Observatorio Vulcanológico y Sismológico de Costa Rica

URI permanente para esta comunidadhttp://10.0.96.45:4000/handle/11056/14933

El Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI) de la Universidad Nacional, es un instituto de investigación universitaria dedicado a la investigación de los volcanes, los sismos y otros procesos tectónicos, con el propósito de encontrar aplicaciones útiles a la sociedad que ayuden a mitigar los efectos adversos de esos fenómenos al desarrollo económico y social. Se trata de un observatorio, por cuanto una cantidad considerable de su esfuerzo va orientada a documentar la actividad sísmica, volcánica y la deformación cortical que, a su vez, retroalimenta a las actividades investigativas propias de un instituto de investigación universitaria.

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Página web: www.ovsicori.una.ac.cr

Correo: ovsicori@una.cr

Teléfono: (506) 2562 4001 / (506) 2261 0611 / (506) 2261 0781

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Mostrando 1 - 20 de 198

50 years of steady ground deformation in the Altiplano-Puna region of southern Bolivia
(Geological Society of America, 2017-12-20) Gottsmann, Joachim; Del Potro, Rodrigo; Muller, Cyril
The Altiplano-Puna Volcanic Complex of the Central Andes is host to an ~150-km-wide, quasi-circular ground deformation anomaly centered on Uturuncu volcano (Bolivia). The precise onset and duration of this deformation is unclear, but geomorphologic studies bracket its initiation at less than a few hundred years ago. Here we report on the deformation history over an ~50 yr period by deriving orthometric height changes from leveling and global navigation satellite system (GNSS) observations at 53 benchmarks along a regional leveling line that crosses the deformation anomaly. The comparison of interferometric synthetic aperture radar (InSAR) line-of-sight (LOS) displacements and LOS-projected orthometric ground velocities in a common reference frame reveal central uplift extending to ~35 km from Uturuncu at a maximum orthometric rate of 1.2 cm yr–1, and peripheral subsidence at a maximum rate of 0.3 cm yr–1 to ~60 km from Uturuncu. This pattern is consistent with the spatial extent and average rate of deformation observed by InSAR. Our interpretation of the data is that long-wavelength ground uplift at Uturuncu has likely occurred at a quasi-constant rate for at least half of a century. This study bridges the observational time spans between modern satellite geodetic observations (up to a few decades) and geomorphological observations (a few centuries and longer) of the recent deformation history of the continental crust in the Central Andes and adds to a select group of case studies of quantifiable long-term volcano deformation worldwide.
A comparison of different indirect techniques to evaluate volcanic intact rock strength
(Springer, 2009) Del Potro, Rodrigo; Hürlimann, Marcel
The efficiency of different methods that are used to evaluate volcanic intact rock strength is compared that includes Schmidt hammers, point load tests and uniaxial compression tests. The Schmidt hammer records the rebound height of the mass (R) on a linear scale and gives an indication of the strength of the material being tested. The two types of Schmidt hammers (L- and N-type), with different impact energies are used that shows results as the rebound height RLand RN for the L- and N-type Schmidt hammers, respectively. The other method used is point load test that provides strength measurements of irregular fragments of rocks or of rock cores, with all rock dimensions being greater than 50 mm. The last method is uniaxial compression that shows little scatter and clear logarithmic increase of uniaxial compressive strength with increasing unit weight. The studies showed that Schmidt hammers can give results similar in quality to uniaxial compression tests.
A Creep Event on the Shallow Interface of the Nicoya Peninsula, Costa Rica Seismogenic Zone
(American Geophysical Union, 2004) Protti, Marino; González, Victor; Kato, Teruyuki; Iinuma, Takeshi; Miyazaki, Shinichi; Obana, Koichiro; Kaneda, Yoshiyuki; LaFemina, Peter; Dixon, Timothy; Schwartz, Susan
Summary of the paper "A Creep Event on the Shallow Interface of the Nicoya Peninsula, Costa Rica Seismogenic Zone"
A long-term record of polythionates in the acid crater-lake of Poás volcano: Changes in the subaqueous input of fumarolic gases
(IAVCEI. General Assembly. Pucón-Chile, 2004-01) Martínez, M.; Van Bergen, M. J.; Fernández, E.; Takano, B.; Malavassi, E.; Barboza, V.; Miura, Y.; Van der Laat, R.; Duarte, E.; Valdés, J.; Sáenz, W.
