Artículos científicos
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Ítem GPS measurements of crustal deformation associated with the 22 April 1991, Valle de la Estrella, Costa Rica Earthquake(Universidad Nacional, Costa Rica, 1993-03-05) Lundgren, Paul R.; Kornreich Wolf, Susan; Protti, Marino; Hurst, Kenneth J.We present analysis results for Global Positioning System (GPS) measurements made at sites in Costa Rica during two campaigns in February and July 1991. GPS solutions for 5 sites (Limon, Liberia, Bratsi, ETCG, and Vueltas) reveal significant horizontal and vertical displacements relative to their February positions. Horizontal displacements relative to Liberia, measured 244.7 ± 0.8, 89.2 ± 0.9, 12.4 ± 1.3, and 1.9 ± 0.9 cm at Limon, Bratsi, Vueltas, and ETCG respectively. Vertical displacements relative to Liberia measured 163 ± 2.1, 15.3 ± 3.0, −10.5 ± 4.4, and −0.6 ±2.1 cm at Limon, Bratsi, Vueltas, and ETCG respectively. We find differences in the GPS derived vertical and horizontal displacements compared to other types of geodetic measurements of uplift in the coastal region and their associated models. To address these differences we compute a dislocation model which fits the GPS measured displacements. A simple uniform planar slip model can not reconcile the differences between the coastal uplift data or the seismic moment, suggesting considerable complexity of the earthquake source.Ítem Strain release at the trench during shallow slow slip: The example of Nicoya Peninsula, Costa Rica(Universidad Nacional, Costa Rica, 2017-05-29) Jiang, Yan; Zhen, Liu; Davis, Earl E.; Schwartz, Susan Y.; Dixon, Timothy H.; Voss, Nick; Malservisi, Rocco; Protti, MarinoThe near-trench behavior of subduction megathrust faults is critical for understanding earthquake hazard and tsunami generation. The shallow subduction interface is typically located in unconsolidated sediments that are considered too weak to accumulate elastic strain. However, the spectrum of shallow fault slip behavior is still elusive, due in large part to the lack of near-field observations. Here we combine measurements from seafloor pressure sensors near the trench and an onshore GPS network in a time-dependent inversion to image the initiation and migration of a well-documented slow slip event (SSE) in 2007 at the Nicoya Peninsula, Costa Rica. Our results show that the shallow SSE initiated on the shallow subduction interface at a depth of ~15 km, where pore fluid pressure is inferred to be high, and propagated all the way to the trench. The migrating event may have triggered a second subevent that occurred 1 month later. Our results document the release of elastic strain at the shallow part of the subduction megathrust and suggest prior accumulation of elastic strain. In conjunction with near-trench shallow slow slip recently reported for the Hikurangi subduction zone and trench breaching ruptures revealed in some large earthquakes, our results suggest that near-trench strain accumulation and release at the shallower portions of the subduction interface is more common than previously thought.Ítem Slow slip events in Costa Rica detected by continuous GPS observations, 2002–2011(American Geophysical Union, 2012-04-18) Jiang, Yan; Wdowinski, Shimon; Dixon, Timothy H.; Hackl, Matthias; Protti, Marino; Gonzalez, VictorUna red de estaciones GPS de registro continuo ha operado en la Península de Nicoya en el norte de Costa Rica desde 2002. Procesamos todos los datos disponibles de esta red para el período 2002-2011 para investigar la ocurrencia de Eventos de Deslizamiento Lento (SSE) en la interfaz de subducción entre las placas de Cocos y Caribe. Con el fin de superar el enmascaramiento de la señal por los altos niveles de ruido troposférico, desarrollamos una nueva técnica que facilita la detección de eventos transitorios en presencia de ruido. Identificamos cinco SSE significativos durante el período 2002-2011, con tiempos medios de evento en 2003, 2005, 2007, 2009 y 2011, con un intervalo de recurrencia promedio de 216 meses. El análisis de series de tiempo muestra que la deformación transitoria imparte una firma