Investigaciones
Simultaneous Inversion for Earthquake Location and Velocity Structure Beneath Central Costa Rica
Autores: Marino Protti, Susan Y. Schwartz, and George Zandt
Abstract: We have imaged the complex crustal and upper mantle structure beneath central Costa Rica using P-wave arrival times from locally recorded earthquakes. Thurber's (1983) iterative inversion method is used to simultaneously estimate velocities along a three-dimensional grid and hypocentral parameters of local earthquakes. Our data consist of over 12,000 arrival times from more than 1300 earthquakes recorded by stations of a permanent seismographic network in Costa Rica . Our resulting velocity model correlates well with mapped geologic units at very shallow depth, and with tectonic features at greater depth. We find low velocities (4.0 to 4.8 km/sec) in the shallow crust (above 10 kin) near the active volcanoes and associated with a NW-SE trending late Cretaceous to late Tertiary sedimentary basin southeast of Herradura peninsula. High velocities (5.4 to 5.7 km/sec) in the shallow crust correlate with outcrops of late Jurassic to early Tertiary ultramafic ophiolitic units and with basic Tertiary volcanic units. At depths between 20 and 30 km , high velocities (6.8 to 7.2 km/sec) are associated with the subducting Cocos plate under Costa Rica and two prominent low-velocity bodies (6.3 to 6.5 krrdsec) are present about 30 km trenchward of the volcanic arc and along the projection of the aseismic Cocos Ridge as it subducts beneath Costa Rica. The thickened oceanic crust of the Cocos Ridge is most likely responsible for its low velocities. The deep low-velocity anomaly located trenchward of the axis of the volcanoes may indicate the presence of a low-density intrusive resulting from an earlier phase of magmatism, possibly the late Miocene episode that produced the Talamanca intrusive complex.
Along-strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica
Autores: A. V. Newman, S. Y. Schwartz, V. Gonzalez, H. R. DeShon, J. M. Protti, and L. M. Dorman
Abstract: 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. I NDEX T ERMS : 3015 Marine Geology and Geophysics: Heat flow (benthic) and hydrothermal processes; 7209 Seismology: Earthquake dynamics and mechanics; 7220 Seismology: Oceanic crust; 7230 Seismology: Seismicity and seismotectonics. Citation: Newman, A. V., S. Y. Schwartz, V. Gonzalez, H. R. DeShon, J. M. Protti, and L. M. Dorman, Alongstrike variability in the seismogenic zone below Nicoya Peninsula , Costa Rica , Geophys. Res. Lett. , 29 (20), 1977, doi:10.1029/ 2002GL015409, 2002.
Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica
Autores: Edmundo Norabuena, Timothy H. Dixon, Susan Schwartz, Heather DeShon, Andrew Newman, Marino Protti, Victor Gonzalez, LeRoy Dorman, Ernst R. Flueh, Paul Lundgren, Fred Pollitz, and Dan Sampson
Abstract: New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge.Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica . In Nicoya , inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short ( _ 1 year) observation period. We also estimate northwest motion of a coastal ‘‘sliver block'' at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with _ 8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costen˜a fold and thrust belt) and _ 1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from _ 10–50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya , with consequent higher coupling and compressive stress in the direction of plate convergence. I NDEX T ERMS : 8150 Tectonophysics: Plate boundary—general (3040); 8102 Tectonophysics: Continental contractional orogenic belts; 1208 Geodesy and Gravity: Crustal movements— intraplate (8110); 1243 Geodesy and Gravity: Space geodetic surveys; 7230 Seismology: Seismicity and seismotectonics; K EYWORDS : seismogenic zone, Costa Rica , geodetic and seismic Citation: Norabuena, E., et al. (2004), Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica, J. Geophys. Res. , 109 , B11403, doi:10.1029/2003JB002931.
Seismogenic zone structure of the southern Middle America Trench , Costa Rica.
