Artículos científicos

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

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Are the ashes from the latest eruptions (2010–2016) at Turrialba volcano (Costa Rica) related to phreatic or phreatomagmatic events?
(Elsevier, 2016-09-15) Mele, Daniela; Alvarado Induni, Guillermo E.; Dellino, Pierfrancesco; de Moor, Joost Maarten; Avard, Geoffroy
The initial eruptive episodes of explosive eruptions are classified as phreatic if the amount of juvenile material (scoria, glass, pumice) is null, and the amount of fresh accidental lithics, and hydrothermally altered lithics, is substantial. Phreatic eruptions have been in some cases recognized as precursory events preceding phreatomagmatic and magmatic eruptive phases. Usually, the lithological features of tephra deposits are investigated and sampled in the field. Investigation of ash samples under binocular microscope or by Back-Scattered Electron (BSE) microscope images of polished sections is usually considered sufficient to typify the fragmentation mechanism of the eruption. The opening eruptive phases at Turrialba volcano, together with the formation of newintracraters (i.e. 2010, 2012, 2014) and the enlargement of theWestern Crater (29 October 2014 to present), were classified, in previous papers and internal reports as phreatic. We studied a series of ash samples erupted from 2010 to 2016, with the aim of understanding the fragmentation processes characterizing the vent opening phases.We used SEM+EDS analyses, in addition to field and microscopic investigation. Results showed a composition of accidental lithics of fresh to hydrothermally altered clasts and secondary minerals (82–98%), besides juvenile andesite fragments (2–18%), which leads us to revisit the classification of the initial eruptive phases of Turrialba as phreatomagmatic. Our method allowed the detection of a juvenile component directly involved in an effective magma-water interaction, which was possible only by a scrutinous examination of the glass surface textures by SEM in the range size between 3 and 3.5 phi. We recommend such a type of investigation when the identification of fresh magma in a new eruption is crucial for the preparedness and hazard evaluation at active volcanoes.
Coda wave attenuation in the Parecis Basin, Amazon Craton, Brazil: sensitivity to basement depth
(Springer, 2011) Vieira Barros, Lucas; Assumpção, Marcelo; Martins Ferreira, Vinicius; Quintero, Ronnie
Small local earthquakes from two aftershock sequences in Porto dos Gaúchos, Amazon craton—Brazil, were used to estimate the coda wave attenuation in the frequency band of 1 to 24 Hz. The time-domain coda-decay method of a single backscattering model is employed to estimate frequency dependence of the quality factor (Qc) of coda waves modeled using Qc = Q0 f η, where Q0 is the coda quality factor at frequency of 1 Hz and η is the frequency parameter.We also used the independent frequency model approach (Morozov, Geophys J Int, 175:239–252, 2008), based in the temporal attenuation coefficient, χ( f ) instead of Q( f ), given by the equation χ( f )=γ + π f Qe, for the calculation of the geometrical attenuation (γ ) and effective attenuation (Q−1e ). Qc values have been computed at central frequencies (and band) of 1.5 (1–2), 3.0 (2–4), 6.0 (4–8), 9.0 (6–12), 12 (8–16), and 18 (12–24) Hz for five different datasets selected according to the geotectonic environment as well as the ability to sample shallow or deeper structures, particularly the sediments of the Parecis basin and the crystalline basement of the Amazon craton. For the Parecis basin Qc = (98 ± 12) f (1.14±0.08), for the surrounding shield Qc = (167 ± 46) f (1.03±0.04), and for the whole region of Porto dos Gaúchos Qc = (99 ± 19) f (1.17±0.02). Using the independent frequency model, we found: for the cratonic zone, γ = 0.014 s−1, Q−1 e = 0.0001, ν ≈ 1.12; for the basin zone with sediments of ∼500 m, γ = 0.031 s−1, Q−1 e = 0.0003, ν ≈ 1.27; and for the Parecis basin with sediments of ∼1,000 m, γ = 0.047 s−1, Q−1 e = 0.0005, ν ≈ 1.42. Analysis of the attenuation factor (Qc) for different values of the geometrical spreading parameter (ν) indicated that an increase of ν generally causes an increase in Qc, both in the basin as well as in the craton. But the differences in the attenuation between different geological environments are maintained for different models of geometrical spreading. It was shown that the energy of coda waves is attenuated more strongly in the sediments, Qc = (78 ± 23) f (1.17±0.14) (in the deepest part of the basin), than in the basement, Qc = (167 ± 46) f (1.03±0.04) (in the craton). Thus, the coda wave analysis can contribute to studies of geological structures in the upper crust, as the average coda quality factor is dependent on the thickness of sedimentary layer.