similar a la caminata aleatoria. La eliminación de los SSE y los errores regionales de modo común de la serie de tiempo redujo la incertidumbre de la velocidad en casi un orden de magnitud. Los datos limitados disponibles para los eventos de 2003, 2005 y 2011 impiden la caracterización detallada de estos eventos. Sin embargo, una buena cobertura espaciotemporal de los eventos de 2007 y 2009 sugiere que ambos eventos tuvieron una duración y distribución irregulares. En el evento de 2007, el deslizamiento lento comenzó en el área costera del noroeste y migró hacia el sureste durante un período de 1 mes. El evento de 2009 tuvo una duración significativamente mayor y un mayor desplazamiento en la superficie. Las estaciones en el área noroeste observaron dos SSE separados en 2008.6 y 2009.4, correlacionándose bien con los episodios de temblores en alta mar, lo que indica un parche de deslizamiento SSE poco profundo con un intervalo de recurrencia más corto. Las diferencias significativas entre los eventos de 2009 y 2007 nos llevan a cuestionar el modelo simple de intervalo de recurrencia para el SSE en Nicoya.Ítem The 6 September 1997 (Mw4.5) Coatzacoalcos-Minatitlán, Veracruz, Mexico earthquake: implications for tectonics and seismic hazard of the region(Universidad Nacional Autónoma de México, 2015-06-30) Singh, Shri Krishna; Pacheco, Javier Francisco; Pérez-Campos, Xyoli; Ordaz, Mario; Reinoso, EduardoAnalizamos el sismo del 6 de septiembre de 1997 (Mw4.5), que se produjo a unos 25 km al SE de Coatzacoalcos, Veracruz. El sismo fue registrado por la estación local TUIG de banda ancha de (tiempo S-P = 5 s). Las polaridades de la onda P a distancias regionales y telesísmicas, y el modelado de las formas de onda de desplazamiento en TUIG sugieren un mecanismo focal inverso (f = 150º; d = 70º; l = 90º). En la misma región ocurrió un sismo destructivo el 26 de agosto de 1959 (Mw6.4), a una profundidad similar y con un mecanismo similar. El análisis del sismo de 1997 refuerza la conclusión anterior de que la corteza inferior bajo la cuenca del Coatzacoalcos-Minatitlán, está en un régimen de esfuerzos de fallamiento inverso, en contraste con la parte superficial de la corteza que se caracteriza por un fallamiento normal; esto implica una permutación con la profundidad de los esfuerzos principales máximo y mínimo. Esto está de acuerdo con las observaciones, en otros sitios, que el estado de esfuerzos en las cuencas sedimentarias pueden ser diferentes del que se tiene a mayor profundidad. Mecanismos focales están disponibles para siete sismos en y cerca del Golfo de México. Todos estos eventos muestran en la región una corteza media y baja en un régimen de fallamiento inverso. La tendencia observada de los ejes P de estos sismos se puede explicar por una o más de las siguientes causas: acoplamiento fuerte a lo largo de la interfase de la placa en subducción fuera de la costa en Tehuantepec; el movimiento absoluto de la placa de América del Norte; y el hundimiento de la litosfera debido a la acumulación de la carga de los sedimentos. Usamos los registros del sismo de 1997 como función de Green empírica para simular los movimientos de tierra en la región epicentral de un sismo de Mw6.4 postulado en la cuenca Comalcalco. Bajo supuestos razonables, los valores esperados de aceleración, velocidad y desplazamiento picos son 120-260 gales, 12 a 28 cm/s, y 6 a 11 cm, respectivamente. La extensa licuefacción reportada en Coatzacoalcos durante el sismo de 1959, Mw6.4, sugiere que los sedimentos de la cuenca se comportan de manera no lineal bajo tal excitación.