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Report on the October 30, 1998 avalanche and breakout flow of
 Casita Volcano, Nicaragua, triggered by Hurricane Mitch
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Author: Jonathan Fink, Arizona State University
Source: Volcanoes listserv, Internet, posted 11/15/98 |
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On October 30, 1998 a disastrous event (called a "mudflow" in the newspapers) occurred on the
south flank of Casita Volcano. According to official reports, the incident killed between 1560
and 1680 people, displaced hundreds more, destroyed several towns and settlements, and
disrupted the Pan American Highway at numerous bridges. For several days there was
uncertainty about the origin of the natural event. On 11-12 Novwmber, the first scientific team
visited the volcano to determine the causes and effects of the disaster. The team examined the
summit area on the first day and made a complete traverse of the devastated zone as far south as
the Pan American Highway on the second day. This report presents the conclusions of the team
and provides some recommendations regarding future risks at this volcano.
Background
Casita Volcano (1405 masl) is within the Cordillera Maribios, a 70 km long volcanic chain that
extends from the northern shore of Lake Managua to the vicinity of Chinandega. Casita is part of
the San Cristobal volcanic complex that consists of 5 principal volcanic edifices (Hazlett, 1987).
San Cristobal, 4 km WNW of Casita, is the largest volcano in Nicaragua (1745 masl). It has
exhibited frequent episodes of historic activity and at the present it is emitting a vigorous
fumarolic plume. For these reasons San Cristobal has been studied in more detail than Casita.
Casita is a composite volcano with deeply dissected morphology. The top of the volcano consists
of a cluster of dacite (?) domes. At its summit is a 1-km diameter crater that could be reached by
a road, now impassable, servicing telecommunication towers. A set of prominent NW trending
normal faults cut the summit area bounding each side of the crater. Explosion craters on the
southern plain are aligned along a conjugate set of fractures trending NW-SE. The domes of the
summit area are autobrecciated and exhibit strong hydrothermal alteration, which is consistent
with low temperature fumarolic activity observed during past decades (Sapper, 1913; Hazlett,
1987). There has been no historic volcanic activity reported here.
Meteorological conditions
Hurricane Mitch was a major factor in the disaster of Casita. Abnormal rainfall related to Mitch,
recorded at the nearby city of Chinandega, began on October 25. By October 27 the precipitation
reached 100 mm per day and continuously increased to its maximum of almost 500 mm per day
on October 30, the day of the avalanche. Subsequently precipitation continuously dropped to
normal levels within three days. The rainfall on October 30 was exceptional. The normal
monthly average for October is 328 mm. In 1998 the October rainfall was 1984 mm, which is
more than 6 times the normal average. |
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