Artículos científicos
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Ítem Sulfur Degassing at Erta Ale (Ethiopia) and Masaya (Nicaragua) Volcanoes : Implications for Degassing Processes and Oxygen Fugacities of Basaltic Systems(Advancing Earth ans Space Sciencess, 2013-10-02) De Moor, J. M.; Fischer, T. P.; Sharp, Z. D.; King, P. L.; Wilke, M.; Botcharnikov, R. E.; Cottrell, E.; Zelenski, M.; Marty, B.; Klimm, K.; Rivard, C.; Ayalew, D.; Ramirez, C.; Kelley, K. A.We investigate the relationship between sulfur and oxygen fugacity at Erta Ale and Masaya volcanoes. Oxygen fugacity was assessed utilizing Fe3þ/ PFe and major element compositions measured in olivine-hosted melt inclusions and matrix glasses. Erta Ale melts have Fe3þ/ PFe of 0.15–0.16, reflecting fO2 of DQFM 0.0 6 0.3, which is indistinguishable from fO2 calculated from CO2/CO ratios in high-temperature gases. Masaya is more oxidized at DQFM þ1.7 6 0.4, typical of arc settings. Sulfur isotope compositions of gases and scoria at Erta Ale (34Sgas 0.5%; 34Sscoria þ 0.9%) and Masaya (34Sgas þ 4.8%; 34Sscoria þ 7.4%) reflect distinct sulfur sources, as well as isotopic fractionation during degassing (equilibrium and kinetic fractionation effects). Sulfur speciation in melts plays an important role in isotope fractionation during degassing and S6þ/ PS is <0.07 in Erta Ale melt inclusions compared to >0.67 in Masaya melt inclusions. No change is observed in Fe3þ/ PFe or S6þ/ PS with extent of S degassing at Erta Ale, indicating negligible effect on fO2, and further suggesting that H2S is the dominant gas species exsolved from the S2-rich melt (i.e., no redistribution of electrons). High SO2/H2S observed in Erta Ale gas emissions is due to gas re-equilibration at low pressure and fixed fO2. Sulfur budget considerations indicate that the majority of S injected into the systems is emitted as gas, which is therefore representative of the magmatic S isotope composition. The composition of the Masaya gas plume (þ4.8%) cannot be explained by fractionation effects but rather reflects recycling of high 34S oxidized sulfur through the subduction zone.Ítem Earth's Magnetic Field Strength and the Cretaceous Normal Superchron: New Data From Costa Rica(American Geophysical Union, 2021-03-12) Di Chiara, A.; Tauxe, L.; Staudigel, H.; Florindo, F.; Protti, M.; Yu, Y.; Wartho, J. A.; Van den Bogaard, P.; Hoernle, K.Limitar la variabilidad a largo plazo y el promedio de la intensidad del campo magnético terrestre es fundamental para comprender las características y el comportamiento del campo geomagnético. Persisten interrogantes sobre la intensidad del campo promedio y la relación entre esta y la frecuencia de inversión, debido a la dispersión de los datos de intervalos de tiempo clave. En este estudio, nos centramos en el Supercrón Normal Cretácico (CNS; 121-84 Ma), durante el cual no se observaron inversiones. Presentamos nuevos resultados de intensidad de 41 sitios de vidrio basáltico submarino (SBG) recolectados en la Península de Nicoya y las Islas Murciélago, Costa Rica. Las nuevas y revisadas restricciones de edad 40Ar/39Ar y bioestratigráficas de estudios previos indican edades de 141 a 65 Ma. Un sitio con una edad de 135,1 ± 1,5 Ma (2σ) arrojó un resultado de intensidad confiable de 34 ± 8 µT (equivalente a un momento dipolar axial virtual, VADM, valor de 88 ± 20 ZAm2), tres sitios de 121 a 112 Ma, que abarcan el inicio del CNS, varían de 21 ± 1 a 34 ± 4 µT (53 ± 3 a 87 ± 10 ZAm2). Estos resultados del CNS son todos superiores al promedio a largo plazo de ∼42 ZAm2 y los datos de Suhongtu, Mongolia (46–53 ZAm2) y son similares a la ofiolita de Troodos, Chipre (81 ZAm2, reinterpretada en este estudio). Junto con los datos reinterpretados, los nuevos resultados de Costa Rica sugieren que la intensidad del campo geomagnético fue aproximadamente la misma antes y después del inicio del SNC. Por lo tanto, los datos no respaldan una correlación estricta entre la longitud del intervalo de polaridad y la intensidad del campo magnético.