High in the Andes
Using remotes sensing and GPS to study volcanoes in Peru
By Frederick Engle, Mark Bulmer, Andrew Johnston, and Guido Salas

In the year 1600, one of the most violent volcanic eruptions in the history of the Western Hemisphere occurred at Huaynaputina in southern Peru. The blast was equivalent to the 1980 Mt. St. Helens eruption in explosive energy and volume of material ejected. However, few historical records exist of Huaynaputina's eruption. At the time, southern Peru was thinly populated with a few Spanish colonists and the remnants of an indigenous population devastated by conquest, famine, and disease. It is known that mudflows racing down from Huaynaputina destroyed several villages and killed about 1,400 people. Today, over one million people live among the volcanoes of southern Peru, and they are a population at risk. The historically active volcanoes in the Andes of southern Peru have the potential to erupt as violently as Huaynaputina. One of these volcanoes is Sabancaya.
    In 1997, a team of scientists from the Smithsonian Institution's Center for Earth and Planetary Studies (CEPS) at the National Air and Space Museum traveled to southern Peru to investigate reports of destructive mudslides triggered by the eruption of Sabancaya. The published reports indicated that the mudslides destroyed one village and killed twenty people.
    The team conducted interviews in the Rio Colca Valley and searched for evidence of mass movements. Based on interviews and physical evidence, it was determined that the reports were erroneous. One village, Maca, was destroyed by an earthquake in July 1991, an event that coincided with the eruption of Sabancaya. Many were injured and twenty people died in Maca, most from collapsing structures. No evidence was found of mudslides entering the village. About one-third of Maca's agricultural terraces were lost in landslides and slumping triggered by the earthquake. Several farmers were killed by falling rocks and collapsing agricultural terraces as the slopes above town gave way. Animals grazing on the terraces were carried down-slope, and about forty head of cattle were buried by earth and rock. Today Maca is just beginning to recover. However, Maca and fourteen other villages in the valley remain under imminent threat from Sabancaya's volcanism and related natural hazards. Based on observations of the volcano and the rural communities it directly threatens, the CEPS team began the Colca Valley Geohazards Project (CVGP).
    CEPS specializes in Earth studies and planetary geology using remote sensing. The CVGP is using RADARSAT as its primary remotely-sensed data source, supplemented with LANDSAT TM, SPOT panchromatic and multispectral, space shuttle, and aerial photography. The analysis of satellite data is combined with fieldwork using differential GPS and direct observation. Peruvian collaborators include geologists and botanists from the Universidad Nacional San Agustin (UNSA) in Arequipa. The project is funded from a variety of sources within the Smithsonian and NASA Stennis Space Center's Commercial Remote Sensing Program (CRSP). Remote sensing education is a major interest of CRSP, and this project is providing an example of how natural hazards scientists use the technology. Remote Sensing Expedition programs produced at the Smithsonian will be broadcast to subscribing middle schools featuring topics such as remote sensing, natural hazards, and volcanism. Videographers accompanied the CVGP team into the Andes of southern Peru to record material for the remote sensing expedition programs.
    Volcanism in the central Andes is the result of the subduction of the Nazca Plate beneath the South American Plate. As the Nazca Plate subducts and melts, lighter molten material, or magma, rises through the overriding crust. In the central Andes, the magma surfaces through point-source eruptions and fissures, much of it as evolved lavas such as andesite and dacite. Volcanoes associated with subduction zones tend to erupt more violently relative to those at spreading centers or hot spots, for example, Iceland and Hawaii. Familiar andesitic volcanoes include Mt. St. Helens, Pinatubo, and Krakatoa. Huaynaputina and Sabancaya are also andesitic volcanoes.
    One of the main objectives of the CVGP is to examine the geologic history of Sabancaya. Very little is known of this remote volcano. Extensive lava flows, that are younger than the last great ice age, originate from its flanks. However, historic records only mention brief activity in the late 1700s. Sabancaya re-awoke in the late '80s and its eruptive activity continues today, signaling its hazard potential to the 25,000 people living in the Colca Valley. During these recent eruptions, ash fall destroyed crops and poisoned water supplies in the valley. The ash covered and contaminated ichu and crespillo grasses on the surrounding altiplano forcing the relocation of alpaca and llama herds. In the valley, cattle died from grazing on ash-laden grass and valley residents suffered severe respiratory problems from inhaling falling ash.
    Ash deposited on nearby Nevado Ampato reduced albedo, thus increasing temperature on the surface, causing the mountain's permanent ice to melt. As Ampato's ice cap contracted, it exposed Juanita, the famous ice mummy discovered by a National Geographic expedition in 1995. Much of Sabancaya's permanent ice cap was buried under a thick layer of ash that insulates against solar radiation. A comparison of SPOT images acquired in July 1989, before the ash eruptions of the early '90s, and September 1998, reveal the changes in ice cover on the Hualca-Sabancaya-Ampato volcano complex. It should be noted that July and September are dry season months, and so seasonal snow extent is minimal. Sudden melting of Sabancaya's buried ice cap, a possibility during an eruption, could trigger fast moving mudflows that may travel down into the Colca Valley. The village of Achoma, population 1,500, lies in the most direct channel into the valley.
