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: 1700m

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.

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).

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.
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.

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.

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:

This report was written by Eduardo Malavassi
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