Ítem Holocene forearc block rotation in response to seamount subduction, southeastern Península de Nicoya, Costa Rica(Geological Society of America, 2001-02) Gardner, Thomas; Marshall, Jeffrey; Merritts, Dorothy; Bee, Bhavani; Burgette, Reed; Burton, Emily; Cooke, Jennifer; Kehrwald, Natalie; Protti, Marino; Fisher, Donald; Sak, PeterThe southeastern tip of the Penı´nsula de Nicoya, Costa Rica, on the Caribbean plate margin lies inboard of the rough bathymetric terrain on the subducting Cocos plate and along the landward projection of the convergence vector for the Fisher seamount group. The southern tip of the peninsula has nearly orthogonal coastlines and extensive, well-preserved, Holocene marine terraces,and is ideally situated to evaluate the spatial distribution of forearc deformation in response to seamount subduction. Two marine terraces that yielded 35 radiocarbon dates give information on the rates, style, and timing of deformation along 40 km of coastline. Ages range from 3.5 to 7.4 ka for a higher terrace and from 0.3 to 2.9 ka for a lower terrace. A maximum uplift rate is ;6.0 m/k.y. along the southeastern tip of the peninsula. Uplift rates decrease linearly to ,1.0 m/k.y. along both orthogonal coastlines and thus landward from the Middle America Trench and away from the line of subducting seamounts. The ;400km2 region along the tip of the peninsula can be approximated as a rotating block with an angular rotation rate of 0.028/k.y. about an axis with an azimuth of 808. Given the modern elevation and dip of the late Quaternary Cobano surface, this style of deformation is limited to a duration of 100–200 k.y. Deformation is occurring in response to seamount bypass or underplating onto the Caribbean plate margin.Ítem Impacto de las politicas estatales en la autonomía de los territorios indígenas en la Zona Sur de Costa Rica el caso del territorio de Salitre(Geo UERJ, 2017-09-14) Vargas Rojas, Guido Alonso; Campos Durán, DanielaEste artículo tiene como objetivo realizar un análisis, mediante revisión bibliográfica, de los conflictos que se han dado entre los indígenas y la población no indígena en la comunidad Bribri de Salitre. Donde se determinó que la principal problemática que enfrenta la población autóctona, son actos de violencia por parte de los no indígenas, a causa de la tenencia de la tierra, quienes han organizado grupos de carácter paramilitar con el objetivo de reprimir a los nativos. Ante esta situación la respuesta del estado ha sido débil y ambigua, ya que no ha planteado soluciones claras al conflicto, dejando abierta la posibilidad a los blancos de ocupar estos territorios, contraponiéndose así a las leyes de la República y convenios internacionales que los defienden, tales como el Convenio 169 “Convenio sobre Pueblos Indígenas y Tribales en Países Independientes” y la Ley N° 6172 “Ley Indígena” de la República de Costa Rica.Ítem Detailed data available for recent Costa Rica earthquake.(University of South Florida, 2013) Dixon, Timothy H.; Schwartz, Susan; Protti, Marino; Gonzalez, Victor; Newman, Andrew; Marshall, Jeff; Spotila, JimOn 5 September 2012 a magnitude 7.6 earthquake occurred beneath the Nicoya Peninsula of northwestern Costa Rica, rupturing the subduction zone between the Cocos and Caribbean plates. In most subduction zones the locus of seismic slip lies far offshore, making it difficult to infer interface seismogenic processes from on-shore observations. In contrast, the Nicoya Peninsula lies close to the trench (within 70 kilometers), allowing observations directly over the earthquake rupture zone.Ítem Microbial Survival in an Extreme Martian Analog Ecosystem: Poás Volcano, Costa Rica(Frontiers, 2022-01-28) Wang, Justin L.; Dragone, Nicholas B.; Avard, Geoffroy; Hynek, Brian M.Past acid-sulfate hydrothermal systems on Mars have promise in their ability to have hosted life for billions of years. One method for analyzing these systems is to study analog environments on Earth. To assess the astrobiological potential of Martian acid-sulfate hydrothermal systems, the crater lake of the active Poás Volcano, Laguna Caliente, was sampled in 2013 and 2017. Laguna Caliente presents an extremely dynamic terrestrial environment with near-ambient to boiling temperatures, pH fluctuations from −0.87 to 1.5, a wide range of chemistries and redox potential, and frequent phreatic-tophreatomagmatic eruptions. Samples of lake fluid, sulfur clumps, and lake bottom sediment underwent 16S rRNA gene sequencing and metagenomic “shotgun” sequencing, which revealed this lake hosts an extremely low biodiversity of microorganisms dominated by Acidiphilium spp. Shotgun metagenomics of the samples suggests this community