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.