremor increased several times [more] than usual. Minor ash emission may
occur…”

Well, that was comforting. After all, if there were to be a supervolcanic eruption, surely Kagiyama-sensei and Utsugi-sensei wouldn’t have left to go to Taiwan… hum, now that I think about it, going to Taiwan didn’t seem like such a bad idea after all. Going to, say, Scotland sounded like an even better idea! But “minor ash emission” didn’t sound that threatening. In fact, I started to worry about something entirely different. What if there was an eruption and I was here all by myself? How would I get pictures? I tried to imagine fighting my way through the crowds (the fools would all be running the wrong way, of course–away from the volcano!), bluffing my way past the cordon of Japanese police and civil-defense workers (”Make way, make way. Let me through. I’m from the University of Idaho! Stand back, you!”), and somehow getting up to…where? It didn’t sound very likely. But then I might miss the eruption! In one little corner of my mind there was a voice that was dolorously wondering about my priorities, but the greater chorus was clamoring to get up to where the action was.

Since there wasn’t much I could do about it, I started to wonder instead about harmonic tremors. What, exactly, is a harmonic tremor? As it turns out, harmonic tremors are sustained, long-period seismic signals that are believed to be caused by magma and volcanic gases moving through fractures deep under an active volcano. Most seismic signals, caused, for example, by the release of mechanical energy during the breaking or sliding of rocks associated with earthquakes, have a characteristic shape that is clearly different from the low-frequency (order of 1-10 Hz), steady-amplitude signature of harmonic tremors (see figure).

Apparently, a scientist in the Menlo Park office of the U.S. Geological Survey named Bernard Chouet first noticed the importance of harmonic tremors in connection with the 1985 eruption of Nevado del Ruiz in Columbia. Because harmonic tremor activity increased dramatically prior to the eruption, he hypothesized that it could be used as a predictive tool. He then went on to successfully predict several eruptions (Mt. Redoubt, 1993, and Popocatepetl, 2000, among others; I actually climbed Popocatepetl a couple of years before it erupted, but that’s another story…). The method has been in wide use ever since to give warning of…well, of “impending doom.”

Tuesday night I woke up at about 11:30 to the slight, but distinct, swaying of a minor earthquake (I’m a light sleeper). A few minutes after that settled down I heard the apartment building I live in begin to creak and groan, and then the swaying started up again. I had felt one tremor earlier in the day, but this series of tremors really made me think. Volcanos and geothermal areas are seismically noisy places, but usually those tremors are below the threshold of human senses. In the dark I padded over to my computer and brought up the Aso live web-cams (courtesy of the Aso Volcanological Museum and RKK television). Both cameras were pitch black; I couldn’t even see the usual scattered glow of the fumaroles in Nakadake crater. Of course, it’s getting cold here now, and steam was probably obscuring the view. Or the cameras were destroyed by an eruption. There was no way to know.  Eventually I went back to bed, but I lay awake a long time, looking at the ceiling and wondering…

As it turns out, Nakadake (the currently active cone in the Aso caldera) didn’t erupt, and the web-cams are still working. You can see for yourself at http://www.rkk.co.jp/mmeye/aso_kakou.html. The “A” camera looks across the crater at Yudamari and the fumaroles; the “B” camera changes, because visitors to the Aso Volcanological Museum can steer the camera and zoom it in and out. The pictures are updated once every minute or two. Anyway, the harmonic tremors ceased after a day, and the earthquakes I felt were located in Fukuoka, in the northern part of Kyushu, not near Aso at all. So I’m still waiting for some “action.” But I’ll be sure to let you know when something happens!

Tags: harmonic tremors, Japan, Jerry Fairley, Kyushu, Mt. Aso, Nakadake

This entry was posted on Friday, November 6th, 2009 at 11:23 pm and is filed under Earth Sciences, Geology. You can follow any responses to this entry through the RSS 2.0 feed. This blog is focused on science and posts and comments are encouraged to stay topical. This is an English-language blog; we are not staffed for translation. Comments and content that appears here is posted by individuals and does not necessarily represent the views or policies of the University of Idaho. You can leave a response, or trackback from your own site.