has numerous genetic adaptations that confer survival, including functional pathways to reduce the effects of toxic metals and numerous metabolic pathways utilizing a variety of simple and complex sugar molecules. The identification of these various metabolic pathways suggests adaptations related to carbon limited environments, fulfillment of high energy requirements, and survival in a hostile volcanic setting. The perseverance of life in Laguna Caliente indicates life on Mars could have thrived in analogous environments, stressing the need for the search for life in relict Martian acid-sulfate hydrothermal systems.Ítem The Helium and Carbon Isotope Characteristics of the Andean Convergent Margin(Frontiers, 2022-06-13) Barry, P. H.; De Moor, J. M.; Chiodi, A.; Aguilera, F.; Hudak, M. R.; Bekaert, D.V.; Curtice, J.; Turner, S. J.; Seltzer, A. M.; Jessen, G. L; Osses, E.; Blamey, J. M.; Amenábar, M. J.; Selci, M.; Cascone, M.; Bastianoni, A.; Nakagawa, M.; Filipovich, R.; Bustos, E.; Schrenk, M. O.; Buongiorno, J.; Ramírez, J. C.; Rogers, T. J.; Lloyd, K. G.; Giovannelli, D.Subduction zones represent the interface between Earth’s interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatile elements are actively cycled between Earth reservoirs by plate tectonics. Helium is a sensitive tracer of volatile sources and can be used to deconvolute mantle and crustal sources in arcs; however it is not thought to be recycled into the mantle by subduction processes. In contrast, carbon is readily recycled, mostly in the form of carbon-rich sediments, and can thus be used to understand volatile delivery via subduction. Further, carbon is chemically-reactive and isotope fractionation can be used to determine the main processes controlling volatile movements within arc systems. Here, we report helium isotope and abundance data for 42 deeply-sourced fluid and gas samples from the Central Volcanic Zone (CVZ) and Southern Volcanic Zone (SVZ) of the Andean Convergent Margin (ACM). Data are used to assess the influence of subduction parameters (e.g., crustal thickness, subduction inputs, and convergence rate) on the composition of volatiles in surface volcanic fluid and gas emissions. He isotopes from the CVZ backarc range from 0.1 to 2.6 RA (n = 23), with the highest values in the Puna and the lowest in the Sub Andean foreland fold-and-thrust belt. Atmosphere-corrected He isotopes from the SVZ range from 0.7 to 5.0 RA (n = 19). Taken together, these data reveal a clear southeastward increase in 3 He/4 He, with the highest values (in the SVZ) falling below the nominal range associated with pure upper mantle helium (8 ± 1 RA), approaching the mean He isotope value for arc gases of (5.4 ± 1.9 RA). Notably, the lowest values are found in the CVZ, suggesting more significant crustal inputs (i.e., assimilation of 4 He) to the helium budget.Ítem The Geothermal Resource in the Guanacaste Region (Costa Rica) New Hints From the Geochemistry of Naturally Discharging Fluids(Frontiers, 2018-06-05) Tassi, Franco; Vaselli, Orlando; Bini, Giulio; Capecchiacci, Francesco; De Moor, J. Marteen; Pecoraino, Giovannella; Venturi, StefaniaThe Guanacaste Geothermal Province (GGP) encompasses the three major volcanoes of northern Costa Rica, namely from NW to SE: Rincón de la Vieja, Miravalles, and Tenorio. The dominant occurrence of (i) SO4-rich acidic fluids at Rincón de la Vieja, (ii) Cl-rich mature fluids at Miravalles, and (iii) HCO−3-rich and low-temperature fluids at Tenorio was previously interpreted as due to a north-to-south general flow of thermal waters and a magmatic gas upwelling mostly centered at Rincón de la Vieja, whereas Miravalles volcano was regarded as fed by a typical geothermal reservoir consistingof a highly saline Na-Cl aquifer. The uniformity in chemical and isotopic (R/Ra andδ 34S) compositions of the neutral Cl-rich waters suggested to state that all the thermal discharges in the GGP are linked at depth to a single, regional geothermal system. In thisscenario, the thermal manifestations related to Tenorio volcano were regarded as a distal and diluted fluid outflow. In this study, a new gas