Autores: H. R. DeShon, S. Y. Schwartz, S. L. Bilek, L. M. Dorman, V. Gonzalez, J. M. Protti,4 E. R. Flueh, and T. H. Dixon
Abstract: 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 M W 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. I NDEX T ERMS : 7209 Seismology: Earthquake dynamics and mechanics; 7220 Seismology: Oceanic crust; 7230 Seismology: Seismicity and seismotectonics; 8123 Tectonophysics: Dynamics, seismotectonics; 8150 Tectonophysics: Plate boundary—general (3040); K EYWORDS : seismogenic zone, Costa Rica , Quepos aftershocks, subduction zone, earthquake location.
An improved P-wave velocity reference model for Costa Rica
Autores: Ronnie Quintero and Edi Kissling
Abstract:We derive a P-wave 1D-velocity model for Costa Rica that may serve for routine high-precision earthquake location and as initial reference model for 3D seismic tomography. The velocity inversions are performed using 822 well-locatable events together with 14774 P-wave observations obtained by merging routine travel time data from 10 335 earthquakes in the period 1984 to 1997 collected by the Universidad Nacional de Costa Rica (OVSICORI), and 3510 earthquakes in the period 1992 to 1998 collected by the Red Sismológica Nacional (RSN) in Costa Rica. Special care is taken during the merging process to reduce the number of errors in the data and, in particular, to update, correct, and complete the station parameter list. Consistency and quality are given priority over completeness of the resulting data set. The final data set for the period 1984 to 1998 in Costa Rica consists of 11 848 local events with 13 2331 P-wave and 86 018 S-wave observations.
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
Autores: Ronnie Quintero, S. Husen and E. Kissling
Abstract: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.
The Mw 6.4 Damas, Costa Rica, Earthquake of 20 November 2004:
Aftershocks and Slip Distribution.
Autores: Javier Fco. Pacheco, Ronnie Quintero, Floribeth Vega, Juan Segura, Walter Jimenez, and Vıctor Gonzalez
Abstract: The earthquake of 20 November 2004 was located north of Damas Island
in the Pacific coast of Costa Rica, within the Costa Rica Deformed Belt. The
earthquake was located at 24 km depth and reported with a magnitude (Mw) of 6.4
and a strike-slip mechanism with a large normal dip-slip motion. This mechanism
agrees with mapped faults in the area that suggest transtensional deformation on the
forearc and the entire western boundary of the Panama microplate. Aftershock locations
do not delineate a preferable plane to distinguish between the two nodal
planes and are distributed between 15 and 25 km depth. The slip distribution during
the mainshock, modeled after teleseismic and local data, pictured a circular rupture
8 km in radius and 0.25 m of average displacement. The fault plane cannot be
distinguished from the two nodal planes from the slip distribution because of the
lack of directivity and resolution for this magnitude earthquake. Weak evidence from
empirical Green’s function analysis suggests that the dextral northwest-oriented fault
could be the causative fault. Depth to the top of the slab, hypocenter location of the
mainshock, its slip distribution, depth distribution of the aftershocks, and Quaternary
fault activity at the surface suggest that deformation takes place throughout the whole
thickness of the crust. This extended deformation might be caused by seamount
subduction and strong basal friction on the upper plate, due to subduction of a thick,
young, and buoyant oceanic plate, rough seafloor, and underplating of large seamounts.
Evento principal y réplicas del sismo del 20 de Noviembre del 2004 (Mw=6.4) en Costa Rica.
Autores: Ronnie Quintero, Juan Segura, Walter Jiménez y Floribeth Vega.