Ítem Constraints on upper plate deformation in the Nicaraguan subduction zone from earthquake relocation and directivity analysis(American Geophysical Union, 2010-03-12) French, S. W.; Warren, L. M.; Fischer, K. M.; Abers, G. A.; Strauch, W.; Protti, J. M.; Gonzalez, V.In the Nicaraguan segment of the Central American subduction zone, bookshelf faulting has been proposed as the dominant style of Caribbean plate deformation in response to oblique subduction of the Cocos plate. A key element of this model is left‐lateral motion on arc‐normal strike‐slip faults. On 3 August 2005, a Mw 6.3 earthquake and its extensive foreshock and aftershock sequence occurred near Ometepe Island in Lake Nicaragua. To determine the fault plane that ruptured in the main shock, we relocated main shock, foreshock, and aftershock hypocenters and analyzed main shock source directivity using waveforms from the TUCAN Broadband Seismic Experiment. The relocation analysis was carried out by applying the hypoDD double‐difference method to P and S onset times and differential traveltimes for event pairs determined by waveform cross correlation. The relocated hypocenters define a roughly vertical plane of seismicity with an N60°E strike. This plane aligns with one of the two nodal planes of the main shock source mechanism. The directivity analysis was based on waveforms from 16 TUCAN stations and indicates that rupture on the N60°E striking main shock nodal plane provides the best fit to the data. The relocation and directivity analyses identify the N60°E vertical nodal plane as the main shock fault plane, consistent with the style of faulting required by the bookshelf model. Relocated hypocenters also define a second fault plane that lies to the south of the main shock fault plane with a strike of N350°E– N355°E. This fault plane became seismically active 5 h after the main shock, suggesting the influence of stresses transferred from the main shock fault plane. The August 2005 earthquake sequence was preceded by a small eruption of a nearby volcano, Concepción, on 28 July 2005. However, the local seismicity does not provide evidence for earthquake triggering of the eruption or eruption triggering of the main shock through crustal stress transfer.Ítem Shear wave anisotropy beneath Nicaragua and Costa Rica: Implications for flow in the mantle wedge(American Geophysical Union, 2009-05-27) Abt, David L.; Fischer, Karen M.; Abers, Geoffrey A.; Strauch, Wilfried; Protti, J. Marino; González, VictorWe present new shear wave splitting data from local events in Costa Rica and Nicaragua recorded by the temporary (July 2004 to March 2006) 48-station TUCAN broadband seismic array. Observed fast polarization directions in the fore arc, arc, and back arc range from arc-parallel to arc-normal over very short distances (<5 km when plotted at raypath midpoints) making the direct interpretation of individual splitting measurements in terms of flow tenuous, even when considering variations in the relationship between lattice-preferred orientation and deformation (e.g., B-type dislocation creep in olivine). Therefore, we tomographically invert the splitting measurements to find a three-dimensional model of crystallographic orientation in the wedge. We assume the elastic constants of olivine and orthopyroxene with hexagonal symmetry and use a damped, iterative least squares approach to account for the nonlinear behavior of splitting when considering three-dimensional ray propagation and distributions of anisotropy. The best fitting model contains roughly horizontal, arc-parallel olivine [100] axes in the mantle wedge down to at least 125 km beneath the back arc and arc, which we interpret to indicate along-arc flow in the mantle wedge. Pb and Nd isotopic ratios in arc lavas provide additional evidence for arc-parallel flow and also constrain the direction (northwest, from Costa Rica to Nicaragua) and minimum flow rate (63–190 mm/a). With only slightly oblique subduction at 85 mm/a of the relatively planar Cocos Plate, the most likely mechanism for driving along-arc transport is toroidal flow around the edge of the slab in southern Costa Rica, generated by greater slab rollback in Nicaragua. Two important implications of this arc-parallel flow are the progressive depletion of the mantle source for arc lavas from Costa Rica to Nicaragua and the possible need for significant decoupling between the wedge and downgoing plate.