A model of degassing and seismicity at Arenal Volcano, Costa Rica
(Universidad Nacional, Costa Rica, 2001-08) Williams Jones, G; Stix, J; Heiligmann, M; Barquero, J; Fernandez, E
Arenal volcano is the most active volcano in Costa Rica and has emitted at least 1.3 Mt of SO2 since its reactivation in July 1968. Gas emissions from the volcano have been both by passive degassing and explosive eruptions, with passive degassing being dominant. Based on correlation spectrometry (COSPEC) measurements made during 1982, 1995 and 1996, the minimum average daily output is 130 ± 60 t d-1 SO2 emitted from Arenal. Arenal is extremely active, with tremor fluctuations showing a distinct correlation with Earth tides; decreased explosive activity and increased tremor appear to coincide with the maximum rate of change in Earth tides. This suggests that a system such as Arenal may be extremely sensitive to small changes in the confining pressure or stress regime of the conduit. The cyclic nature of explosive activity also may be caused by corresponding fluctuations in the extrusion rate of lava. At high extrusion rates, lava from the non-explosive conduit may overflow into the explosive conduit, temporarily blocking it with a resulting pressurisation of the system. Arenal is likely tapping a deep to mid crustal magma chamber and, unlike many volcanoes, there is a comparatively small difference between petrological and COSPEC SO2 estimates (0.41 vs. 1.3 Mt, respectively, since 1968), suggesting that Arenal is being continuously supplied by fresh magma. © 2001 Elsevier Science B.V. All rights reserved.
A model of diffuse degassing at three subduction-related volcanoes
(Universidad Nacional, Costa Rica, 2000) Williams-Jones, G; Stix, J; Heiligmann, M; Charland, A; Sherwood Lollar, B; Arner, N; Gustavo Garzón, V; Barquero, J; Fernandez, E
Radon, CO2 and δ13C in soil gas were measured at three active subduction-related stratovolcanoes (Arenal and Poas, Costa Rica; Galeras, Colombia). In general, Rn, CO2 and δ13C values are higher on the lower flanks of the volcanoes, except near fumaroles in the active craters. The upper flanks of these volcanoes have low Rn concentrations and light δ13C values. These observations suggest that diffuse degassing of magmatic gas on the upper flanks of these volcanoes is negligible and that more magmatic degassing occurs on the lower flanks where major faults and greater fracturing in the older lavas can channel magmatic gases to the surface. These results are in contrast to findings for Mount Etna where a broad halo of magmatic CO2 has been postulated to exist over much of the edifice. Differences in radon levels among the three volcanoes studied here may result from differences in age, the degree of fracturing and faulting, regional structures or the level of hydrothermal activity. Volcanoes, such as those studied here, act as plugs in the continental crust, focusing magmatic degassing towards crater fumaroles, faults and the fractured lower flanks.
A New Sulfur and Carbon Degassing Inventory for the Southern Central American Volcanic Arc: The Importance of Accurate Time-Series Data Sets and Possible Tectonic Processes Responsible for Temporal Variations in Arc-Scale Volatile Emissions
(AGU Publications, 2017-12-12) Muller, C.; Saballos, A.; Ibarra, M.; LaFemina, Peter; de Moor, Joost Maarten; Kern, Christoph; Avard, Geoffroy; AIUPPA, Alessandro; Protti, Marino; Fischer, Tobias
This work presents a new database of SO2 and CO2 fluxes from the Southern Central American Volcanic Arc (SCAVA) for the period 2015–2016. We report 300 SO2 flux measurements from 10 volcanoes and gas ratios from 11 volcanoes in Costa Rica and Nicaragua representing the most extensive available assessment of this 500 km arc segment. The SO2 flux from SCAVA is estimated at 6,24061,150 T/d, about a factor of three higher than previous estimations (1972–2013). We attribute this increase in part to our more complete assessment of the arc. Another consideration in interpreting the difference is the context of increased volcanic activity, as there were more eruptions in 2015–2016 than in any period since 1980. A potential explanation for increased degassing and volcanic activity is a change in crustal stress regime (from compression to extension, opening volcanic conduits) following two large (Mw>7) earthquakes in the region in 2012. The CO2 flux from the arc is estimated at 22,50064,900 T/d, which is equal to or greater than estimates of C input into the SCAVA subduction zone. Time-series data sets for arc degassing need to be improved in temporal and spatial coverage to robustly constrain volatile budgets and tectonic controls. Arc volatile budgets are strongly influenced by short-lived degassing events and arc systems likely display significant short-term variations in volatile output, calling for expansion of nascent geochemical monitoring networks to achieve spatial and temporal coverage similar to traditional geophysical networks.
