Artículos científicos

URI permanente para esta colecciónhttp://10.0.96.45:4000/handle/11056/17882

Examinar

Mostrando 1 - 20 de 74

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 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 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.
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.
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.
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.
ANTOLOGÍA SOBRE POLITIONATOS: COMPUESTOS DE AZUFRE EXISTENTES EN LOS LAGOS CRATÉRICOS DE VOLCANES ACTIVOS
(Revista de Vulcanología, Sismología y Tectónica., 1999) Martínez-Cruz, María
En la siguiente recopilación bibliográfica sobre los oxianiones de azufre llamados politionatos, con fórmula general SxO6 2-, se contemplan algunas de sus propiedades físico-químicas, y se hace énfasis principalmente en el hecho de que se ha encontrado que los cambios en la concentración total de los politionatos son un buen indicador de cambios en la actividad fumarólica subacuática de varios lagos cratéricos. La variación en la concentración de los ácidos politiónicos presentes en los lagos cratéricos, se ha observado y relacionado con actividad sísmica y la producción de erupciones freáticas en los volcanes Poás en Costa Rica; Kusatsu Shirane en Japón, y el Ruapehu en Nueva Zelanda. Los oxianiones conteniendo al azufre en varios estados de oxidación han sido encontrados en ambientes tales como fumarolas volcánicas, fuentes termales, en el agua sumamente ácida de algunos lagos cratéricos, sedimentos de lagos, aguas residuales de industrias mineras y solfataras. El monitoreo de la variación en la concentración de ciertas especies químicas en los sistemas volcánicos constituye una herramienta de gran importancia en la predicción de actividad volcánica.
Base computarizada de datos sobre Tectónica y Geología estructural de Centroamerica y el Caribe (Tectobase)
(Universidad de Costa Rica, 1985) Aguilar D., Álvaro; Alvarado V., Fernando; Obando A., Luis; Protti, Marino
Presentación de proyecto de banco computarizado sobre la bibliografía publicada sobre sobre Tectónica y Geología Estructural de Centroamérica y el Caribe (TECTOBASE)
Carbon and sulfur isotopes in tree rings as a proxy for volcanic degassing
(Geological Society of America, 2019-07-05) D’Arcy, Fiona; Boucher, Étienne; De Moor, J. Maarten; Hélie, Jean-François; Piggott, Robert; Stix, John; de Moor, Joost Maarten; Hélie, Jean-François
Trees are useful archives of past atmospheric conditions. They have most commonly been used to infer large-scale changes in climate, industrial pollution, and the magnitude and frequency of geological hazards. While geochemical changes in tree rings have been linked to localized anthropogenic smelter pollution, their potential to track geochemical changes in volcanic degassing has not yet been fully realized. Here, we applied a new proxy using sulfur and carbon isotopes in tree rings to examine fluctuations in gas emission at Turrialba volcano, Costa Rica. Since 2009, Turrialba has emitted a persistent gas plume and increasingly frequent explosions and ash eruptions as activity has accelerated. We collected cores from a species of alder tree, Alnus acuminata, at several locations surrounding the volcano. Biannual isotopic analysis of rings demonstrated a notable δ34S shift of –5.2‰ and a similarly sharp δ13C shift of +1.3‰ in trees downwind of the plume following the onset of strong degassing in 2009. We propose that these shifts in the isotopic values of the tree correspond to those of the volcanic SO2 and CO2, and in the case of the δ13C, an additional fractionation caused by leaf impairment from exposure to volcanic SO2. This new proxy can be applied to other volcanoes as a novel method of obtaining a temporal record of degassing, a crucial tool for volcano monitoring.
