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.