Investigaciones Sismología

Abstract Subduction of the Cocos plate beneath the Nicoya Peninsula, Costa Rica, generates large underthrusting earthquakes with a recurrence interval of about 50 yrs. The most recent of these events occurred on 5 September 2012 (Mw 7.6). A vigorous sequence of more than 6400 aftershocks was recorded by a local seismic network within the first four months after the mainshock. We determine locations and focal mechanisms for as many aftershocks as possible with M ≥1:5 occurring within the first nine days of the mainshock, all aftershocks with M ≥3 through the end of 2012, and all events with M ≥4 through the end of 2015. We determine faulting geometries using regional full waveform moment tensor (MT) inversion for the largest events (M ≥4) and P-wave first-motion polarities for smaller events, producing a mechanism catalog with 347 earthquakes. Sixty percent of these events are identified as underthrusting, and their locations are compared with spatial distributions of mainshock slip, afterslip, prior interplate seismicity, and slow-slip phenomena to better understand the mechanical behavior of the plate interface. Most of the aftershocks on the megathrust occur up-dip of the coseismic slip, where afterslip is large, and between coseismic slip and shallow slow-slip patches. The pattern of interplate seismicity during the interseismic period is similar to that for the aftershocks but does not extend to as great a depth. The coseismic slip extends even deeper than the interplate aftershocks, suggesting that the mainshock ruptured a strongly locked patch driving down-dip slip into the conditionally stable part of the deep plate interface that also hosts slow slip. About 80% of the aftershocks have one nodal plane oriented favorably to promote failure from static stress changes following the mainshock and early afterslip, whereas most others occur in regions of large afterslip.

Abstract. In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise–based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [Mw (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations.

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Abstract: We use acceleration data from the Observatorio Vulcanologico y Sismologico, Universidad Nacional de Costa Rica (OVSICORI-UNA) and Laboratorio de Ingenieria Sismica, Universidad de Costa Rica (LIS-UCR) seismic network for the relocation and moment-tensor solution of the September 5, 2012, 14:42:03.35 UTC, Nicoya, Costa Rica earthquake (Mw 7.6 GCMT). Using different relocation methods we found a stable earthquake hypocenter, near the original OVSICORI-UNA location in the Nicoya Peninsula, NW Costa Rica at Lat 9.6943°N, Lon 85.5689°W, depth 15.3 km, associated with the subduction of the Cocos plate under Caribbean plate. Acceleration records at OVSICORI-UNA and LIS-UCR stations (94–171 km), at 0.03 < f < 0.06 Hz were used in the waveform inversion for a single-point centroid moment tensor (CMT). Using spatial grid search the centroid position was found at the depth of 30 km, situated at Lat 10.0559°N, Lon 85.4778°W, i.e. of about 41 km NNE from the epicenter. The centroid time is 14:42:18.89 UTC, i.e. 15.54 s later relative to the location-based origin time. The nodal plane (strike 318°, dip 27° and rake 115°) is the fault plane that agrees with the geometry of the subducted slab at Nicoya, NNW Costa Rica. Increasing the maximum studied frequency from 0.06 to 0.15 Hz, the multiple point source inversion model leads to two subevents. The first one was located near the centroid and the second subevent was situated 20 km along strike and 10 km down dip from the first subevent and 6 s later. The uncertainty of the source model was carefully examined using complementary inversion methods, viz the iterative deconvolution and non-negative least squares.

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.

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.

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.

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.