Ítem Strong along-arc variations in attenuation in the mantle wedge beneath Costa Rica and Nicaragua(American Geophysical Union, 2008-10-09) Rychert, C. A.; Fischer, K .M.; Abers, G. A.; Plank, T.; Syracuse, E.; Protti, J. M.; Gonzalez, V.; Strauch, W.La estructura de atenuación en la zona de subducción de Centroamérica se visualizó utilizando eventos locales registrados por el conjunto Tomography Under Costa Rica and Nicaragua, un despliegue de 20 meses (julio de 2004 a marzo de 2006) de 48 sismómetros que abarcaron las regiones de antearco, arco y trasarco de Nicaragua y Costa Rica. Las formas de onda P y S se invirtieron por separado para la frecuencia de esquina y el momento de cada evento y para el operador de atenuación promediado por trayectoria (t*) de cada par evento-estación, asumiendo que la atenuación depende ligeramente de la frecuencia (/ = 0,27). Luego, se realizaron inversiones tomográficas para la atenuación S y P (QS1 y QP1). Dado que las amplitudes de la onda P reflejan tanto el módulo de cizallamiento como el de volumen, también se realizaron inversiones tomográficas para determinar la atenuación de cizallamiento y volumen (QS1 y Qk1), la pérdida de energía por ciclo debido al cizallamiento y la compresión uniforme, respectivamente. El amortiguamiento y otros parámetros tomográficos de inversión se variaron sistemáticamente. Como es típico en los estudios de atenuación de la zona de subducción, se obtuvieron imágenes de una losa, placa superior y esquina de cuña menos atenuantes y una cuña del manto más atenuante. Además, se observaron diferencias de primer orden entre los mantos debajo de Nicaragua y Costa Rica. La losa en Nicaragua es más atenuante que la losa en Costa Rica. Una zona más grande de mayor atenuación por cizalladura también caracteriza la cuña del manto nicaragüense. Dentro de la cuña, los valores máximos de atenuación a 1 Hz corresponden a Qs = 38-73 debajo de Nicaragua y Qs = 62-84 debajo de Costa Rica, y los valores promedio son Qs = 76-78 y Qs = 84-88, respectivamente. Las variaciones de atenuación se correlacionan con las tendencias a lo largo del arco en los indicadores geoquímicos que sugieren que la fusión debajo de Nicaragua ocurre en condiciones más hidratadas, y posiblemente a mayores extensiones y profundidades, en relación con el norte de Costa Rica.Ítem Tracking Formation of a Lava Lake From Ground and Space: Masaya Volcano (Nicaragua), 2014–2017(American Geophysical Union, 2018-02-22) AIUPPA, Alessandro; de Moor, J. Marteen; Arellano, Santiago; Coppola, Diego; Francofonte, Vincenzo; Galle, Bo; Giudice, Gaetano; Liuzzo, Marco; Mendoza, Elvis; Saballos, Armando; Tamburello, Giancarlo; Battaglia, Angelo; Bitetto, Marcello; Gurrieri, Sergio; Laiolo, Marco; Mastrolia, Andrea; Moretti, RoberttoA vigorously degassing lava lake appeared inside the Santiago pit crater of Masaya volcano (Nicaragua) in December 2015, after years of degassing with no (or minor) incandescence. Here we present an unprecedented-long (3 years) and continuous volcanic gas record that instrumentally characterizes the (re)activation of the lava lake. Our results show that, before appearance of the lake, the volcanic gas plume composition became unusually CO2 rich, as testified by high CO2/SO2 ratios (mean: 12.2 6 6.3) and low H2O/CO2 ratios (mean: 2.3 6 1.3). The volcanic CO2 flux also peaked in November 2015 (mean: 81.3 6 40.6 kg/s; maximum: 247 kg/s). Using results of magma degassing models and budgets, we interpret this elevated CO2 degassing as sourced by degassing of a volatile-rich fast-overturning (3.6–5.2 m3 s21) magma, supplying CO2-rich gas bubbles from minimum equivalent depths of 0.36–1.4 km. We propose this elevated gas bubble supply destabilized the shallow (<1 km) Masaya magma reservoir, leading to upward migration of vesicular (buoyant) resident magma, and ultimately to (re)formation of the lava lake. At onset of lava lake activity on 11 December 2015 (constrained by satellite-based MODIS thermal observations), the gas emissions transitioned to more SO2-rich composition, and the SO2 flux increased by a factor 40% (11.4 6 5.2 kg/s) relative to background degassing (8.0 kg/s), confirming faster than normal (4.4 versus 3 m3 s21) shallow magma convection. Based on thermal energy records, we estimate that only 0.8 of the 4.4 m3 s21 of magma actually reached the surface to manifest into a convecting lava lake, suggesting inefficient transport of magma in the near-surface plumbing system.