Active Subduction on Both Coasts of Costa Rica Does not Represent an Important Tsunami Hazard
(2007-05-22) Protti, Marino; González, V.
Active Thrusting in the Inner Fore arc of Middle America, Costa Rica
(American Geophysical Union, 2004) Sitchler, J. C.; Fischer, D. M.; Gardner, T. W.; Protti, Marino
Summary of the paper "Active Thrusting in the Inner Fore arc of Middle America, Costa Rica"
La actividad explosiva del volcán Turrialba (Costa Rica) en el período 2010 - 2016
(Universidad de Costa Rica, 2016-12-18) Brenes-André, José; Barrantes, Manuel; Vega, Eduardo; De Moor, J. Maarten; Avard, Geoffroy; Dellino, Pierfrancesco; Mele, Daniela; DeVitre, Charlotte; Di Piazza, Andrea; Rizzo, Andrea L.; Carapezza, Maria L.; Alvarado, Guillermo E.
The most recent eruptive activity of Turrialba volcano began on the 5th of January 2010, after more than a century of dormancy. The fragmentation process and aerodynamic behavior of the ash from Turrialba’s vulcanian eruptions were investigated by combining grain-size, petrography, mineralogy, Scanning Electron Microscopy (SEM) and Energy Dispersive System (EDS) analyses. The ash components include variable percentages of accessory fresh (no necessary juvenile) to hydrothermally altered lithics (15 - 50 % vol.) with hydrothermal minerals (≤ 12 % vol.: anhydrite, gypsum, bassanite, alunite, hexahydrite, pyrite, heulandite, native sulfur), clay minerals (8 - 17 % vol.: montmorillonite, halloysite, allophane), and a smaller quantity of fresh glassy ashes (5 - 49 % vol.) as fragments and shards (3 - 20 % vol. tackylite and 2 - 26 % vol. sideromelane), as well as primary and eroded/recycled phenocrysts (3 - 13 % vol.: 1 - 5% vol. plagioclase, 1 - 7 % vol. pyroxene, 0 - 1 % vol. olivine, 0 - 6 % vol. opaques, cristobalite and tridimite), and xenocrysts (≤ 1 % vol.: riebeckite and biotite). The secondary minerals were sourced from the deeper to surficial hydrothermal system. Textural features identified in ash particles (90 - 350 μm) suggest that they were formed by brittle fragmentation of vesicle-poor magma/water interaction; molten structures seems to be related to the ductile behavior of some fragments, probably due to the high temperature (> 600 °C) of the fumarolic/magmatic system. The percentage of juvenile components was low (1 - 2 % vol.) at the first opening eruptive phase in January 2010, and it increased steadily until the end of 2016 (ca. 12 - 25 % vol.) . The ash eruptions in the Western Crater from 2014 to 2016 were related to one and later two or three simultaneously active vents fed by distinct conduit branches. The alternation of volcanic explosions (VEI: 0 - 2), from closed conduit and the formation of new craters, to open system with phreatomagmatic events, and the repose intervals (inter-eruptive exhalative degassing), were controlled by the rate at which magma ascended and remained in the volcanic edifice. The recent tephra sequence consists of a complex succession of layers generated by contrasting fragmentation and transportation dynamics. They resulted from fully diluted, low temperature (< 300 °C), pyroclastic density currents (wet surge deposits), originated by short-lived, singlepulse, column collapse of phreatomagmatic columns, which traveled short distances (< 1000 m) from the vent area and surmounted topographic obstacles, and simultaneous fallout and ballistic ejecta. The fine material, in continuous suspension within the uppermost part of the convective plume, was dispersed into the atmosphere and finally settled down on the Valle Central. The quiescent phases could be related to a temporal cooling of the magmatic dike system or to a waning of magmatic activity at depth. Sequential fragmentation/transport theory (SFT) was used to decompose grain-size distributions into five different sub-populations. A new way of using the resultant fragmentation coefficient to assign sub-populations to different fragmentation mechanisms, even in cases when modes overlapped, is presented. For the first time the corresponding results are consistent with the phreatomagmatic eruptions, as well as with those derived from ab initio fractal model.
