by Jeff Johnson
photography by Benjamin Phillips
On the last day of 2011 our group heads in to Santiaguito dome for two nights and three days of seismo-acoustic-tilt station installation. Access to the domes is challenging and requires a descent of about 1000 m, first on a traverse through cloud forest, then through a tunnel of vegetation (those who are tall suffer the most), and finally down a steep ravine called the canaleta. The canaleta is comprised of water-polished lava coated with a thin layer of ash and can be treacherous. Upon reaching the bottom we are on the playon, a vast flat area between the Santa Maria (SM) caldera rim and the Santiaguito (SG) domes. The playon is littered with poorly sorted lahar debris ranging from refrigerators to fine sand.
Our guide Armando leads us boulder-hopping down the playon to a hidden camp nestled in the woods near the caldera wall. It is a excellent site with views of Brujo Dome across the playon. It is also shielded from winds and also has flowing water. Tents go up and we plan our next moves. Because field work at the Santiaguito domes is tough and just getting to the campsite is an accomplishment a subset of our group is ready to call it a day. A few others are just too excited to be back here in this volcanic Disneyland so four of us head out to carry loads and conduct reconnaissance.
Each of us loads packs with batteries (53 Amp-hours each) and we head toward the deployment site. Our route wends along the "moat" separating the SM edifice and SG domes (imagine the domes as the castle). Just beyond the entrance to the canaleta the moat steepens as it goes over a constricted passage. Car-sized boulders choke the gulley and we climb 300 m in a series of rubbly steps. I like to refer to the passage as the Khumbu rock fall in honor of its similarities to the ice fall at Mount Everest. Above the Khumbu rock fall the gap between SG and SM broadens and becomes flat. Some 15 years ago there was a lake here, but it is presently filled with sediments following debris avalanches from SM. By now it is dusk and as we turn the corner around the Mitad dome we finally see the glow from Caliente dome (the currently active dome of SG). Caliente towers 400 m above us and is about 800 m distant. Here we stash our batteries and sit down to await an explosion.
The dome glow is strong suggesting an active, and perhaps enhanced, effusion of lava. Occasionally a degassing burst will send a vapor column hundreds of meters above the summit. The plume is lit both by incandescence and the moon. Frequent rock fall can be seen, but not heard, originating from both SG and SM, which towers 1500 m above us. Although we don't observe our anticipated explosion prior to heading down it is still a downright awe-inspiring place to spend a New Year's Eve. We finally decide to downclimb the Khumbu rock fall using our headlamps and arrive at camp at 10:00 PM. Everyone there is asleep.
On day two of the dome excursion the Italian team (see Corada's report) head out to climb the domes while we head back up to Khumbu rock fall to deploy. Our goal is to install three stations in a 150-m aperture tripartite configuration. Two stations will be equidistant from the vent (about 500 m away) and will be comprised of triaxial broadband seismometers and biaxial tiltmeters. A third station will record seismic and infrasound from a pair of microphones. In order to deploy the seismic and tilt deep holes must be excavated to isolate seismo-tilt from meteorological influences.
The deeper the hole the better. John, Jake, and Andrew, who comprise the seismic team, are charged with digging the three holes. John and Jake vie for deeper holes and are both only satisfied when they are a meter deep and large enough to bury a small horse. We decide to coin station names (SJON, SJAK, and SAND) as a contraction of the volcano (Santiaguito) and the digger's name.
Deploying tilt is especially difficult because the sensor must be oriented and level and is very sensitive. One component can be satisfactorily leveled, but then fall out of true as the second component is leveled. At station SJAK the leveling of the tilt is particularly onerous. We read off voltages as John, stuck in the hole gently tweaks the instrument and packs tephra around it. At one point we are interrupted by an eruption. We hear the boom before we see it. Looking up we see bombs raining out of the column. One bomb lands on the slopes and rolls in our direction coming to its resting place, which is not all that far away.
Finally, after dark, the tiltmeter is leveled. We push the button to start acquisition and retreat to a safer vantage point to wait for another explosion.
Two hours later and still there is no bang. Its getting cold and we decide that we will leave at 9:00 PM explosion or not. At 8:56 we get what we waited for. Incandescent projectiles are thrown onto the flanks of the upper dome and a plume shoots upward from the summit. We navigate the Khumbu Rock Fall and return to camp by 10:30 PM.
On the third day we must head back to Xela. One group heads out directly while another smaller group goes to check on the seismic sites before ascending the canaleta. In order to verify that our stations are functioning properly we bring a laptop and a tent in which to view the data. Some of the stations require some minor corrections, e.g., reduction of gain on one of the channels, so we are grateful to be able to make these corrections before leaving the stations for a week's worth of data acquisition.
The hike out from SG is, as always, brutal. We retrace our steps up the slippery canaleta, through the ash-covered vegetation, and back to the waiting microbus, which bumps and convulses its way down to Xela. Tonight we get to eat (Tecum) hamburgers and drink beer. Tomorrow we will go visit the lava flows on the other side of the volcano.
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