Ítem The Magmatic Gas Signature of Pacaya Volcano, With Implications for the Volcanic CO2 Flux From Guatemala(American Geophysical Union, 2018-03-10) Battaglia, A.; Bitetto, M.; Aiuppa, A.; Rizzo, A. L.; Chigna, G.; Watson, I. M.; D’Aleo, R.; Juarez Cacao, F. J.; de Moor, M. J.Pacaya volcano in Guatemala is one of the most active volcanoes of the Central American Volcanic Arc (CAVA). However, its magmatic gas signature and volatile output have received little attention to date. Here, we present novel volcanic gas information from in-situ (Multi-GAS) and remote (UV camera) plume observations in January 2016. We find in-plume H2O/SO2 and CO2/SO2 ratios of 2-20 and 0.6-10.5, and an end-member magmatic gas signature of 80.5 mol. % H2O, 10.4 mol. % CO2, and 9.0 mol. % SO2. The SO2 flux is evaluated at 885 6 550 tons/d. This, combined with co-acquired volcanic plume composition leads to H2O and CO2 fluxes of 2,230 6 1,390 and 700 6 440, and a total volatile flux of 3,800 tons/d. We use these results in tandem with previous SO2 flux budgets for Fuego and Santiaguito to estimate the total volcanic CO2 flux from Guatemala at 1,160 6 600 tons/day. This calculation is based upon CO2/total S (St) ratios for Fuego (1.5 6 0.75) and Santiaguito (1.4 6 0.75) inferred from a gas (CO2/St ratio) versus traceelement (Ba/La ratio) CAVA relationship. The H2O-poor and low CO2/St ratio (1.0-1.5) signature of Pacaya gas suggests dominant mantle-wedge derivation of the emitted volatiles. This is consistent with 3 He/4 He ratios in olivine hosted fluid inclusions (FIs), which range between 8.4 and 9.0 Ra (being Ra the atmospheric 3 He/4 He ratio) at the upper limit of MORB range (8 6 1 Ra). These values are the highest ever measured in CAVA and among the highest ever recorded in arc volcanoes worldwide, indicating negligible 4 He contributions from the crust/slab.Ítem Multiscale postseismic behavior on a megathrust: the 2012 Nicoya earthquake, Costa Rica(American Geophysical Union, 2015-06-17) Malservisi, Rocco; Schwartz, Susan Y.; Voss, Nicholas; Protti, Marino; Gonzalez, Victor; Dixon, Timothy H.; Jiang, Yan; Newman, Andy V.; Richardson, Jacob; Walter, Jacob I.; Voyenko, DenisLa Península de Nicoya, en el noroeste de Costa Rica, se encuentra sobre una sección del megaembotamiento de subducción a lo largo de la Fosa Mesoamericana. El 5 de septiembre de 2012, un megaembotamiento de magnitud de momento 7,6 se produjo bajo una densa red de estaciones GPS y sísmicas continuas. Muchas de las estaciones GPS registraron el evento a alta frecuencia, 1 Hz o mejor. Analizamos la evolución temporal y espacial de la deformación superficial tras el terremoto. Nuestros resultados muestran que la ruptura principal fue seguida por un deslizamiento posterior significativo dentro de las primeras 3 h posteriores al evento principal. El comportamiento del desplazamiento superficial puede representarse mediante procesos de relajación con tres tiempos característicos: 7, 70 y más de 400 días. Suponemos que el tiempo de relajación largo corresponde a la relajación viscoelástica y el tiempo de relajación intermedio corresponde al deslizamiento posterior en la falla principal. El tiempo de relajación corto puede representar una combinación de deslizamiento posterior rápido, ajuste poroelástico en la corteza superior u otros procesos. Durante los primeros meses posteriores al terremoto, el deslizamiento posterior probablemente liberó una cantidad significativa del déficit de deslizamiento aún presente tras la ruptura cosímica, en particular el buzamiento ascendente de la ruptura. El deslizamiento posterior parece estar limitado por el buzamiento ascendente de las regiones afectadas por deslizamientos lentos previos al terremoto, lo que sugiere que ambos procesos se ven influenciados por diferentes propiedades de fricción.