Hazard-Zone Maps and Volcanic RiskLiving With Volcanoes: The U.S.Geological Survey's Volcano Hazards Program: USGS Circular 1073, 57p. |
|
The purpose of hazard-zone maps is to give accurate information on the type and
frequency of volcanic eruptions and consequent volcanic and hydrologic processes
that could impact a given area -- information vital to sound land-use planning.
In 1951, two decades before the first comprehensive hazard assessment for
volcanoes in Hawaii, a developer could have felt secure in placing a
subdivision on Kilauea's lower east rift zone knowing that: (1) Kilauea had not
erupted at all fro the past 17 years, and (2) Kilauea had not erupted on its
lower east rift zone since 1840, more than a century of inactivity. Such time
periods are long by human standards and commonly influence judgment in land-use
decisions. But he would have made a mistake. Kilauea became active again, at
its summit, in 1952 and over the succeeding 38 years there were 13 separate east
rift eruptions, two of which persisted longer than 4 years. From the first
return of activity at the lower east rift zone in 1955 to the present, almost 30
percent of the land surface between the rift zone and a 51-kilometer stretch of
coastline has been covered by lava.
Geologic mapping and dating of volcanic deposits at Kilauea form the basis for an astonishing statistic -- over 90 percent of the land surface of Kilauea has been covered by lava since the time of arrival of the Hawaiians, about 1,500 years ago. Had the developer been made aware of the entire history of Kilauea's activity, the risk of developing the east rift zone would have been understood to be higher than could be inferred from just considering the record of the preceding 111 years. The 123-year period of inactivity at Mount St. Helens also created a false sense of security regarding future activity. however, in this case, a hazard assessment was made in 1978 which correctly inferred an end to the period of repose and alerted people to the possibility of renewed activity.
|
The 1985 Ruiz eruption offers another, much more tragic example of the need to
understand the entire history of a volcano in assessing hazards. The town of
Armero, Colombia -- buried by mudflows triggered by the 1985 eruption at Nevado
del Ruiz -- was located on a debris fan that was overrun by destructive mudflows
in the year 1595, shortly after the arrival of the Spanish colonists, and again
in 1845, killing hundreds of people in each instance. During the ensuing
140-year period of inactivity, people forgot and the town was rebuilt at the
same site and grew in population. Although a preliminary hazard-zone map for
Ruiz, completed one month before the November 1985 eruption, clearly delineated
Armero as being especially vulnerable to mudflows, emergency-response measures
taken during the eruption were entirely inadequate to save the more than 23,000
lives lost when the mudflows struck.
