Cast: M. Garces (PI, scribe), Morrissey-san (USGS, Colorado), Sudo-sensei (Aso Volcanological Laboratory), Ishihara-sensei, Iguchi-san (both at Sakurajima Volcanological Observatory), Wakasugi-san, and Nakamura-san (both at Aichi University).
I'm sitting in the car, chewing on my grilled eel and picking with my chopsticks on the obligatory pickled prune atop a mound of rice. From station SAKG, 2 km away from the active crater of Sakurajima Volcano, I can see the steam and ashes are rising to a height of about 1 km. In the moonless night, the ash column reflects the yellow luster from the city of Kagoshima, which is 10 km West of the active vent. During the last three days, Sakurajima Volcano has evolved from quiescence to a fairly impressive level of activity, and I am just waiting for the story to unfold.
Looking back, it had been a serendipitous experiment with an unpromising start. Sakurajima (Cherry Blossom Island), a stratovolcano on the Southern tip of Kyushu Island, has been called Japan's most active volcano, although in recent years it has not been excessively active. In May 10, 1998, we installed a six-element infrasonic and seismic array within 3 km of the active vent. An additional four element infrasonic and seismic network was also deployed with a wider aperture but still within the same region on the Western side of Sakurajima island. To my knowledge, this is the largest infrasonic experiment performed at an active volcano to date, although it almost didn't happen because, according to reliable sources, the volcano was quiet. From May 11 to May 16 the volcano was indeed relatively calm, and in addition we had a storm which wreaked havoc with our data quality. On the morning of May 17, the wind died, the sky cleared, and at 6:05 AM we had a large explosion with triggered on every station. This was the beginning of an increased level of activity which was accompanied by a drizzle of reddish ash concentrating on the South side of the volcano. The older ash on the ground was dark and granular, in striking contrast to the fine-grained, ochre material which now covered the region. The local farmers were quite occupied dusting their plants and protecting their delicate crops, which had recently begun to ripen.
Our daily routine consisted of downloading and interpreting the data twice a day, and keeping the crows off our instruments. After the May 17 explosion we observed that the number of seismic and infrasonic events increased in number and changed in character. From May 18 to May 19 we recorded video images and over one hundred infrasonic events associated with the increased intensity of activity at Sakurajima. We observed these events gradually merge into an almost continuous oscillation of the ground and the atmosphere. In the morning of May 19, Morrissey-san, seven months pregnant and visibly growing, had to return to Colorado. In the afternoon of May 19 everybody had left but me, so I removed the array but kept the two nearest-vent microphones recording in conjunction with the video camera. The experiment was originally scheduled to end in May 18, but the temporal changes in the infrasonic wavefield were far too alluring to abandon. When the sun set behind the ash-laden backdrop of Kagoshima city on the evening of May 19, it was clear the recordings had to go on for one more night.
In fact, dusk was terminated rather quickly because of the continuously falling ashes. The wind had shifted and now blew West, so that while driving at 6:30 PM to the nearest station W of the volcano I had to put on the windshield wipers and the high lights. The height of the steam-ash column had increased from 700 m to over 1 km, and the ash fall became significantly denser as I approached station SAKG. This station was the quietest and the nearest to the vent, and was located in a gargantuan structure of steel and concrete constructed for the purpose of minimizing erosion and controlling floods, lahars, and possibly pyroclastic flows. These dams, or series of sieves (called Sabos in Japan) are only one of the many fine examples of the inventive uses of concrete in Japan.
So it is at this Sabo dam that I eat my eel while I listen to the continuous roar of the volcano and the occasional explosion. Notebook at hand, I keep a log of observations. There is very little incandesence, which suggest that there was not much juvenile material being ejected. Only one of the bigger explosions launches some large glowing blocks, which add some luminosity to the flanks of the volcano. By this time I have switched from triggered recording to continuous recording on the last two remaining stations, so every event is captured and preserved by some of the best instruments money can buy. Tired after a weeks' work, I recline on the car seat and receive the distinct physical impression that the atmosphere was steadily vibrating. The last log entry was at 21:47.
At 22:17 I wake up suddenly because I cannot breathe. For some reason I looked at my watch as I choked on the SO2-rich, oxygen-deprived cloud of gas that come flowing down the Sabo dam lining the valley. The contingency plan came quickly into action as I started up the car and drove towards the coast as promptly as possible. The car drove rather roughly, partly because I drove off with the emergency brake on, but perhaps also because the air composition was a bit altered. Away from SAKG, the air was still quite noxious but a bit more breathable. Based on deformation of Sakurajima, at 22:17 of May 19 the Japan Metereological Agency issued an official eruption warning, and if I my portable satellite fax had been operating at the time, I may have had a chance to be forewarned about my possible impending death. After catching some O2 near the coast, I headed back to my two operating stations to download the data and restart the measurements again for the next 2.5 hour recording interval.
At 22:35 I arrive at my second station, SAKH, where a siren suddenly begins to alert the population of what? The changing chemical content? The siren stops after a few minutes, and nobody comes out of the houses. I suppose this is quite common in this neck of the woods. For the next four hours, repeated outbursts of foul gas roll down the flanks of the volcano. The gas is not as acrid as the first outburst, but enough to induce some discomfort. The process is quite captivating. Back in SAKG, I watch a cloud of gas rise, pause, and then collapse and flow down the valleys towards the coast. At the front and bottom of the cloud the air is slightly opaque, and on the top rides a white layer that extends like a gloved finger. It flows down the valley slowly, so I can leisurely evacuate the area before it covers me. At 02:03 of May 20, as I'm servicing stations, I am almost run over by gang of kids racing their cars up and down the paved road near the Sabo dam. I hear their engines opening up from SAKG, and hope that I don't have to make any rapid exits. Hell of a night to pick for drag racing - only in Japan.
After 3:30 AM the activity calms down, and I mercifully fall asleep until approximately 5 AM. The morning is clear as I remove my last two remaining stations and prepare for the drive back to Aso. The experiment is now over, and we have collected 10 days worth of seismo-acoustic data which encompass numerous explosions a clear eruption sequence. The village near the ferry harbor is covered with a few millimeters of ash, and the early risers are busy clearing walkways, cars, boats, and buildings. I drive the car into the ferry, and as we begin to steam across the bay to Kagoshima an ash plume rises to a height of 2 km. And the gentle morning light colors in orange the diffuse streak of ashes that floats Westwards towards the city.
Milton A. Garces, Ph.D. Visiting Associate Professor of Volcanology Aso Volcanological Laboratory and Beppu Geothermal Research Laboratory, Kyoto University Choyo-son, Aso-gun, Kumamoto 869-1400, Japan
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