geochemical dataset, including both chemical and isotopic (δ 13C-CO2 and R/Ra) parameters of fluid discharges from the three volcanoes, is presented and discussed. Particular attention was devoted to the Tenorio thermal manifestations, since they were poorly studied in the past because this area has been considered of low geothermal potential. The aim is to provide insights into the magmatic-hydrothermal fluid circulation and to verify the spatial distribution of the heat fluid source feeding the fluid manifestations. According to this new dataset, CO2, i.e., the most abundant dry gas in the fluid manifestations, is mostly produced by limestone, whereas the mantle CO2 contribution is ≤3.3%. Strongly acidic gas compounds from magma degassing were absent in the discharged fluids, being scrubbed by secondary processes related to prolonged fluid-rock interactions and mixing with shallow aquifers. Our results only partially confirm the previously depicted model, because the geochemical and isotopic features (e.g., relatively high concentrations of temperature-dependent gases and high R/Ra values) shown by fluids seeping out from the southern sector of Tenorio volcano are more representative of medium-to-high enthalpy volcanic systems than those typically occurring in distal areas. This implies that the geothermal potential in the south of the GGP is higher than previously thought.Ítem Sampling across large-scale geological gradients to study geosphere–biosphere interactions(Frontiers, 2022-10-31) Giovannelli, Donato; Barry, Peter H.; De Moor, J. Maarten; Jessen, Gerdhard L.; Schrenk, Matthew O.; Lloyd, Karen G.Despite being one of the largest microbial ecosystems on Earth, many basic open questions remain about how life exists and thrives in the deep subsurface biosphere. Much of this ambiguity is due to the fact that it is exceedingly difficult and often prohibitively expensive to directly sample the deep subsurface, requiring elaborate drilling programs or access to deep mines. We propose a sampling approach which involves collection of a large suite of geological, geochemical, and biological data from numerous deeply-sourced seeps—including lower temperature sites—over large spatial scales. This enables research into interactions between the geosphere and the biosphere, expanding the classical local approach to regional or even planetary scales. Understanding the interplay between geology, geochemistry and biology on such scales is essential for building subsurface ecosystem models and extrapolating the ecological and biogeochemical roles of subsurface microbes beyond single site interpretations. This approach has been used successfully across the Central and South American Convergent Margins, and can be applied more broadly to other types of geological regions (i.e., rifting, intraplate volcanic, and hydrothermal settings). Working across geological spatial scales inherently encompasses broad temporal scales (e.g., millions of years of volatile cycling across a convergent margin), providing access to a framework for interpreting evolution and ecosystem functions through deep time and space. We propose that tectonic interactions are fundamental to maintaining planetary habitability through feedbacks that stabilize the ecosphere, and deep biosphere studies are fundamental to understanding geo-bio feedbacks on these processes on a global scale.Ítem Seismogenic zone structure of the southern Middle America Trench, Costa Rica(Journal of Geophysical Research, 2003-10) DeShon, H.R.; Schwartz, S.Y.; Bilek, S.L.; Dorman, LeRoy; González, Victor; Protti, Marino; Flueh, E.R.; Dixon, T.H.The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located 300 events that occurred following the MW 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three-dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19 that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30–35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level.