Ítem Fore-arc motion and Cocos Ridge collision in Central America(American Geophysical Union, 2009-05-07) LaFemina, Peter; Dixon, Timothy H.; Govers, Rob; Norabuena, Edmundo; Turner, Henry; Saballos, Armando; Mattioli, Glen; Protti, Marino; Strauch, WilfriedWe present the first regional surface velocity field for Central America, showing crustal response to interaction of the Cocos and Caribbean plates. Elastic half-space models for interseismic strain accumulation on the dipping subduction plate boundary fit the GPS data well and show strain accumulation offshore and beneath the Nicoya and Osa peninsulas in Costa Rica but not in Nicaragua. Since large subduction zone earthquakes occur in Nicaragua, we suggest that interseismic locking in Nicaragua and some other parts of Central America occurs but is mainly shallow, <20 km depth, too far offshore to be detected by our on-land GPS measurements. Our data also show significant trench-parallel motion for most of the region, generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates. However, trench-parallel motion is also observed in central Costa Rica, where plate convergence is normal to the trench, and in the Nicaraguan fore arc, where trench-parallel motion is fast, up to 9 mm a 1, but mechanical coupling is low. A finite element model of collision (as opposed to subduction) involving the aseismic Cocos Ridge also fits the GPS surface velocity field, most significantly reproducing the pattern of trench-parallel motion. We infer that buoyant, thickened CNS-2-Cocos Ridge crust resists normal subduction and instead acts as an indenter to the Caribbean plate, driving crustal shortening in southern Costa Rica and contributing to trench-parallel fore-arc motion in Costa Rica and perhaps Nicaragua as a type of tectonic escape.Ítem Seismic tomography and earthquake locations in the Nicaraguan and Costa Rican upper mantle(American Geophysical Union, 2008-07-30) Syracuse, Ellen M.; Abers, Geoffrey A.; Fischer, Karen; MacKenzie, Laura; Rychert, Catherine; Protti, Marino; González, Víctor; Strauch, WilfriedThe Central American subduction zone exhibits large variations in geochemistry, downgoing plate roughness and dip, and volcano locations over a short distance along the arc. Results from joint inversions for Vp, Vp/Vs, and hypocenters from the Tomography Under Costa Rica and Nicaragua (TUCAN) experiment give insight into its geometry and structure. In both Costa Rica and Nicaragua, the intermediate-depth seismic zone is a single layer no more than 10 to 20 km thick. Tomographic images show that throughout Nicaragua and Costa Rica the slowest mantle P wave velocities appear below and behind the volcanic front, indicating likely zones of highest temperature extending 80 to 120 km depth. A sheet of high Vp/Vs, thought to be caused by melt, is imaged directly beneath the Nicaraguan volcanoes, whereas a weaker, broader anomaly is imaged beneath the Costa Rican volcanoes, potentially indicating a greater extent of melting beneath Nicaragua. Within the downgoing plate, anomalously low velocities occur at least 20–30 km below Wadati-Benioff zone seismicity, to depths of 140 km beneath Nicaragua and to 60 km depth beneath Costa Rica. They indicate 10–20% serpentinized upper mantle of the downgoing plate beneath Nicaragua, similar to that inferred from refraction seaward of the trench, but continuing to subarc depths. This unusually hydrated lithosphere may introduce more water into the Nicaraguan mantle, initiating increased amount of melting and fluid flux to the arc.Ítem Horizontal subduction and truncation of the Cocos Plate beneath central Mexico(American Geophysical Union, 2008-09-27) Pérez-Campos, Xyoli; Kim, YoungHee; Husker, Allen; Davis, Paul M.; Clayton, Robert W.; Iglesias, Arturo; Pacheco, Javier F.; Singh, Shri K.; Manea, Vlad Constantin; Gurnis, MichaelBased on analysis of data from a trans-Mexico temporary broadband seismic network centered on Mexico City, we report that the subducting Cocos Plate beneath central Mexico is horizontal, and tectonically underplates the base of the crust for a distance of 250 km from the trench. It is decoupled from the crust by a very thin low viscosity zone. The plate plunges into the mantle near Mexico City but is truncated at a depth of 500 km, probably due to an E-W propagating tear in the Cocos slab. Unlike the shallow slab subduction in Peru and Chile, there is active volcanism along the Trans Mexican Volcanic Belt (TMVB) that lies much further inland than regions to either side where subduction dip is not horizontal. Geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction geometry.