Constraints on inner forearc deformation from balanced cross sections, Fila Costen˜ a thrust belt, Costa Rica
(American Geophysical Union, 2007-12-21) Sitchler, Jason C.; Fisher, Donald M.; Gardner, Thomas W.; Protti, Marino
The Fila Costeña thrust belt in the forearc of Costa Rica is accommodating a significant portion of the convergence of the Cocos plate and Panama microplate. Geologic mapping of the thrust belt depicts a duplex with three horses that incorporate Eocene limestones and Oligocene to early Miocene clastics inboard of the subducting Cocos Ridge axis. By constructing a cross section at this location along a NE-SW trending transect perpendicular to the thrust belt, we constrain a shortening rate of approximately 40 mm/a and propose that as much as 50% of the total plate convergence rate is taken up in the inner forearc. The Eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the Cocos Ridge axis owing to decrease in slip on faults and a lateral ramp in the basal décollement. The thrust belt terminates near the Panama border at the onland projection of the subducting Panama Fracture Zone. These observations suggest that shortening is propagating to the east with the migration of the Panama triple junction and the onset of shallow subduction of the thickened edge of the Cocos plate. The absence of similar features in the Nicaraguan forearc, where the subducting crust is older, subducts more steeply, and lacks incoming ridges and seamounts, indicates that deformation of the forearc basin in Costa Rica reflects greater couplin between converging plates inboard of the Cocos Ridge. Copyright 2007 by the American Geophysical Union.
Mercury fluxes from volcanic and geothermal sources: An update
(The Geological Society of London vol. 410 263 – 285. 2015, 2015-03-01) BAGNATO, E.; TAMBURELLO, Giancarlo; AVARD, G.; MARTINEZ-CRUZ, M.; ENRICO, M.; FU, X.; SPROVIERI, MARIO; Sonke, Jeroen; BAGNATO, e.
We review the state of knowledge on global volcanogenic Hg emissions to the atmosphere and present new data from seven active volcanoes (Poás, Rincón de la Vieja, Turrialba, Aso, Mutnovsky, Gorely and Etna) and two geothermal fields (Las Pailas and Las Hornillas). The variability of Hg contents (c. 4-125 ng m-3) measured in gaseous emissions reflects the dynamic nature of volcanic plumes, where the abundances of volatiles are determined by the physical nature of degassing and variable air dilution. Based on our dataset and previous work, we propose that an average Hg/SO2 plume mass ratio of c. 7.8 × 10-6 (± 1.5 × 10-6; 1 SE, n = 13) is best representative of open-conduit quiescent degassing. Taking into account the uncertainty in global SO2 emissions, we infer a global volcanic Hg flux from persistent degassing of c. 76 ± 30 t a-1. Our data are derived from active volcanoes during non-eruptive periods and we do not have any direct constraint on the Hg flux during periods of elevated SO2 flux associated with large-scale effusive or explosive eruptions. This suggests that the time-averaged Hg flux from these volcanoes is even larger if the eruptive contribution is considered. Conversely, closed-conduit degassing and geothermal emissions contribute modest amounts of Hg.
Mercury fluxes from volcanic and geothermal sources: an update
(Geological Society, London, Special Publications, 2014-08-02) Bagnato, E.; Tamburello, G.; Avard, G.; Martínez-Cruz, M.; Enrico, M.; Fu, X.; Sprovieri, M.; Sonke, J. E.