Ítem Seafloor structural observations, Costa Rica accretionary prism(1996-04-15) McAdoo, Brian G.; Orange, Daniel L.; Silver, Eli; McIntosh, Kirk; Abbott, Lon; Galewsky, Joseph; Kahn, Leslie; Protti, MarinoBy studying seafloor morphology we can make associations between near surface defornation, fluid flow and the overakk structural framework of accretionary prisms. In February, 1994 a DS/RV ALVIN program to the Costa Rica accretionary prism investigated the relationship of fluid seepage and sediment deformation by using the distribution of chemosynthetic communities and heat flow anormalies as indicators of fluid flow. The active normal faults that cut the hemipelagic section on the Cocos plate may provide conduits for fluids that cause the regional heat flow to be extremely low. These normal faults intersect the toe of the prism at an oblique angle, creating localized regions of increased deformation. Positive heat flow anormalies observed at the deformation. Positive heat flow anormalies observed at the deformation front indicate of flow, however, we discovered ni seep communities indicative of focused flow. The seaward-most seep communities discovered are in a region of active out-of-sequence thrusts that cut a sediment apron which covers the complex to within 3 km of the prism toe. Vents occur consistently at the base of the fault scarps. Dives in a mud diapir show extensive seep communities, pock marks, and authigenic carbonates. Evidence of a fluid release is on the crest which implies a low viscosity fluid migrating upward in the center of the structure. Normal faults on the upper slope can be seen in cross- section in the walls of a submarine canyon. The faults cut the slope apron and displace the seafloor, actively maintaining the critical taper of the prism.Ítem The March 25, 1990 (Mw=7.0 Ml=6.8) earthquake at the entrance of the Nicoya Gulf, Costa Rica: its prior activity, foreshocks, aftershocks and triggered seismicity(1995-10-10) Protti, Marino; McNally, Karen; Pacheco, Javier; González, Victor; Montero, Carlos; Segura, Juan; Brenes, Jorge; Barboza, Vilma; Malavassi, Eduardo; Güendel, Federico; Simila, Gerald; Rojas, Daniel; Velasco, Aaron; Mata, Antonio; Schillinger, WalterOn March 25, 1990 a large earthquake( Mw=7.0, ML=6.8) occurreda t the entranceo f the Nicoya Gulf, CostaR ica, at 1322:55.6U TC, producingc onsiderabled amagei n centralC osta Rica andg eneratingm uchi nteresta boutw hethero r not the Nicoya seismicg ap (Nishenko,1 989) had broken.T he local country-wides eismographince tworkr ecorded6 yearso f activityp rior to thisl argee arthquake1, 6 hourso f foreshockst,h e mainshocka, ndi ts aftershocksT.h is networki s operatedjo intly by the CostaR ica Volcanologicala nd SeismologicaOl bservatorya t the National University( OVSICORI-UNA), andt he CharlesF . RichterS eismologicaLl aboratorya t the Universityo f California,S antaC ruz (CFRSL-UCSC).W e obtainedh igh resolutionlo cations from thisn etworka ndl ocatedt hem ainshocka t 9ø38.5'N,8 4ø55.6'W( depthi s 20.0 km) andt he largestf oreshock(M w=6.0,M arch2 5, 1990,a t 1316:05.8U TC) at 9ø36.4'N,8 4ø57.1'W( depth is 22.4 km). We find that the aftershockz one abutst he southeasbt oundaryo f the Nicoya seismic gap, suggestingth at the seismicg ap did not rupture.S incet he installationo f the local networki n April 1984 to March 24, 1990, nearly 1900 earthquakews ith magnitudesfr om 1.7 to 4.8 (318 with magnitude3 .0 or larger)h aveb eenl ocateda t the entranceo f the Nicoya Gulf, one of the mosta ctiver egionsi n CostaR ica. The March 25 earthquakeo ccurreda t the northweste dgeo f this region,w herea sequencoe f foreshockbs egan1 6 hoursp rior to the mainshockT. he spatialtemporadl istributiono f aftershockasn dd irectivitya nalysiso f the mainshockru pturep rocess usingt eleseismicre cordsb othi ndicatea southeasptr opagatingru pture.T he mainshockru ptured ana speriotyf approximat6e0ly0k m2 ofa reaw, itht hisa reae xpanditnog4 000k m2 afte7r days. We presenet videnceth ats uggesttsh att her uptureda sperityis producedb y the subductioonf a seamountI.n versiono f teleseismicb roadbanda ndl ong-periodP and SH wavesy ieldsa thrust faultingm echanismw ith the shallowp lanes triking2 92ø , dipping2 6ø , andw ith a rakeo f 88ø , in agreemenwt ith the subductiono f the Cocosp lateu ndert he Caribbeanp late.L ocal first motions for the largestf oreshocka ndt he mainshocka greew ith thiss olutionW. e alsop resente vidence suggestintgh at the March 25, 1990,e arthquaketr iggereda ndr eactivateds everals eismics warmsin centralC ostaR ica andt emporallyd ecreasedth e activityi n the epicentraal reao f the July 3, 1983 (Ms=6.2), P6rez Zeled6n earthquake.