Ítem Central American subduction system(GEOPHYSICISTS, 2007-10) Protti, Marino; Gonzalez, Victor; Alvarado, Guillermo; Hoernle, Kaj; Plank, Terry; Silver, EliThe driving force for great earthquakes and the cycling of water and climate-influencing volatiles (carbon dioxide, sulfur, halogens) across the convergent margin of Central America have been a focus of international efforts for over 8 years, as part of the MARGINS pro gram of the U.S. National Science Foundation, the Collaborative Research Center (SFB 574) of the German Science Foundation, and the Cen tral American science community. Over 120 scientists and students from 10 countries met in Costa Rica to synthesize this intense effort spanning from land to marine geological and geophysical studies.Ítem Subduction-zone structure and magmatic processes beneath Costa Rica constrained by local earthquake tomography and petrological modelling(Royal Astronomical Society, 2003-10-01) Husen, S.; Quintero, R.; Kissling, E.; Hacker, B.A high-quality data set of 3790 earthquakes were simultaneously inverted for hypocentre locations and 3-D P-wave velocities in Costa Rica. Tests with synthetic data and resolution estimates derived from the resolution matrix indicate that the velocity model is well constrained in central Costa Rica to a depth of 70 km; northwestern and southeastern Costa Rica are less well resolved owing to a lack of seismic stations and seismicity. Maximum H2O content and seismic wave speeds of mid-ocean ridge basalt and harzburgite were calculated for metamorphic phase transformations relevant to subduction. Both the 3-D P-wave velocity structure and petrological modelling indicate the existence of low-velocity hydrous oceanic crust in the subducting Cocos Plate beneath central Costa Rica. Intermediate-depth seismicity correlates well with the predicted locations of hydrous metamorphic rocks, suggesting that dehydration plays a key role in generating intermediate-depth earthquakes beneath Costa Rica. Wadati– Benioff zone seismicity beneath central Costa Rica shows a remarkable decrease in maximum depth toward southeastern Costa Rica. The presence of asthenosphere beneath southeastern Costa Rica, which entered through a proposed slab window, may explain the shallowing of seismicity due to increased temperatures and associated shallowing of dehydration of the slab. Tomographic images further constrain the existence of deeply subducted seamounts beneath central Costa Rica. Large, low P-wave velocity areas within the lower crust are imaged beneath the southeasternmost volcanoes in central Costa Rica. These low velocities may represent anomalously hot material or even melt associated with active volcanism in central Costa Rica. Tomographic images and petrological modelling indicate the existence of a shallow, possibly hydrated mantle wedge beneath central Costa Rica.Ítem The synchronous occurrence of shallow tremor and very low frequency earthquakes offshore of the Nicoya Peninsula, Costa Rica(American Geophysical Union, 2013-03-05) Walter, Jacob I.; Schwartz, Susan Y.; Protti, Marino; Gonzalez, VictorThe occurrence of transient, shallow slow slip at seismogenic zones has important implications for earthquake and tsunami hazards. Here we provide evidence that a tremor and slow slip event occurred at shallow depth offshore of the Nicoya Peninsula, Costa Rica, in August 2008. The temporal coincidence of offshore tremor, very low frequency earthquakes (VLFEs), motions consistent with slow slip on the plate interface on western coastal GPS stations, and a pressure transient in an IODP borehole all indicate slow slip occurring at shallow depths. Large ocean loading stresses on the shallow plate interface modulate tremor activity, with the peak Coulomb stress forced by semi-diurnal ocean tides correlating with tremor productivity. Based on beamforming data, we constrain that the VLFE activity occurs in the same region as the tremor and slow slip. The presence of slow slip at shallow depth has important implications for the up-dip extent of earthquake rupture. The proximity of the 5 September 2012, Mw 7.6 megathrust earthquake to slow slip, tremor, and VLFE activity in the 2008 event suggests abrupt frictional transitions from locked to conditionally stable behavior on the plate interface offshore of the Nicoya Peninsula.