The information presented here represents preliminary research as the result of ten-weeks of investigation in-residence at the National Museum of Natural History. This is not an official publication of the information.

As preliminary information, results and/or findings should not be cited as part of conclusive work. Please contact the authors first if you wish to utilize the information presented here.

ABSTRACT

Modern technology, along with the exponentially growing population has led to human travel and occupancy in areas of the world never inhabited before. The major issues here are rare events such as great earthquakes and certain volcanic eruptions that now result in fatalities more frequently.

The focus here is an event at Arenal Volcano, Costa Rica, where two individuals sustained fatal injuries in a pyroclastic flow erupted on August 23, 2000, in spite of active seismic monitoring. Arenal volcano has been continuously active since it first erupted in 1968, killing 78 individuals. In order to identify any predictive tools useful in preventing this type of tragedy, seismic data from the event were examined. Individual signals were characterized and plotted in order to create a 7-month history before the event.

The August 23rd event has a distinct signal that also occurred briefly in June and July, but not nearly with the length or magnitude of that of the fatal event. While the number of events steadily increased in the three months leading up to the August 23rd event, we found no patterns that could be used to predict pyroclastic flows.

Samples from the flow were examined with an electron microprobe using WDS and EDS in order to gain information on water content. Classically, the presence of higher levels of calcium within plagioclase grains indicates higher water content upon crystallization. A higher water content is often an indicator of a higher level of explosive potential. Thus, comparing samples from the fatal pyroclastic flow to a "normal" lava flow will indicate whether the magma composition could have been responsible for the pyroclastic flow.

In this study, the Ca levels of the pyroclastic flow block were not statistically different from that of the lava flow, and thus the mechanism of eruption could have been simply an abnormally large volume of buoyant magma moving out of the crater, responsible for the unusual seismic signal (X).

While the prediction of hazardous pyroclastic flows using scientific data was not illustrated through this research, the methods of data reduction engaged here were only some of the few available to predict and study events.

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WHAT IS A RARE EVENT?

One of the most difficult phenomena to predict and manage are certain rare natural events. The important concept here is that pyroclastic flows are particularly common. However, a pyroclastic flow as violent and deadly as that of August 23rd is quite noteworthy, and the combination of circumstances with which it occurred is rare. In order to prevent such devastation in the future, the event itself must be examined thoroughly for evidence of precursors.

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HISTORY

Arenal Volcano is a small stratovolcano that lies in the Alajeula Province of Costa Rica and the youngest cone in the Arenal-Chato system (MAP). Continuously active, although with decreasing intensity since 1968, the volcano displays continuous explosive activity and lava flows with occasional pyroclastic flows, activities typical of strombolian eruptions.

1968: Initial and Most Intense Destructive Phase

- Explosively ejected blocks, hot avalanches and base surges cause vast rainforest and arable land damage and 78 deaths

- Warning signs including earthquakes began 12 hours prior to eruption

- Thought to be inactive so no preventative damage actions taken
Later Pyroclastic Flows

- Substantial events in August 1993, May 1998, August 2000 and May 2001

- No recorded warning signals

- Scientists estimate 3.6 pyroclastic flows every year and one of substantial magnitude every 4.6 years
August 23rd, 2000

- Two American tourists and guide were ascending and viewing summit on a well-traveled Northeast slope path when margin of flow moving 60 mph engulfed them

- Costa Rican guide carried young girl down to base and died from severe burns

- Girl was airlifted to the States and died days later of severe burns; Mother survived

Map of Costa Rican Volcanoes

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GEOCHEMISTRY / PETROLOGY

Samples from the August 23rd pyroclastic flow as well as samples from a "normal" lava flow were analyzed using an electron microprobe in order to detect differences in their explosive potential, which increases with water content. Both samples were basaltic andesite with phenocrysts of plagioclase, orthopyroxene and clinopyroxene . The compositions of the margins of the plagioclases in the two samples were found to be indistinguishable by electron microprobe analyses, suggesting similar water contents and thus, similar explosive potential. In light of these data, the most likely cause of the August 23rd event was the upwelling of a large volume of buoyant magma.

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SEISMIC DATA

In order to characterize the events of August 23rd, a classification system was designed to separate the signals of January through August of 2000 into nine distinct categories. Later, the categories were combined to aid in the detection process . The goal was to identify any signal that could potentially act as a precursor to pyroclastic flows. The first occurrence in 2000 of any signal that could be characterized as similar to the August 23rd signal, was on June 2nd. The second occurrence was on July 7th. Both events lasted approximately 2 minutes, which is much shorter than the signal on the 23rd which occurred in 4 blocks from 7:13-8:04 GMT, for a total of about 11 minutes. Overall, the signals like those of the fatal event were extremely rare, and were not predicted by any identifiable precursors.

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Seismic categories

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Summary graph of all seismic signals, special attention to signal X and mystery tremor storm in September

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CONCLUSION

We found no evidence that would have allowed us to predict the August 23rd pyroclastic flow. After the August 23rd disaster, the Costa Rican government received enough pressure to begin taking action. The National Emergencies Commission, in conjunction with the Volcanology and Seismology Observatory of Costa Rican has been given the power to restrict tourist activity around the volcano. Further construction around the danger areas has been strongly discouraged, despite the fact that the largest resort currently stands atop the 1968 pyroclastic flow. In order to prevent further fatalities, on-site monitoring of the area must begin, along with documentation and research into precursory conditions and public education of the properties of rare, potentially destructive pyroclastic flows.

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SUGGESTIONS FOR FURTHER RESEARCH

As the world's population continues to expand, more and more individuals will be forced to reside in areas with active volcanoes. Further seismic studies, along with advancements in hazard mitigation and continued engineering advancement will help protect the residents and tourists that reside in these areas. We encountered a number of unexplained signals, including symmetrical repeating signals, that are worthy of further study. Perhaps the largest factor in protecting the individuals as well as the valuable farmland and irreplaceable forest that surrounds these volcanoes is public education, which will explain and illustrate the damage that these pyroclastic flows can cause so that action can be taken to prevent further catastrophes.

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ENTOMOLOGY

Sarah Garrett
"An Inventory of Leafroller Species of Moths of the Great Smoky Mountains National Park"

Abstract / Summary

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