Ítem 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, KarenHigh- 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.Ítem Interpretación preliminar de la secuencia sísmica de potrero grande (marzo 1988) y su relación con la secuencia sísmica de sierpe (marzo de 1987)(Universidad Nacional, Costa Rica, 1992) Protti, Marino; Guendel, Federico; Montero, Carlos; Brenes, JorgeEl sismo de magnitud 5.3 grados en la escala de Ritcher se convierte en el más importante después de los terremotos de abril y junio de 1983, dado que es el primero que se registra en la recién instalada Red Sismográfica Nacional, operada por el OVSICORI-UNA. Este documento realiza una interpretación preliminar sobre las características epicentrales e ipocentrales del sismo. Así como el estudio de las intensidades del evento principal. Se presenta el mapa de intensidades según la escala de Mercalli, combinando la información del mapa con una gira efectuada a la zona de mayores intensidades. En la gira a la zona sur del país se utilizó un cuestionario de intensidades suministrado por los reporteros sísmicos del OVSICORI-UNA, como parte del proyecto de investigación "Sistematización de reportes de intensidad sísmica (UNA-CONICIT)". Durante el trabajo de campo se utilizó una encuesta para determinar la intensidad máxima alcanzada por el sismo en los lugares visitados.Ítem Regional characteristics of observable foreshocks(Seismological Society of America (SSA), 2022) Wetzler, Nadav; Brodsky, Emily; Chaves, Esteban; Goebel, Thomas H.W.; Lay, ThorneMeasures of foreshock occurrence are systematically examined using earthquake catalogs for eight regions (Italy, southern California, northern California, Costa Rica, Onshore Japan, Alaska, Turkey, and Greece) after imposing a magnitude ≥ 3.0 completeness level. Foreshocks are identified using three approaches: a magnitude-dependent space + fixed-time windowing method, a nearest-neighbor clustering method, and a modified magnitude-dependent space + variable-time windowing method. The method with fixed-time windows systematically yields higher counts of foreshocks than the other two clustering methods. We find similar counts of foreshocks across the three methods when the magnitude aperture is equalized by including only earthquakes in the magnitude range M*−2≤ M< M*, in which M* is the mainshock magnitude. For most of the catalogs (excluding Italy and southern California), the measured b-values of the foreshocks of all region-specific mainshocks are lower by 0.1–0.2 than b-values of respective aftershocks. Allowing for variable-time windows results in relatively high probabilities of having at least one foreshock in Italy ( ∼ 43%–56%), compared to other regional catalogs. Foreshock probabilities decrease to 14%–41% for regions such as Turkey, Greece, and Costa Rica. Similar trends are found when requiring at least five foreshocks in a sequence to be considered. Estimates of foreshock probabilities for each mainshock are method dependent; however, consistent regional trends exist regardless of method, with regions such as Italy and southern California producing more observable foreshocks than Turkey and Greece. Some regions with relatively high background seismicity have comparatively low probabilities of detectable foreshock activity when using methods that account for variable background, possibly due to depletion of near-failure fault conditions by background activity.