Actividad sismotectónica en Costa Rica durante el 2025
(Universidad Nacional, Costa Rica, 2025-12) Vega Solano, Floribeth; Urrutia Jiménez, Walter; Sequeira Arguedas, Jose María; Núñez Alpízar, Evelyn; Campos Salas, Nahomy; Protti, Marino; Hajaji Salgado, Sonia; Chaves, Esteban J
Durante el 2025 el Observatorio Vulcanológico y Sismológico de Costa Rica, de la Universidad Nacional, localizó 14481 sismos en territorio costarricense, tanto terrestre como marítimo. De estos, únicamente 123 fueron percibidos por la población, siendo el ocurrido el 21 de octubre, a las 21:57 horas, el más grande, con una magnitud ( de 𝑀𝐿)6,2 y ubicado a 17 km de profundidad, justo en la interfaz entre la placa del Coco y la microplaca de Panamá. El epicentro de este evento se localizó a 19 km hacia el suroeste de Manuel Antonio, Quepos, en el Pacífico Central. De la actividad sísmica del 2025, lo más extraordinario fue la ocurrencia de 5 secuencias sísmicas importantes: 1) en el flanco sur del volcán Tenorio al norte de Tilarán, 2) en el Parque Nacional Juan Castro Blanco, 3) en los alrededores de Barrio Aranjuez en San José, durante los meses de agosto y septiembre, 4) en Quepos, Pacífico Central y sobre todo 5) una secuencia de más de 1000 eventos frente a la costa de Dominical en el Pacífico Sur de la Provincia de Puntarenas.Finalmente, el mes con la mayor cantidad de eventos sísmicos corresponde con el mes de octubre, mientras que el mes con la menor actividad sísmica fue el mes de enero. En su mayoría, la actividad sísmica en Costa Rica correspondió con actividad sísmica de fondo y la magnitud promedio de sismos diarios fue de . 2,2± 0,3
Aerial strategies advance volcanic gas measurements at inaccessible, strongly degassing volcanoes
(American Association for the Advancement of Science, 2020-10-30) Liu, Emma; Bitetto, Marcello; Clarke, Robert; Edmonds, Marie; Hayer, Catherine; Nowicki, Scott; Schipper, C Ian; Aiuppa, Alessandro; Alan, Alfredo; Arellano, Santiago; Bobrowski, Nicole; Carn, Simon A.; Corrales, Ernesto; Moor, J. Maarten de; Díaz, Jorge Andrés; Fischer, Tobias P.; Freer, Jim E.; Fricke, G. Matthew; Galle, Bo O.; Itikarai, Ima; Jones, J.; Mason, Emily; Mulina, Kila; Rahilly, Kristen E; Rüdiger, Julian; Watson, I. Matthew; Gerdes, Gustav; Giudice, Gaetano; Gutmann, Alexandra; McCormick, Brendan; Richardson, Thomas
Volcanic emissions are a critical pathway in Earth’s carbon cycle. Here, we show that aerial measurements of volcanic gases using unoccupied aerial systems (UAS) transform our ability to measure and monitor plumes remotely and to constrain global volatile fluxes from volcanoes. Combining multi-scale measurements from ground-based remote sensing, long-range aerial sampling, and satellites, we present comprehensive gas fluxes—3760 ± [600, 310] tons day−1 CO2 and 5150 ± [730, 340] tons day−1 SO2—for a strong yet previously uncharac terized volcanic emitter: Manam, Papua New Guinea. The CO2/ST ratio of 1.07 ± 0.06 suggests a modest slab sediment contribution to the sub-arc mantle. We find that aerial strategies reduce uncertainties associated with ground-based remote sensing of SO2 flux and enable near–real-time measurements of plume chemistry and carbon isotope composition. Our data emphasize the need to account for time averaging of temporal variability in volcanic gas emissions in global flux estimates
Aftershocks of the 2012 Mw 7.6 Nicoya, Costa Rica, earthquake and mechanics of the plate interface
(Seismological Society of America (SSA), 2017-05-02) Chaves, Esteban; Duboeuf, Laure; Schwartz, Susan; Lay, Thorne; Kintner, Jonas
Subduction of the Cocos plate beneath the Nicoya Peninsula, Costa Rica, generates large underthrusting earthquakes with a recurrence interval of about 50 yrs. The most recent of these events occurred on 5 September 2012 (Mw 7.6). A vigorous sequence of more than 6400 aftershocks was recorded by a local seismic network within the first four months after the mainshock. We determine locations and focal mechanisms for as many aftershocks as possible with M ≥1:5 occurring within the first nine days of the mainshock, all aftershocks with M ≥3 through the end of 2012, and all events with M ≥4 through the end of 2015. We determine faulting geometries using regional full waveform moment tensor (MT) inversion for the largest events (M ≥4) and P-wave first-motion polarities for