Resumen: La red sismológica del OVSICORI-UNA ubicada en Costa Rica, localizó un sismo de magnitud
Mw=6.4 Harvard CMT, que ocurrió el 20 de Noviembre del 2004 a las 08:07 UTC. El sismo fue
localizado en el Pacífico Central de Costa Rica, a 100 km de la Fosa Mesoamericana y a una
profundidad de 25 km. El sismo ocurrió en la parte más profunda de la corteza y con réplicas
distribuidas hacia la superficie. La actividad sísmica fue relocalizada usando la técnica de la
doble diferencia. La falla delimitada por la secuencia sísmica es parte de un sistema de fallas que
marcan el límite oeste entre la placa Caribe y la Microplaca de Panamá.
Subduction-zone structure and magmatic processes beneath
Costa Rica constrained by local earthquake tomography
and petrological modelling.
Autores: S. Husen, R. Quintero, E. Kissling B. and Hacker
Abstract 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 iswell 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 lowvelocities 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.
Key words: crustal structure, earthquake location, mineralogy, seismic tomography,
subduction.
Actividad sismica frente a las costas de Quepos Pacifico Central, Costa Rica
Autores: Federico Güendel y Marino Protti, Observatorio Vulcanológico y Sismológico de Costa Rica y Universidad Nacional.
Resumen: Actividad sísmica de magnitud moderada (5.0 ≤ mb ≤ 5.8) ha venido ocurriendo, al suroeste de Puerto Quepos en la región del Pacífico central de Costa Rica, historicamente. Esta actividad sísmica se presenta a manera de secuencias en las cuales generalmente ocurren al menos dos eventos principales con sus patrones de réplicas respectivos, usualmente espaciados por uno o más días. La característica más importantes de estas secuencias sísmicas, tal como lo propusieran Güendel y McNally (1981), son los períodos de quietud inter-sísmica, de entre 2 y 4 años, que anteceden y preceden a dicha actividad. Durante la revisión e impresión del presente artículo ocurrieron de manera similar, dos nuevos eventos en la misma región. Esta nueva secuencia sísmica inició con un evento de magnitud mb=5.1 el día 10 de agosto de 1999 y 10 días después, el 20 de agosto se presentó el evento de mayor magnitud mb=5.7, Ms=6.7, Mw=7.0. Estos dos nuevos eventos sísmicos ocurren tras tres años de “quietud sísmica” que precedieron a los eventos ocurridos en agosto y setiembre de 1996 y que sirvieron como motivación del presente estudio.
Afinamiento del Potencial Sísmico y Monitoreo de la Brecha Sísmica de Nicoya.
Autores: Víctor González y Marino Protti,Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional Y Grupo Académico Informal OBCIKORY Provisional.
Resumen: La brecha sísmica de Nicoya es un segmento de la zona de subducción en el noroeste de Costa Rica. Las últimas tres rupturas produjeron grandes sismos en 1853, 1900 y 1950. Las evidencias que indican la existencia de un fuerte acople sísmico en el segmento de Nicoya son: a) un bajo nivel de sismicidad de fondo; b) el cese repentino de la propagación de las réplicas de los sismos de 1990 y de 1992 justo en los bordes de la brecha sísmica de Nicoya; c) el rápido movimiento de la Península de Nicoya hacia el NE aunado a subsidencia de su costa oeste y levantamiento de la costa este, compatibles con el patrón de deformación elástica resultante de un acoplamiento a lo largo de la interfase por debajo de esa península. Nuevos resultados de investigaciones científicas desde el 2001 han permitido afinar aun más el potencial de esta brecha sísmica. Mantenemos una red de monitoreo geodinámico en y alrededor de la península de Nicoya para el registro de las deformaciones pre-, co- y post-sísmicas.
Instrumentación Guralp CMG-6TD OVSICORI-UNA
Autores: Ronnie Quintero
Resumen: Calibración del Instrumento CMG-6TD. Remover la respuesta del Instrumento
Conversión de la repuesta del Instrumento de Hz a Radianes Conversión de Velocidad a desplazamiento. Remover la Repuesta del Instrumento usando SAC y aplicación del filtro Wood-Anderson. Repuesta del Instrumento en Formato SEISAN y su uso para cálculo de magnitud
El terremoto de Nicoya: un posible escenario.
