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OBSERVATORIO VULCANOLOGICO Y SISMOLOGICO DE COSTA RICA
UNIVERSIDAD NACIONAL
OVSICORI-UNA
Fax (506) 261-0303, Email ovsicori@irazu.una.ac.cr
ERUPTIVE ACTIVITY OF RINCON DE LA VIEJA VOLCANO, COSTA RICA
November 4-13, 1995
(Latitude: 10 49.40 Longitude: 85 19.42 Elevation: 1700 m)
Rincon de la Vieja is an active stratovolcano located in
northwestern Costa Rica on the Guanacaste volcanic range. The
area is remote making volcano monitoring difficult, and very few
population centers are located within a distance of 10 Km from
the volcano.
Since its last magmatic eruption in 1966-67, this volcano
has been having moderate to very vigorous fumarolic activity,
with periods of increased activity in which phreatic eruptions
are emitted from the active crater lake. Eruptions consist
mainly of cypresoidal jets of hot water, wet ash and water vapor.
The largest eruptions fall outside of the active crater.
The muddy hot water issued by the eruption normally descends
along the streams, that drain the steep area located north of the
active crater, producing hot mudflows or lahars. The drainages
under lahar hazard are the Penjamo and the Azul rivers. Both
rivers merge to flow north under the name of Pizote river.
Lahars sediments traveled in 1991, up to 18 Km from the source
along the Pizote river, which flows north to Lake Nicaragua.
On November 6, 1995 at 15:04 hr. local time (21:04 hr. GMT)
RincĒn de la Vieja volcano started a period of increased activity
which had its highest peak during November 8, 1995. After that,
the volcano continued to have very strong fumarolic activity and
small to moderate size vapor eruptions. This is a review of the
volcanic activity prepared by faculty of the Costa Rican
Volcanological and Seismological Observatory at Universidad
Nacional, Heredia, Costa Rica (OVSICORI-UNA).
Volcano Monitoring:
OVSICORI-UNA monitors Rincon de la Vieja volcano with a
permanent vertical short period seismographic station
(approximate gain of 40.000) which transmits to the main
laboratory in Heredia. The seismographic station is located five
kilometers southwest of the main crater. Seismic monitoring is
complemented with periodic field observations and geodetic
measurements. Geochemistry of ground water and gas condensates,
when samples can be collected, is also used to monitor changes in
volcanic activity. The base line of OVSICORI-UNA at Rincon de la
Vieja volcano was initiated around 1978, but seismic (real time)
monitoring started later in November, 1984.
Precursors of the Eruption:
Rincon de la Vieja volcano had moderate fumarolic activity
since the periods of increased fumarolic activity recorded in
1992 and 1991. The 1991 eruption produced lahars that destroyed
two bridges on the Penjamo and Azul rivers, north of the volcano.
In early October a group of park rangers from the Guanacaste
Conservation Area (GCA), affiliated with the Costa Rican National
Park System visited the volcano. The group reported to OVSICORI
an increment in the fumarolic activity of the volcano and also
the occurrence of landslides on the main crater walls.
Personnel from the GCA stationed at different sites near the
Interamerican Highway, 28 Km west-southwest of the volcano,
reported smelling sulfur on several occasions during the two days
prior to the eruption. These sites are located downwind from the
volcano.
During 1995, the total seismicity (low and high frequency)
recorded by the Rincon de la Vieja seismographic station was less
than 10 earthquakes per month until June and no tremor was
recorded during the same period. During July, August and
September, the monthly totals of seismicity were respectively 78,
40 and 26 earthquakes (low and high frequency added together). In
addition, the monthly totals of polychromatic tremor (medium
frequency: 2.0-2.3 Hz) ranged between 1-1.5 hours for July,
August and September.
Unfortunately, OVSICORI-UNA seismographic station was out of
service during a large portion of the month of October. The
station was repaired and started to work on October 31. The
seismic records available only cover six days prior to the
eruption. Low frequency earthquakes gradually increased from 4
(11/1) to 18 (11/5), but decreased to 11 on November 6. In
contrast, high frequency earthquakes were recorded only after
November 3, and decreased from 3 events (11/4) to 1 event (11/6).
Tremor was not recorded during November, prior to the beginning
of the eruption.
Initiation of the Activity:
The first eruption, started on November 6 at 15:04 hr.
(21:04 GMT), consisted of a moderate vapor eruption with
subordinate ash. The eruption was probably not higher than 1.5
Km above the summit of the volcano. The first eruption lasted
130 seconds and constant, disarmonic tremor produced by very
vigorous fumarolic activity was recorded for several hours after
the eruption.
DATE ----NUMBER OF ERUPTIONS RECORDED
11/6 --2
11/7 ----8 ---Largest eruptions a.m.
11/8 -- --24 --- Largest eruptions a.m.
11/9 ----27
11/10 ---- 23
11/11 ----17
11/12 ----22
11/13 ----22
Only two eruptions occurred during the first 17 hours after
the initial eruption. After that period, the volcano started to
erupt again, producing 8 eruptions on November 7 and 24 eruptions
on November 8.
The climax of the eruption occurred in the morning of
November 8 when the frequency and strength of eruptions was
largest. The eruptions occurred at 07:29 and 08:21 hr. on
November 8 produced eruption columns up to 3 kilometers high
above the active crater, with a rich content of water and wet
ash.
