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) Rincon 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).
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.
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.=20 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.
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.
Brief History
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.
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.
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 =20
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.
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.
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:
This report was written by Eduardo Malavassi
email: fguendel@irazu.una.ac.cr
or emalava@irazu.una.ac.cr
Best Regards,
Enrique Coen
Departamento de Fisica, Universidad Nacional, Heredia; Costa Rica
P.O.Box 86 , Heredia , 3000
Phone: (506)2773347
Fax: (506)2601197
email:ecoen@irazu.una.ac.cr
http://www.una.ac.cr/u/ecoen/