Autores: Carlos Montero y Henry Rodríguez.
Resumen: El terremoto esperado por debajo de la península de Nicoya se calcula tendrá una magnitud de 7.7. Por la sola comparación con el terremoto de Limón que alcanzó una magnitud de 7.6 grados, esta diferencia de magnitud significa que el terremoto de Nicoya liberaría una cantidad de energía 1.5 veces mayor que la del terremoto de Limón. De acuerdo con esto tendríamos que imaginarnos daños mucho mayores a los ocasionados por el terremoto de Limón, pero esto en realidad podría no ser así.
Potencial de instalación de un sistema en el NW de Costa Rica para la alerta temprana al valle central al momento de ocurrencia de sismos fuertes por debajo de la península de Nicoya.
Autor: Marino Protti, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional,OVSICORI-UNA.
Resumen: Una brecha sísmica madura existe por debajo y frente a la península de Nicoya en el Pacífico norte de Costa Rica. Esta brecha, la brecha sísmica de Nicoya, es un segmento de la Fosa Mesoamericana donde la placa del Coco se subduce bajo la placa del Caribe. Terremotos de gran magnitud han ocurrido en este segmento en 1853, 1900 y 1950. La distribución de réplicas de terremotos ocurridos en la década de los 90s en los segmentos aledaños a esta brecha han permitido afinar su ubicación geográfica y determinar las dimensiones de la misma. Sin deslizamiento sísmico importante desde 1950, con una taza de convergencia de 88 mm/año y un área comprendida entre los 5000 y los 10000 km2, la brecha sísmica de Nicoya tiene ya potencial para generar un terremoto con magnitud superior a los 7.5 grados. El Valle Central de Costa Rica, donde se concentra la mayor concentración de su población e infraestructura, se encuentra entre 100 y 250 km de distancia del área potencial de ruptura de la brecha sísmica de Nicoya. Estas distancias están dentro del rango de sistemas de alerta temprana para terremotos que han probado ya ser efectivos, haciendo de esta región un excelente sitio para la operación de uno de esos sistemas de alerta.
Delimitación de la Zona Sismogénica (penínsulas de Osa y Nicoya).
Autores: Universidad de Miami (UM), Universidad de California en Santa Cruz (UCSC), Universidad de California en San Diego (UCSD), Observatorio Vulcanológico y Sismológico de Costa Rica de la Universidad Nacional (OVSICORI-UNA), Laboratorio de Propulsión a Chorro de la NASA (JPL) y GEOMAR.
Resumen: El objetivo principal de este proyecto es la definición geográfica de los límites superior e inferior de la zona sismogénica (la parte bloqueada o parcialmente bloqueada de la zona de contacto entre las placas que producen grandes terremotos) y definir su naturaleza en esta zona de subducción. Este no es un proyecto de predicción sísmica ni tiene como objetivo la captura del próximo terremoto de Nicoya (si el sismo ocurriera durante el período de instrumentación sería de gran valor, pero no es requerido para el éxito del proyecto). El proyecto es financiado por la Fundación Nacional de Ciencias de los Estados Unidos (NSF) a través del subprograma "Seismogenic Zone Experiment" (SEIZE) del programa MARGINS. La península de Nicoya fué uno de los sitios escogidos por SEIZE para los experimentos sobre zonas sismogeneradoras. La península de Osa se ha incluido en este proyecto con el fin de apoyar el trabajo que el grupo alemán de GEOMAR realiza en esa región, a cambio de que ellos provean el barco y la logística para la instalación y recolección de los sismómetros de fondo oceánico. Esto le ahorraría una suma importante de dinero al proyecto y fue fundamental para que NSF lo financiara. Las penínsulas de Osa y Nicoya son casi únicas en el mundo por cuanto nos permiten instalar instrumentación justo sobre la zona sismogénica; este sería el primer transecto completo sobre una zona de subducción.