Abstract Small local earthquakes from two aftershock sequences in Porto dos Gaúchos, Amazoncraton—Brazil, were used to estimate the coda wave attenuation in the frequency band of 1 to 24 Hz. The time-domain coda-decay method of a single backscattering model is employed to estimate frequency dependence of the quality factor (Qc) of coda waves modeled using Qc = Q0 f η, where Q0 is the coda quality factor at frequency of 1 Hz and η is the frequency parameter.We also used the independent frequency model approach (Morozov, Geophys J Int, 175:239–252, 2008), based in the temporal attenuation coefficient, χ( f ) instead of Q( f ), given by the equation χ( f )=γ + π f Qe , for the calculation of the geometrical attenuation (γ ) and effective attenuation (Q−1e ). Qc values have been computed at central
frequencies (and band) of 1.5 (1–2), 3.0 (2–4), 6.0(4–8), 9.0 (6–12), 12 (8–16), and 18 (12–24) Hz for five different datasets selected according to the geotectonic environment as well as the ability to sample shallow or deeper structures, particularly the sediments of the Parecis basin and the crystalline basement of the Amazon craton. For the Parecis basin Qc = (98 ± 12) f (1.14±0.08), for the surrounding shield Qc = (167 ± 46) f (1.03±0.04), and for the whole region of Porto dos Gaúchos Qc = (99 ± 19) f (1.17±0.02). Using the independent
frequency model, we found: for the cratonic zone, γ = 0.014 s−1, Q−1 e = 0.0001, ν ≈ 1.12; for the basin zone with sediments of ∼500 m,
γ = 0.031 s−1, Q−1 e = 0.0003, ν ≈ 1.27; and for the Parecis basin with sediments of ∼1,000 m, γ = 0.047 s−1, Q−1e= 0.0005, ν ≈ 1.42. Analysis of the attenuation factor (Qc) for different values of the geometrical spreading parameter (ν) indicated that an increase of ν generally causes an increase in Qc, both in the basin as well as in the craton. But the differences in the attenuation between different geological environments are maintained for different models of geometrical spreading. It was shown that the energy of coda waves is attenuated more strongly in the sediments, Qc = (78 ± 23) f (1.17±0.14) (in the deepest part of the basin), than in the basement, Qc = (167 ± 46) f (1.03±0.04) (in the craton). Thus, the coda wave analysis can contribute to studies of geological structures in the upper crust, as the average coda quality factor is dependent on the thickness of sedimentary layer.

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.

Abstract Many seismological studies depend on the accuracy of timing of seismological data. In seismic tomography, travel-time residuals defined as differences between the observed and calculated arrival times of seismic phases are minimized to constrain 3D velocity structure. Inconsistencies and large errors in data sets that result from incorrect station coordinates, errors in the timing acquisition system, errors
in the merging procedure, inconsistency in the picking and phase misidentification can also generate travel-time residuals, and because of their systematic nature, these errors cannot be treated as random noise even by exploiting a large number of travel times.
While the inverse problem is perfectly set up to deal with random noise, systematic errors lead to significant artifacts in the solution, but may not be detected by a posterior error assessment. For this reason, detecting and removing systematic travel-time errors from data sets before inversion is crucial for seismic tomography studies.
We present a methodology based on the use of a minimum 1D model to detect and remove systematic errors in travel-time data by detailed analysis of station delays and observation residuals and apply it to a local earthquake data set from Costa Rica. The determination of the exact nature of detected inconsistencies needs further investigations in each individual case. If the cause of detected systematic errors cannot be determined beyond any doubt and the afflicted data may not be corrected, they must be deleted from the data set. To assess the extent of influence of systematic errors on hypocenter locations and their uncertainties, we present two examples showing the effects of station mislocation.

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.

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.

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

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.

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.

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 N W - S E 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.

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.

he largest earthquake observed in the stable continental interior of the South American plate occurred in Serra do Tombador, Mato Grosso state — Brazil, on January 31, 1955 with a magnitude of 6.2 mb. Since then no other earthquake has been located near the 1955 epicentre. However, in Porto dos Gaúchos, 100 km northeast of Serra do Tombador, a recurrent seismicity has been observed since 1959. Both Serra do Tombador and Porto dos Gaúchos are located in the Phanerozoic Parecis basin. Two magnitude 5 earthquakes occurred in Porto dos Gaúchos, in 1998 and 2005, with intensities up to VI and V, respectively. These two main shocks were followed by aftershock sequences lasting more than three years each. Local seismic stations have been deployed by the Seismological Observatory of the University of Brasilia since 1998 to study the “Porto dos Gaúchos” seismic zone (PGSZ). A local seismic refraction survey was carried out with two explosions to help define the seismic velocity model. Both the 1998 and 2005 earthquake sequences occurred in the same WSW–ENE oriented fault zone with right-lateral strike-slip mechanisms. The epicentral zone is in the Parecis basin, near its northern border where there are buried grabens, generally trending WNW–ESE, such as the deep Mesoproterozoic Caiabis graben which lies partly beneath the Parecis basin.
However, the epicentral distribution indicates that the 1998 and 2005 sequences are related to a N60°E fault which probably crosses the entire Caiabis graben. The 1955 earthquake, despite the uncertainty in its epicentre, does not seem to be directly related to any buried graben either. The seismicity in the Porto dos Gaúchos seismic zone, therefore, is not directly related to rifted crust. The probable direction of the maximum horizontal stress near Porto dos Gaúchos is roughly E–W, consistent with other focal mechanisms further south in the Pantanal basin and Paraguay, but seems to be different from the NW–SE direction observed further north in the Amazon basin. The recurrent seismicity observed in Porto dos Gaúchos, and the large 1955 earthquake nearby, make this area of the Parecis basin one of the most important seismic zones of Brazil.

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