Carbon isotope systematics of Turrialba volcano, Costa Rica, using a portable cavity ring-down spectrometer
(AGU Publications, 2017-07-27) Stix, J.; Chu, K.; Sherwood Lollar, B.; Malowany, Kalina; de Moor, Joost Maarten; Lacrampe-Couloume, Georges
Over the past two decades, activity at Turrialba volcano, Costa Rica, has shifted from hydrothermal to increasingly magmatic in character, with enhanced degassing and eruption potential. We have conducted a survey of the d13C signatures of gases at Turrialba using a portable field-based CRDS with comparison to standard IRMS techniques. Our d13C results of the volcanic plume, high-temperature vents, and soil gases reveal isotopic heterogeneity in the CO2 gas composition at Turrialba prior to its recent phase of eruptive activity. The isotopic value of the regional fault system, Falla Ariete (–3.460.1&), is in distinct contrast with the Central crater gases (–3.960.1&) and the 2012 high-temperature vent (–4.460.2&), an indication that spatial variability in d13C may be linked to hydrothermal transport of volcanic gases, heterogeneities in the source composition, or magmatic degassing. Isotopic values of CO2 samples collected in the plume vary from d13C of 25.2 to 210.0&, indicative of mixing between atmospheric CO2 (–9.260.1&), and a volcanic source. We compare the Keeling method to a traditional mixing model (hyperbolic mixing curve) to estimate the volcanic source composition at Turrialba from the plume measurements. The predicted source compositions from the Keeling and hyperbolic methods (–3.060.5&and 23.960.4&, respectively) illustrate two potential interpretations of the volcanic source at Turrialba. As of the 29 October 2014, Turrialba has entered a new eruptive period, and continued monitoring of the summit gases for d13C should be conducted to better understand the dominant processes controlling d13C fractionation at Turrialba.
Chlorine isotope and Cl–Br fractionation in fluids of Poás volcano (Costa Rica): Insight into an active volcanic–hydrothermal system
(Elsevier, 2016-10-01) Rodríguez, Alejandro; Eggenkamp, Hans; Martínez Cruz, María; Van Bergen, Manfred
Halogen-rich volcanic fluids issued at the surface carry information on properties and processes operating in shallow hydrothermal systems. This paper reports a long-term record of Cl–Br concentrations and δ37Cl signatures of lake water and fumaroles from the active crater of Poás volcano (Costa Rica), where surface expressions of magmatic-hydrothermal activity have shown substantial periodic changes over the last decades. Both the hyperacid water of its crater lake (Laguna Caliente) and subaerial fumaroles show significant temporal variability in Cl–Br concentrations, Br/Cl ratios and δ37Cl, reflecting variations in the mode and magnitude of volatile transfer. The δ37Cl signatures of the lake, covering the period 1985–2012, show fluctuations between + 0.02 ± 0.06‰ and + 1.15 ± 0.09‰. Condensate samples from adjacent fumaroles on the southern shore, collected during the interval (2010–2012) with strong changes in gas temperature (107–763°C), display a much larger range from − 0.43 ± 0.09‰ to + 14.09 ± 0.08‰. Most of the variations in Cl isotope, Br/Cl and concentration signals can be attributed to interaction between magma-derived gas and liquid water in the volcanic–hydrothermal system below the crater. The δ37Cl were lowest and closest to magmatic values in (1) fumarolic gas that experienced little or no interaction with subsurface water and followed a relatively dry pathway, and (2) water that captured the bulk of magmatic halogen output so that no phase separation could induce fractionation. In contrast, elevated δ37Cl can be explained by partial scavenging and fractionation during subsurface gas–liquid interaction. Hence, strong Cl isotope fractionation leading to very high δ37Cl in Poás’ fumaroles indicates that they followed a wet pathway. Highest δ37Cl values in the lake water were found mostly in periods when it received a significant input from subaqueous fumaroles or when high temperatures and low pH caused HCl evaporation. It is concluded that combined monitoring of δ37Cl and Br/Cl in Laguna Caliente and adjacent fumaroles provides valuable information on activity in the subsurface hydrothermal system with significant relevance for volcanic surveillance of Poás.
Co-ideation of disaster preparedness strategies through a participatory design approach: Challenges and opportunities experienced at Turrialba volcano, Costa Rica
(Elsevier, 2015) Van Manen, Saskia; Avard, Geoffroy; Martínez-Cruz, María
Disaster preparedness is key to coping and adaptation during the immediate aftermath of a natural hazard, but the majority of those at risk do not feel prepared. In this participatory action research we investigate the use of a participatory design approach to increase disaster preparedness around Turrialba volcano, Costa Rica. We present a case study of two ideation workshops and explore the process, outcomes, challenges and opportunities during ideation. Socio-cognitive dimensions, specifically risk and responsibility transfer, appear to be important factors influencing the uptake of self-protective measures. Challenges in workshop facilitation were of a human, cultural and resource nature. However, the overall process was successful with participants showing indications of empowerment and a number of pressing design opportunities identified.