Ítem Effect of subducting sea-floor roughness on fore-arc kinematics, Pacific coast, Costa Rica(Geological Society of America, 1998-05-01) Fisher, Donald M.; Gardner, Thomas W.; Marshall, Jeffrey S.; Sak, Peter B.; Protti, MarinoFault kinematics and uplift in the Costa Rican fore arc of the Middle America convergent margin are controlled to a large extent by roughness on the subducting Cocos plate. Along the northwest flank of the incoming Cocos Ridge, seafloor is characterized by short wavelength roughness related to northeast-trending seamount chains. Onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore-arc thrust belt and separate blocks with contrasting uplift rates. Trunk segments of Pacific slope fluvial systems typically follow these margin-perpendicular faults. Regionally developed marine and fluvial terraces are correlated between drainages and acrossfaults along the Costa Rican Pacific coast. Terrace separations across block-bounding faults reveal a pattern of fore-arc uplift that coincides roughly with the distribution of incoming seamounts. Magnitude and distribution of Quaternary uplift along the Costa Rican Pacific coast suggests that, despite a thin incoming sediment pile, the inner fore arc shows an accumulation of mass—a characteristic that may be due to underplating of seamounts beneath the fore-arc high.Ítem Persistent tremor within the northern Costa Rica seismogenic zone(American Geophysical Union Advancing Earth and Space Sciences, 2011-01-13) Walter, Jacob I.; Schwartz, Susan Y.; Protti, Marino; Gonzalez, VictorWe identify tremor using a spectral detection method and characterize its occurrence over a period of four years (2006–2009) in the vicinity of the Nicoya Peninsula, Costa Rica. Although a few major tremor events accompanied by geodetic slow slip occur, much of the tremor record consists of minor episodes with short duration and no detectable geodetic slip. Its persistent occurrence suggests that some portion of the interface is experiencing slow slip nearly continuously driving small patches to fail in accelerated slip. Locations indicate that much of the tremor occurs at shallow depth, in freely slipping regions of the seismogenic zone. This result is significant in that locations of slow slip and tremor at other subduction zones are largely limited to the downdip frictional transition. Tremor locations may help to refine the heterogeneous distribution of locked and freely slipping patches within the Costa Rica seismogenic zone.Ítem Quantifying recent pyroclastic and lava flows at Arenal Volcano, Costa Rica, using medium-footprint lidar(American Geophysical Union Advancing Earth and Space Sciences, 2006-11-03) Hofton, M. A.; Malavassi, E.; Blair, J. B.Arenal volcano is a small, active stratovolcano in Costa Rica. In 1998 and 2005, NASA’s Laser Vegetation Imaging Sensor (LVIS) was used to collect wide-swath 3- dimensional topographic images of the volcano. The LVIS is a full-waveform, scanning, medium-sized footprint, airborne laser altimeter system. By digitally recording the shape of the returning laser pulse (waveform), the LVIS provides a precise and accurate view of both the sub-canopy and canopy-top topographies as well as the vertical and horizontal structure of vegetation at 15– 25 m horizontal resolution. By comparing georeferenced waveform data collected in 1998 and 2005, we mapped lava and pyroclastic flows deposited during this period. The active crater grew by 3.82 m yr 1. A flow volume estimate of 2.19 107 m3 (Dense Rock Equivalent of 1.89 107 m3 or 0.085 m3s 1) Nwas obtained for the period 1998 to 2005. Precise elevation and elevation change data such as those provided by the LVIS are essential to calculate eruption volume and to study magma-supply dynamics, as well as assess the danger posed by the volcano to the local population from hazards such as pyroclastic flows.