Two scientists from OVSICORI visited the summit area of the
volcano on November 7, between 9:00 and 11:30 hr., finding
craters up to 2 meter in diameter, produced by impacts from
blocks with diameters ranging between 0.5-1 m. Erupted blocks
were still hot 90 minutes after eruption, recording temperatures
up to 130 Celsius. Laboratory inspection of the ejected blocks
and ash indicated that no juvenile material was ejected, despite
the high temperature of some of the blocks. Other eruptions,
similar in size to those observed during the morning occurred
during the afternoon.
The scientific team from OVSICORI observed several eruptions
that morning when the team was located less than 500 meters from
the crater rim. The team described those eruptions as typical
phreatic eruptions extruded through a crater lake. The eruptions
produced jets of very dark, wet ash and hot water with
cypresoidal appearance that fell outside the active crater and
produced mudflows along the drainages of the Penjamo and Azul
rivers. After the fall back of wet ashes and blocks about one
kilometer in diameter from the active crater, only white vapor
clouds with small contents of ash will be transported by the
prevailing winds.
The highest frequencies and seismic signal amplitudes of
eruptions were recorded during the morning of November 8. Other
high amplitude events occurred the morning of November 7 and in
early morning November 9.
Avalanches related to lahars caused partial damage to the
bridge located on the Penjamo river, north of the volcano, along
the main road that partially surrounds the volcano on November 8,
near Dos Rios and Buenos Aires de Upala. In the upper part of
both drenaiges, erosion of up to several meters occurred as a
consequence of the lahar movement along the river bed. At lower
elevations deposition of mudflows occurred at the slope break.
Avalanches reported on November 7, were richer in water and
cooler than avalanches reported on November 8.
The speed at which lahars descended to the main road on the
north flank of the volcano near Buenos Aires de Upala, was
calculated using reports of the arrival of the fronts of
avalanches to the bridge on the Penjamo river by different
sources. The speeds ranged between 23-93 Km/h, with an average
of 44 Km/h. The time for the avalanche fronts to travel the 11
Km of drainage from the moment of onset of individual eruptions
to the moment of arrival to the bridge ranged between 7-28
minutes, with averages ranging between 15-20 minutes.
The shock wave of an eruption on November 9, 4:11 hr., was
felt in Liberia located 25 Km southwest of the volcano. Glowing
blocks were also reported for the same eruption by observers
located on the northern flank of Rincon de la Vieja. This report
confirmed the report of the OVSICORI field team regarding the
ejection of very hot blocks, that was made earlier the previous
day.
Eruptions seismic signals decreased in amplitude, had no
appreciable contents of ash and stopped producing mudflows
during the morning of November 9. Constant (24 hours/day) tremor
amplitudes decreased an order of magnitude by November 10 and the
duration of the tremor decreased to less than 1 hour by November
13.
Seismicity Recorded during the Eruption:
The trend to increase the low frequency events, reported
during the six days before the eruption, continued after the
onset of the eruption, when 19 events were recorded for a daily
total of 30 events on November 6. In contrast, only one high
frequency event was recorded that day. On November 7 and 8, the
number of low frequency events decreased to 13 events per day,
but the high frequency events increased to approximately 8 events
per day. On November 9, the number of high frequency events
decreased to 8 events and the number of high frequency events
decreased to 1. Beginning November 10, no more high frequency
events occurred and (1-5) intermediate frequency events started
to be recorded daily for the following days. Low frequency
activity continued to range between 7-3 events per day.
After the first eruption, constant, disarmonic tremor
produced by strong fumarolic activity was recorded for several
hours. The first tremor had medium frequency of 1.0 Hz, maximum
seismic amplitudes of 11 mm. and lasted for 105 minutes.
Immediately after, the amplitude of the tremor diminished to 1mm.
for 27 minutes. Beginning at 15:33 hr. tremor amplitude increased
to 6 mm for 75 minutes, followed by a gradual reduction of
amplitudes to 2.0-4.0 mm. Between 17:21 and 18:27 Hr.. tremor
increased its frequency to 2.3 Hz, with amplitudes of 1.0-4.0 mm.
Beginning at 18:27 Hr. on November 6, tremors were
constantly recorded (24 hours) until November 8, when only 10
hours of tremors were recorded. Tremors had frequencies of
1.0-1.5 Hz. and amplitudes of 1-8 mm. Tremor activity increased
again to 24 hours on November 9, with similar frequencies and
amplitudes of 1-4 mm. An order of magnitude reduction in tremor
amplitudes (.1-1.0 mm) occurred November 10-12, despite that
frequencies and daily hours remained unchanged. On November 13,
less than 1 hour of tremor was recorded.
Characterization of the Eruption:
Eruptions occurred between November 6-13 at Rincon de la
Vieja volcano are similar to previous eruptions of that volcano,
i.e.. 1991. The presence of jets of water carrying sediments
from the crater lake, cypresoidal shaped eruption columns, lahar
avalanches produced several minutes immediately after many of the
eruptions, and the absence of juvenile material suggests that
these eruptions were phreatic eruptions. A transition to
phreatomagmatic was perhaps beginning to occur, at the moment
when the intensity of the eruption diminished, as there is a
report of glowing blocks for one of the eruptions, but the
transition was never completed.
FACULTY AND STAFF OF OVSICORI THAT COVERED THE RINCON DE LA VIEJA
VOLCANIC CRISIS 6-13 NOVEMBER, 1995: Erick Fernandez, Vilma
Barboza, Eliecer Duarte, Rodolfo Van der Laat, Eduardo Malavassi,
Henry Rodriguez, Antonio Mata, Ana Arguedas, Zaida Campos, Jorge
Brenes, Federico Guendel, Carlos Montero, Victor Gonzalez, Tomas
Marino, Enrique Hernandez. This report was written by Eduardo
Malavassi.
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