We review the state of knowledge on global volcanogenic Hg emissions to the atmosphere and present new data from seven active volcanoes (Poa´s, Rinco´n de la Vieja, Turrialba, Aso, Mutnovsky, Gorely and Etna) and two geothermal fields (Las Pailas and Las Hornillas). The variability of Hg contents (c. 4–125 ng m23) measured in gaseous emissions reflects the dynamic nature of volcanic plumes, where the abundances of volatiles are determined by the physical nature of degassing and variable air dilution. Based on our dataset and previous work, we propose that an average Hg/SO2 plume mass ratio of c. 7.8 × 10-6 (+1.5 × 10-6; 1 SE, n= 13) is best representative of open-conduit quiescent degassing. Taking into account the uncertainty in global SO2 emissions, we infer a global volcanic Hg flux from persistent degassing of c. 76+30 t a-1. Our data are derived from active volcanoes during non-eruptive periods and we do not have any direct constraint on the Hg flux during periods of elevated SO2 flux associated with large-scale effusive or explosive eruptions. This suggests that the time-averaged Hg flux from these volcanoes is even larger if the eruptive contribution is considered. Conversely, closed-conduit degassing and geothermal emissions contribute modest amounts of Hg.
Nicoya earthquake rupture anticipated by geodetic measurement of the locked plate interface
(Nature Research, 2013-12-22) Protti, Marino; González, Victor; Newman, Andrew; Dixon, Timothy H.; Schwartz, Susan; Marshall, Jeffrey S; Feng, Lujia; Walter, Jacob; Malservisi, Rocco; Owen, Susan E.
The Nicoya Peninsula in Costa Rica is one of the few places on Earth where the seismically active plate interface of a subduction zone is directly overlaid by land rather than ocean. At this plate interface, large megathrust earthquakes with magnitudes greater than 7 occur approximately every 50 years. Such quakes occurred in 1853, 1900 and 1950, so another large earthquake had been anticipated. Land-based Global Positioning System (GPS) and seismic measurements revealed a region where the plate interface was locked and hence accumulated seismic strain that could be released in future earthquakes. On 5 September 2012, the long-anticipated Nicoya earthquake occurred in the heart of the previously identified locked patch. Here we report observations of coseismic deformation from GPS and geomorphic data along the Nicoya Peninsula and show that the magnitude 7.6 Nicoya earthquake ruptured the lateral and down-dip extent of the previously locked region of the plate interface. We also identify a previously locked part of the plate interface, located immediately offshore, that may not have slipped during the 2012 earthquake, where monitoring should continue. By pairing observations of the spatial extent of interseismic locking and subsequent coseismic rupture, we demonstrate the use of detailed near-field geodetic investigations during the late interseismic period for identifying future earthquake potential.
The intraplate Porto dos Gaúchos seismic zone in the Amazon craton — Brazil
(Elsevier, 2009-01-15) Vieira Barros, Lucas; Assumpção, Marcelo; Quintero, Ronnie; CAIXETA, DANIEL
The largest earthquake observed in the stable continental interior of the South American plate occurred in Serra do Tombador, Mato Grosso state — Brazil, on January 31, 1955 with a magnitude of 6.2 mb. Since then no other earthquake has been located near the 1955 epicentre. However, in Porto dos Gaúchos, 100 km northeast of Serra do Tombador, a recurrent seismicity has been observed since 1959. Both Serra do Tombador and Porto dos Gaúchos are located in the Phanerozoic Parecis basin. Two magnitude 5 earthquakes occurred in Porto dos Gaúchos, in 1998 and 2005, with intensities up to VI and V, respectively. These two main shocks were followed by aftershock sequences lasting more than three years each. Local seismic stations have been deployed by the Seismological Observatory of the University of Brasilia since 1998 to study the “Porto dos Gaúchos” seismic zone (PGSZ). A local seismic refraction survey was carried out with two explosions to help define the seismic velocity model. Both the 1998 and 2005 earthquake sequences occurred in the same WSW–ENE oriented fault zone with right-lateral strike-slip mechanisms. The epicentral zone is in the Parecis basin, near its northern border where there are buried grabens, generally trending WNW–ESE, such as the deep Mesoproterozoic Caiabis graben which lies partly beneath the Parecis basin. However, the epicentral distribution indicates that the 1998 and 2005 sequences are related to a N60°E fault which probably crosses the entire Caiabis graben. The 1955 earthquake, despite the uncertainty in its epicentre, does not seem to be directly related to any buried graben either. The seismicity in the Porto dos Gaúchos seismic zone, therefore, is not directly related to rifted crust. The probable direction of the maximum horizontal stress near Porto dos Gaúchos is roughly E–W, consistent with other focal mechanisms further south in the Pantanal basin and Paraguay, but seems to be different from the NW–SE direction observed further north in the Amazon basin. The recurrent seismicity observed in Porto dos Gaúchos, and the large 1955 earthquake nearby, make this area of the Parecis basin one of the most important seismic zones of Brazil.