Ítem Magma plumbing processes for persistent activity at Poás volcano, Costa Rica(American Geophysical Union Advancing Earth, 2005-04-21) Rymer, Hazel; Locke, Corinne A.; Brenes, jorge; Williams-Jones, GlynNew microgravity data from the active crater of Poás volcano, Costa Rica, collected in 2002 – 2004 extends the existing dataset to provide a unique 20-year time series. These data show that gravity has decreased monotonically in the north and east of the crater over the last 5 years, whilst it has increased to the west and remained approximately constant in the south. These changes are interpreted in terms of convective recharge within dendritic intrusions beneath the crater, with overall down-welling in the north and up-welling in the west. The data reveal a 5 –10 year periodicity in sub-crater mass movement, but overall, the upper part of the conduit system appears to have maintained a state of mass equilibrium.Ítem High-resolution gravity survey: Investigation of subsurface structures at Poás volcano, Costa Rica(Universidad Nacional, Costa Rica, 2003-08-03) Fournier, Nicolas; Rymer, Hazel; Williams-Jones, Glyn; Brenes, JorgeBouguer gravity surveys have long been used to investigate sub-surface density contrasts. The main sources of error in previous surveys have been the determination of relative elevations of stations and the effect of topography (removed via the terrain correction). The availability of high precision Kinematic GPS data now facilitates generation of high-resolution Digital Elevation Models that can help to improve the accuracy of relative elevation determination and the terrain correction. Here we describe a highresolution gravity survey at Poás volcano, Costa Rica. Our gravity modelling identifies small pockets of magma at shallow depths which relate to successive magma intrusion through time and shows that the persistent degassing in the eastern part of the crater is related to local deformation at the top of the volcano and changes in the fracture network, rather than to the presence of a shallow magma intrusion.Ítem New insights into the magmatic-hydrothermal system and volatile budget of Lastarria volcano, Chile: Integrated results from the 2014 IAVCEI CCVG 12th Volcanic Gas Workshop(Geological Society of America, 2018-05-07) Lopez, Taryn; Aguilera, Felipe; Tassi, Franco; De Moor, J. Marteen; Bobrowski, Nicole; Aiuppa, Alessandro; Tamburello, Giancarlo; Rizzo, Andrea L.; Liuzzo, Marco; Viveiros, Fátima; Cardellini, Carlo; Silva, Catarina; Fischer, Tobias; Philippe, Jean-Baptiste; Kazayaha, Ryunosuke; Hidalgo, Silvana; Malowany, Kalina; Lucic, Gregor; Bagnato, Emanuela; Bergsson, Baldur; Reath, Kevin; Liotta, Marcello; Carn, Simon; Chiodini, GiovanniRecent geophysical evidence for large-scale regional crustal inflation and localized crustal magma intrusion has made Lastarria volcano (northern Chile) the target of numerous geological, geophysical, and geochemical studies. The chemical composition of volcanic gases sampled during discrete campaigns from Lastarria volcano indicated a well-developed hydrothermal system from direct fumarole samples in A.D. 2006, 2008, and 2009, and shallow magma degassing using measurements from in situ plume sampling techniques in 2012. It is unclear if the differences in measured gas compositions and resulting interpretations were due to artifacts of the different sampling methods employed, short-term excursions from baseline due to localized changes in stress, or a systematic change in Lastarria’s magmatic-hydrothermal system between 2009 and 2012. Integrated results from a two-day volcanic gas sampling and measurement campaign during the 2014 International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Commission on the Chemistry of Volcanic Gases (CCVG) 12th Gas Workshop are used here to compare and evaluate current gas sampling and measurement techniques, refine the existing subsurface models for Lastarria volcano, and provide new constraints on its magmatic-hydrothermal system and total degassing budget. While compositional differences among sampling methods are present, distinct compositional changes are observed, which if representative of long-term trends, indicate a change in Lastarria’s overall magmatic-hydrothermal system. The composition of volcanic gases measured in 2014 contained high proportions of relatively magma- and water-soluble gases consistent with degassing of shallow magma, and in agreement with the 2012 gas composition. When compared with gas compositions measured in 2006–2009, higher relative H2O/CO2 ratios combined with lower relative CO2/St and H2O/St and stable HCl/St ratios (where St is total S [SO2 + H2S]) are observed in 2012 and 2014.