Comparación de la composición química de partículas PM10 y PM2,5 colectadas en ambientes urbanos y zonas volcánicas del área metropolitana de Costa Rica
(Revista Ciencias Ambientales, 2014-12) Herrera, Jorge; Rojas, José Félix; Martínez, María; Avard, Geoffroy; De Moore, Martin; Sáenz, Wendy; Beita, Víctor H.; Rodríguez, Agustín; Agüero, Alejandro
Se muestrearon, en forma simultánea, partículas PM10 y PM2,5 en ambientes urbanos y volcánicos del área metropolitana de Costa Rica, durante octubre y noviembre de 2012. En las áreas industriales y comerciales de alto flujo vehicular (La Uruca, Heredia y Belén) se presentaron concentraciones másicas significativamente superiores (42-29 μgm-3) de PM10 y PM2,5 en comparación con las registradas en la zona volcánica. Las concentraciones diarias de PM10 y PM2,5, obtenidas en el volcán Poás variaron entre 3-14 μgm-3 y 2-11 μgm- 3, respectivamente. Sin embargo, la acidez de las partículas colectadas en el Poás es mayor a la acidez correspondiente a ambientes urbanos, debido, probablemente, a una ocurrencia menor de tasas de neutralización. El aporte de iones secundarios resultó ser la principal contribución (40%) a la composición de las PM10 volcánicas, a diferencia de las colectadas en centros urbanos.
Constraints on the sulfur subduction cycle in Central America from sulfur isotope compositions of volcanic gases
(European Association of Geochemistry, 2022-01-20) de Moor, Joost Maarten; Fischer, Tobias; Plank, Terry
The sulfur cycle at convergent margins remains poorly constrained yet is fundamentally important for understanding the redox state of Earth's reservoirs and the formation of ore deposits. In this study we investigate the sulfur isotope composition of high temperature volcanic gases emitted from the Nicaraguan (average of +4.8 ± 1.3‰) and Costa Rican (average of +2.3 ± 1.3‰) arc segments contributing to emissions from the Southern Central American Volcanic Arc (SCAVA; average of +3.8 ± 1.7‰). Along-arc variations in geochemical tracers at SCAVA are widely accepted to reflect variations in subduction parameters and deep fluid sources and correlations between these parameters and gas S isotope compositions are observed. These correlations suggest that gas emissions are sourced from a mixture of mantle S with δ34S ~ 0‰ and isotopically heavy slab-derived sulfur with δ34S ≥ ~ +8‰. We employ Monte Carlo mass balance modeling to constrain S inputs to the subduction zone and relative contributions from mantle and slab to arc sulfur emissions. The models indicate that bulk subduction input in Nicaragua has a S isotope composition of +1.4 ± 0.5‰ compared to −0.2 ± 0.4‰ in Costa Rica, requiring preferential release of isotopically heavy oxidized S from the slab to explain the relatively high δ34S observed in arc outputs. We show that the flux of S from the slab is sufficient to oxidize the entire mantle wedge within the lifetime of the arc, indicating that S is a primary oxidizing agent in subduction zones. Furthermore, the preferential removal of heavy S from the slab requires retention of isotopically light S in the residual slab. Subduction-scale fractionation of S isotopes is fundamentally important in explaining why Earth's bulk surface reservoirs are isotopically positive.
Constraints on upper mantle anisotropy surrounding the Cocos slab from SK(K)S splitting
(American Geophysical Union, 2010-06-24) Abt, David L.; Fischer, Karen M.; Abers, Geoffrey A.; González, Victor; Strauch, Wilfried; Protti, Marino
SKS and SKKS splitting observations are used to constrain the pattern of mantle flow in the Central American subduction zone beneath Costa Rica and Nicaragua. After removing the effects of shallow wedge anisotropy on SK(K)S waveforms, a best fitting model of anisotropy beneath the Cocos Plate and in the deeper mantle wedge is determined. Fast polarization directions and model symmetry axis orientations in both regions (as well as the shallow wedge) are dominated by roughly arc‐parallel azimuths and, therefore, are not consistent with sublithospheric mantle flow entrained by the subducting Cocos Plate or simple two‐dimensional corner flow in the wedge. In conjunction with geochemical data and local S splitting tomography, the SK(K)S splitting observations and anisotropy models are consistent with flow to the WNW within the mantle wedge on the Caribbean side of the Cocos Plate, possibly drawn through a slab window beneath Panama and southern Costa Rica. Anisotropy in the Pacific mantle beneath the Cocos Plate is also best explained by flow with a component that is roughly parallel to the strike of the slab, although the absolute direction of this flow is not uniquely constrained
Correlation between the age of the subducting Cocos Plate and the geometry of the Wadati-Benioff zone under Nicaragua and Costa Rica.