Tomographic evidence for a subducted seamount beneath the Gulf of Nicoya, Costa Rica: The cause of the 1990 Mw = 7.0 Gulf of Nicoya earthquake
(American Geophysical Union, 2002-04-26) Husen, S.; Kissling, Edi; Quintero, Ronnie
Tomographic images constrain the existence of a subducted seamount beneath the Gulf of Nicoya, Costa Rica. The subducted seamount is found at a depth of 30 km within the rupture area of the March 25, 1990, Mw = 7.0 Gulf of Nicoya earthquake. The Gulf of Nicoya earthquake was a typical thrust-type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting Cocos plate and the overriding plate. Precise relocation of the mainshock and its aftershocks in a 3-D P-wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount. Most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate. We interpret the subducted seamount as an asperity whose rupture caused the 1990 Gulf of Nicoya earthquake. INDEX TERMS: 1734 History of Geophysics: Seismology; 6982 Radio Science: Tomography and imaging; 7209 Seismology: Earthquake dynamics and mechanics; 7230 Seismology: Seismicity and seismotectonics
Transport properties of glassy and molten lavas as a function of temperature and composition
(Elsevier, 2016-11-15) Hofmeister, Anne M.; Sehlke, Alexander; Avard, Geoffroy; Bollasina, Anthony J.; Robert, Geneviève; Whittington, Alan
We provide measurements of thermal diffusivity (D), heat capacity (CP), and viscosity (η) for 12 remelted natural lavas and 4 synthetic glasses and melts, ranging in composition from leucogranite to low-silica basalt, and calculate their thermal conductivity. Both viscosity and the glass transition temperature decrease with decreasing melt polymerization. For basaltic glasses, D is low, ~ 0.5 mm2 s− 1 at room temperature, decreases slightly with increasing temperature, and then drops upon melting to ~ 0.25 to 0.35 mm2 s− 1. Other samples behave similarly. Despite scatter, clear correlations exist between D of glass or melt with Si content, density, NBO/T, and, most strongly, with fragility (m). Glass thermal diffusivity is represented by D = FT− G + HT, where F, G and H are fitting parameters. For melts, ∂ D/∂ T was resolved only for dacite-andesite and MORB: a positive slope is consistent with other iron-bearing samples. Glass and liquid CP depend on density and other physical properties, but not exactly in the same manner as D. We calculate thermal conductivity (k) from these data and demonstrate that k for glasses is described by a Maier-Kelly formula. Large scatter exists for k at 298 K, but silicic to intermediate melts have k between 1.8 and 1.3 Wm− 1 K− 1, whereas basaltic melts are constrained to ~ 1.4 ± 0.1 Wm− 1 k− 1. Low values for thermal diffusivity and viscosity for basaltic melts suggests that basalts transfer heat much more efficiently by advection than by conduction alone, and that partially molten zones in the mantle quickly become more thermally insulating than non-molten zones, potentially contributing to melt localization during decompression melting.

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