    With imagery and two field seasons of ground truth data, the CVGP team has begun to write the geologic history of Sabancaya. In the 1970s, researchers mapped Sabancaya's lava flows using Landsat MSS and their maps indicated eleven separate flows. CVGP researchers recently mapped the flows using higher resolution satellite imagery, including RADARSAT and oblique aerial photos taken from an aircraft chartered for observation flights over the study area. The CVGP maps show thirty nine lava flows and fifty one flow lobes. During the August and September 1998 field season, the CVGP team used a real-time differential GPS to collect data on the lava flows. The Trimble 4800 Total GPS base station was located on the east side of the volcano at the team's base camp, about 1 kilometer (km) from the distal edge of the lava flow and at an elevation of 4,400 meters (m). The base station receiver and UHF radio were powered by one of the teams' vehicles. The base and roving teams communicated by hand-held VHF radio.
    Working conditions at high altitudes can be difficult. Soroche, or altitude sickness, was avoided by spending several days acclimatizing at 2,400m and 3,600m elevation. Ascending and descending the lava flows with the roving GPS unit and other equipment could have triggered soroche, so the acclimatization period was time well spent. The team worked as high as 5,000m without serious problems from altitude sickness. The dry season weather was generally good, but high winds occasionally blasted the site. On these occasions, volcanic ash from previous eruptions was entrained by the wind and visibility dropped to several meters. The ash penetrated most of the equipment and seriously damaged the CCDs of one video camera. The GPS equipment's performance was not degraded by the ash. During the field work, Sabancaya's activity varied from fumarolic emissions to ash eruptions with plumes rising 1km. The activity did not impact data collection on the lava flows, but it did prevent the team from ascending to the summit crater.
    High-precision topographic profiles were obtained on a flow located on the eastern side of Sabancaya. The profiles are used to interpret the emplacement mechanism for thick lava flows characterized by surface ridges. During the topographic survey, background slope variations between the proximal, distal, and lateral margins were also measured. The coarse-scale surface structures along and across the flow were characterized using 5 transects with topographic data collected about every 3m. Fine-scale surface structure was determined from 3- 15m transects with topographic data collected at 10 centimeter intervals. With the differential GPS, we achieved a relative accuracy of about 1cm horizontally and 2cm vertically. The dominant cross-flow structures are well defined in the coarse-scale transects, and the profiles clearly show ridges, levees, and channels. The dependence of height variations between points a given distance apart was examined for each of the 5 coarse-scale transects. These data are being used to make statistical comparisons of surface roughness, where dimensions are greater than 1m, with a range of other lava flow surfaces. The influence of surface roughness on optical, infrared, and microwave measurements is fundamentally important to geologic interpretation of remote sensing data.
    Parameters such as lava flow thickness, length, and width are being used to derive information about yield strength, emplacement time, and viscosity. These are important factors to determine in any assessment of risks that future lava flows might pose to people living near the volcano. Other parameters such as ridge height and spacing between ridges on lava flows are being used in flow models to gain a better understanding of conditions effecting ridge formation. RADARSAT and ERS data for the volcano have been obtained at 44o and 23o incidence angles respectively. Quantitative comparisons of surface properties with other microwave datasets for thick lava flows may be possible in the future if active calibrators are deployed at Sabancaya during the project's next RADARSAT acquisition.
    The CVGP is using space technology in the form of satellite imagery, and ground truthing with GPS and other tools, to study a remote and potentially deadly volcano. Active volcanoes in southern Peru pose a serious threat to rural populations and large urban centers. Understanding the histories of these volcanoes, including characteristics of past eruptions, is essential to assessing the hazards they pose today and in the future. Results from the Colca Valley Geohazards Project will be shared with civil defense officials at the local, provincial, and national level in Peru. The data will be available for disaster planning as GIS technology comes into use by civil defense planners in Peru.

Acknowledgments
Trimble Navigation donated a Total 4800 system to CEPS for use in Peru and for other projects. American Airlines donated tickets for travel to Peru. Mountain Safety Research and Mountain Hard Wear provided equipment for the CVGP base camp. Special thanks to NASA Stennis Space Center's Commercial Remote Sensing Program under the direction of David Brannon and through David Powe for supporting CVGP field work and education programs. And finally, thanks to Don Tracia and Sean Elwell of the Smithsonian's Natural Partners Office for persevering at high altitudes and filming our work.

About the Authors:
Frederick Engle ([email protected]) is a geographer at the Smithsonian Institution's Center for Earth and Planetary Studies (CEPS) at the National Air and Space Museum. Mark Bulmer is a volcanologist at CEPS. Andrew Johnston is a research specialist at CEPS. Guido Salas is a professor of geology at the Universidad Nacional San Agustin in Arequipa, Peru.

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