Ítem The 27 February 2022 Lop Nor earthquake: detectability, location, and discrimination(Seismological Society of America (SSA), 2022) Gibbons, Steven J.; Chaves, Esteban; Fisk, MarkA seismic event with mb 4.8 (Mw 4.2) was detected close to the Chinese Lop Nor nuclear test site on 27 February 2022. Waveforms recorded at regional and far regional distances in central Asia indicate greater likeness with previous earthquakes in the region than with historical nuclear tests. We investigate signal-to-noise ratio (SNR) at regional and global stations, and find the best signals in central Asia and Alaska. Lower SNR at stations in China, Europe, and Australia is likely related to the radiation pattern. A joint probabilistic location of the 2022 event and well-constrained historical nuclear tests indicates an epicenter near 41.88° N and 88.10° E, about 25 km northwest of the tunnel portion of the test site. A moment tensor inversion using high-quality regional signals indicates a nearly deviatoric source with a 72% double couple and a reverse fault mechanism. The centroid depth is 20–25 km, consistent with depth phases recorded in Alaska. The observed faulting geometry and source composition for the 2022 Lop Nor event is consistent with previous earthquakes in the region and the spatial alignment of local geomorphological features, indicating tectonic and not anthropogenic origin.Ítem Yield estimates for the six North Korean nuclear tests from teleseismic P wave modeling and intercorrelation of P and Pn recordings(AGU Advancing Earth and Space Sciences, 2019) Voytan, Dimitri; Lay, Thorne; Chaves, Esteban; Ohman, John T.The yields of the six declared underground nuclear tests at the North Korean test site are estimated using high-frequency teleseismic P wave amplitude modeling and waveform equalization of short-period teleseismic P waves and regional Pn signals. Average amplitudes of the first cycle of high-frequency (>4 Hz) filtered P wave displacements for each event, adjusted for station sampling relative to the 3 September 2017 event, are modeled using Mueller-Murphy explosion source models for granite and a constant-Q attenuation operator with t* = 0.78 ± 0.03 s. The yield estimates range from 2.6 to 230 kt. Intercorrelation, a waveform equalization procedure that accounts for source function and depth-phase variations between events, is applied to large sets of filtered (>0.8 Hz) teleseismic P and regional Pn seismograms. Searching over yield and burial depth for both events gives optimal parameters by simultaneous waveform equalization of multiple stations. Using specified burial depths spanning from 430 to 710 m for all events based on estimated locations in the source topography assuming tunneling with 4% grade, along with allowing for reduction in source region velocity due to weathering, rock layering, and damage zones, gives yield estimates ranging from 1.4 to 250 kt. Comparison of predicted and observed spectral ratios of Pn phases at station MDJ establishes that these source models are reasonable. Using the preferred yield estimates from intercorrelation, WIC, a yield-calibrated relation of mbNEIC = 0.9 log10WIC + 4.13 is determined for the North Korean test site.Ítem Repeating earthquakes record fault weakening and healing in areas of megathrust postseismic slip(American Association for the Advancement of Sciences (AAAS), 2020) Chaves, Esteban; Schwartz, Susan; Abercrombie, RachelRepeating earthquakes (REs) rupture the same fault patches at different times allowing temporal variations in the mechanical behavior of specific areas of the fault to be interrogated over the earthquake cycle. We study REs that reveal fault weakening after a large megathrust earthquake in Costa Rica, followed by fault recovery. We find shorter RE recurrence intervals and larger slip areas immediately following the mainshock that both gradually return to pre-earthquake values. RE seismic moments remain nearly constant throughout the earthquake cycle. This implies a balance between fault weakening (reducing slip) and transient embrittlement (increasing rupture area by converting regions from aseismic to seismic slip), induced by the increased loading rate following the mainshock. This interpretation is consistent with positive, negative, and constant moment versus RE recurrence interval trends reported in other studies following large earthquakes and with experimental work showing slip amplitudes and stress drop decrease with loading rate.