smaller events, producing a mechanism catalog with 347 earthquakes. Sixty percent of these events are identified as underthrusting, and their locations are compared with spatial distributions of mainshock slip, afterslip, prior interplate seismicity, and slow-slip phenomena to better understand the mechanical behavior of the plate interface. Most of the aftershocks on the megathrust occur up-dip of the coseismic slip, where afterslip is large, and between coseismic slip and shallow slow-slip patches. The pattern of interplate seismicity during the interseismic period is similar to that for the aftershocks but does not extend to as great a depth. The coseismic slip extends even deeper than the interplate aftershocks, suggesting that the mainshock ruptured a strongly locked patch driving down-dip slip into the conditionally stable part of the deep plate interface that also hosts slow slip. About 80% of the aftershocks have one nodal plane oriented favorably to promote failure from static stress changes following the mainshock and early afterslip, whereas most others occur in regions of large afterslip.
Algunas Observaciones en el volcán Poás: 16 de diciembre de 2015. (Reporte de campo)
(Universidad Nacional, Costa Rica, 2015) Duarte, Eliecer
Durante la visita del 16 de diciembre al volcán Poás se realizaron observaciones en el lago, el domo y alrededores de la cavidad cratérica. En este informe de campo se compilan descripciones de hasta 7 puntos que se muestran en la figura siguiente y en la que se ubican los sitios visitados (o descritos) para fines de orientación geográfica.
Algunos Efectos Proximales y Distales por Acumulación de la Ceniza: Volcán Turrialba (Reporte de campo: 22-24 de setiembre de 2016)
(Universidad Nacional, Costa Rica, 2016) Duarte, Eliecer
En este informe de campo se presenta un resumen de observaciones generales realizadas a raíz de la actividad volcánica con emanación de ceniza entre el 19 y 24 de setiembre del 2016.
Along-Arc Variations in Attenuation in the Nicaragua-Costa Rica Mantle Wedge
(2006) Rychert, Catherine; Fischer, K. M.; Abers, G. A.; Syracuse, E.; Protti, Marino; González Salas, V.; Strauch, Wilfried
Along-strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica
(American Geophysical Union, 2002) Newman, Andrew V.; Schwartz, Susan; González Salas, Victor; Deshon, Heather R.; Protti, Marino; Dorman, LeRoy
At the subduction zone in northwestern Costa Rica, the seismogenic zone lies directly beneath the Nicoya Peninsula, allowing for near source seismic studies of earthquake activity. We located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the Nicoya Peninsula. Using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth, 10 km in the south to 20 km in the north, that occurs where the subducting oceanic crust changes from warmer Cocos-Nazca Spreading center (CNS) origin to colder East Pacific Rise (EPR) origin. We argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath Nicoya, Costa Rica; subducting colder oceanic crust deepens the seismogenic updip limit.
An improved P-wave velocity reference model for Costa Rica
(Universidad Nacional Autónoma de México, 2001-01-01) Quintero, Ronnie; Kissling, Edi
Derivamos un modelo de velocidad unidimensional para la onda P en Costa Rica, el cual puede servir en la rutina de localización de eventos sísmicos y como modelo de referencia para tomografía en 3 dimensiones. La inversión para la velocidad es realizada usando 822 sismos con buena ubicación y 14 774 observaciones de la onda P, las cuales fueron obtenidas combinando datos de tiempos de arribo de 10 335 eventos en el periodo 1984-1997 colectados por el Observatorio Vulcanológico y Sismológico de la Universidad Nacional de Costa Rica (OVSICORI-UNA) y 3510 eventos en el periodo 1992-1998 colectados por la Red Sismológica Nacional (RSN). Durante el proceso de fusión, se tomó un cuidado estricto para reducir el número de errores en los datos, y en particular, para adaptar, corregir y completar los parámetros de las estaciones sísmicas. Un modelo de velocidad unidimensional es prerequisito para tal proceso de fusión cuando la consistencia y calidad tienen prioridad sobre la totalidad del resultante juego de datos. Los datos finales para el periodo 1984-1998 en Costa Rica consiste de 11 848 eventos locales con 13 2331 observaciones de ondas P y 86 018 de ondas S.