(Universidad Nacional, Costa Rica, 1995) Protti, Marino; Güendel, Federico; McNally, Karen
High- resolution seismicity data from Costa Rica and southern Nicaragua have been used to image the geometry of the Wadati-Benioff zone under southern Central America. The data include 9,514 events with computed horizontal and vertical errors smaller than 4 and 5 km respectively. We found that under the Nicaragua- Costa Rica border the Wadati- Benioff zone contorts (from steep to shallow dip angles, northwest to southeast) but does not show evidence of a brittle tear, as postulated by others. Further to the southeast-northeast from Puerto Caldera- the Wadati-Benioff zone does, however, show a tear (the Quesada Sharp Contortion) at intermediate depths (h >70 km). Overall the dip angle of the Wadati-Benioff zone decreases from 84° under Nicaragua to 60° under central Costa Rica. The maximum depth of interaplate Wadati-Benioff zone earthquakes also decreases from northwest (-200 km under Nicaragua) to southeast (-125 km under central Costa Rica). In southern Costa Rica, east of 83°55 W, we find no evidence of the Wadati-Benioff zone below 50 km. This geometry of the Wadati-Benioff zone and other tectonic features related to the subduction of the Cocos plate beneath the Caribbean plate have been integrated into a model that correlates them with along- trench variations in age of the subducted Cocos plate. These tectonic featuresare (1) the shallowing of the Middle America Trech bathymetry from northwest to southeast, (2) differences in coupling between Cocos and Caribbean plates, (3) the termination of the Central America Volcanic Chain in central Costa Rica, and (4) distinct stress field variations on the overriding Caribbean plate. The subduction of the Cocos Ridge under southern Costa Rica is partially responsible for somo of these features (shallowing of the Middle America Trench in southern Costa Rica) and for the high uplift rates of the outer arc, However, since the presence of the Panama Fracture Zone limits the subducted extension of the Cocos Ridge to less that 100 km from the trench, we propose that the overall geometry of the Wadati- Benioff zone is controlled by abrupt along trench changes in the age of the subducted Cocos plate.
Crustal structure along the southern Central American volcanic front
(American Geophysical Union, 2008-08-20) Mackenzie, Laura; Abers, Geoffroy A.; Fischer, Karen M.; Protti, Marino; González, Victor
Subduction alters continents several ways, including accretion, magmatic addition, mantle wedge serpentinization, and crustal differentiation. These changes affect seismic velocities, so characterizing upper plate crust establishes a baseline for composition and continental growth. Teleseismic P and PP arrivals from a temporary deployment of broadband seismometers in Central America have been used to estimate crustal thickness and Vp/Vs ratio from receiver functions and to image crust across the active arc. Crustal thickness ranges from 25 to 44 km with formal errors of 1.6–9.2 km. The thinnest crust (24.6 ± 3.5 km) lies directly beneath the Nicaraguan arc, whereas the thickest crust lies in the Nicaraguan back arc (43.5 ± 2.5 km) and beneath the Costa Rican arc (37.9 ± 5.2 km). Crustal structure and Vp/Vs show sharp transitions at terrane boundaries. The Moho exhibits strong velocity contrasts throughout the study area of ∼0.5–1.0 km/s, even beneath arc and fore arc, precluding extensive serpentinization or ponded melt below the Moho. Crust is thicker beneath the Costa Rican arc, consistent with 10–23 km3/km/Ma crustal growth. The crust is thinner by 11–18 km beneath the large depression in central Nicaragua, with the thinnest crust beneath the arc. There, the relationship between thin crust, arc location, and deeper seismic velocities suggests that upper plate structure plays a critical role in focusing magma to the surface.

Examinar

Examinando Artículos científicos por browse.metadata.rights "Acceso abierto"