Análisis de la actividad sísmica en Costa Rica durante el 2010 y resumen de los eventos sísmicos más importantes presentados en Costa Rica de 1983 - 2012
(Universidad de Costa Rica, 2014-01) Segura Torres, Juan; Quintero, Ronnie; Burgoa Rosso, Billy; Jiménez Urrutia, Walter
El objetivo de este trabajo es analizar la actividad sísmica registrada por la red sismográfica del OVSICORI-UNA en el año 2010; además de recopilar información de los sismos más importantes sentidos por la población del país desde 1984. Para la localización de los sismos se usaron los arrivos de las ondas de P y S: así como los primeros impulsos y las formas de onda para analizar características de la fuente sísmica. A causa del proceso de subducción de la Placa del Coco por debajo de la placa Caribe, se produjeron dos sismos de magnitudes 6.2 y 6.1 Ml al suroeste de Puerto Quepos y otro de 5.3 Ml en la entrada del Golfo de Nicoya; éstos no generaron daños pero fueron sentidos fuerte por la población. Se localizaron dos enjambres sísmicos corticales; uno en las vecindades del Volcán Irazú, con magnitud máxima de 3.2 Ml y el segundo, en agosto de 2010 en Ujarrás y Durika de Buenos Aires, Puntarenas, con magnitudes entre 2.9 y 4.0 Ml. Otra zona que estuvo activa sísmicamente fue el punto de junción entre las placas de Nazca, Coco y Bloque de Panamá; en las afueras de Punta Burica. En total 107 sismos fueron sentidos por la población y más de 4000 sismos fueron registrados por la red sísmica de OVSICORI-UNA a lo largo del 2010. Los datos sísmicos históricos muestran que la magnitud máxima alcanzada ha sido de Mw 7.6 e intensidades de IX grados en la escala Mercalli Modificada.
Analysis of seismic and acoustic observations at Arenal Volcano, Costa Rica, 1995–1997
(Elsevier Science, 2000-08) Hagerty, M. T.; Schwartz, S.Y.; Garcés, M. A.; Protti, Marino
In November 1995, we installed five, three-component broadband seismometers and electronic tiltmeters around the circumference of Arenal Volcano, a young stratovolcano in Costa Rica that exhibits strombolian activity. With the addition of two continuous-recording GPS receivers deployed in May 1995, these instruments provide continuous monitoring of seismicity and ground deformation at an active volcano over a very wide bandwidth. In addition, during April–May 1997, we deployed a small, linear array of co-located three-component seismometers and broadband microphones. This paper presents a comprehensive analysis of all the seismic and acoustic data collected thus far. Seismic signals are primarily of two types: (1) longperiod (1–3 Hz) transients associated with summit explosions; and (2) harmonic tremor that contains regularly spaced spectral peaks (0.9, 1.8, 2.7, 3.6, 4.5, 5.4, 6.3, and 7.1 Hz) and lasts up to several hours. The explosion signals appear to originate in a small volume that is located at shallow depth beneath the vent and does not migrate with time. No unambiguous long-period seismic signals T . 5 s associated with volcanic processes at Arenal have been observed during the three-year deployment period. The spectra of summit explosions show distinct signatures at each site, suggesting significant path and/or site modification of the waveforms. In contrast, the harmonic tremor signals show no variation in the frequency content at the five sites, nor on the three seismic components at each site (Hagerty et al., 1997). This, and the fact that harmonic tremor is recorded in the acoustic channels as well, demonstrates that harmonic tremor is not a seismic propagation effect and that pressure disturbances propagate within the magma–gas mixture inside of volcanic conduits. These pressure waves are sensitive to the flow velocity and to small changes in the gas content of the magma–gas mixture. Observations and synthetic tests are presented that challenge the notion that harmonic tremor is a superposition of repeated gas explosions at shallow depth. We propose that equilibrium degassing of the melt creates a stable, stratified magma column where the void fraction increases with decreasing depth and that disruption of this equilibrium stratification is responsible for observed variations in the seismic efficiency of explosions. q2000 Elsevier Science B.V. All rights reserved.

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