Subduction-zone structure and magmatic processes beneath Costa Rica constrained by local earthquake tomography and petrological modelling

Abstract

SUMMARY 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 is well 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 low velocities 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.

Detection of Systematic Errors in Travel-Time Data Using a Minimum 1D Model: Application to Costa Rica Seismic Tomography

“…In subduction zones, such as Costa Rica, a delay of 1 sec is not unrealistically large and, hence, may not be easily detected in the data. A systematic error of 1 sec at one station will certainly introduce a large local velocity variation, but this anomaly will not appear as an obvious artifact in comparison with the velocity variations normally expected in complex regions such as subduction zones, which can be easily in the order of 10% (see, e.g., Husen et al, 2003). In summary, systematic errors such as the ones documented in this article will likely go undetected in the data and the results.…”

“…To have an idea of the effect of station mislocations on earthquake hypocenter locations, we used two events recorded by station PAL as an example. We computed synthetic arrival times for all stations that recorded those two events using the 3D velocity model calculated by Husen et al (2003). To be more realistic, we introduced normal distributed Gaussian noise with a variance equal to the quality class Period 1a.…”

“…To estimate the precision of the relocated hypocenter location, we calculated the hypocenter error ellipsoid for every kilometer of station mislocation. The error ellipsoids are computed from the nonlinear probabilistic solution of the earthquake location problem by the shareware NonLinLoc (Lomax et al, 2000;Husen et al, 2003). This is an important point since NonLinLoc computes the full set of uncertainties, which are more reliable than uncertainties computed by traditional, linearized methods.…”

Inversión de efectos de sitio y factor Q utilizando cocientes espectrales

“…RALT posee menor amplificación que RMOI, pero mientras que RALT no posee picos claramente identificables, RMOI posee un valor de amplificación bastante notorio cercano a los 1,2 Hz. Este valor pico sería importante para eventos fuertes y lejanos, que son frecuentes en la costa pacífica de Costa Rica (DeShon et al, 2003;Husen et al, 2003; Protti et al, 1995), y que pueden excitar ese punto de vibración del suelo y tener repercusiones en estructuras elevadas o que contienen líquidos como lo son los tanques de almacenamiento de combustible. SISD está ubicada al pie del valle de San Isidro de El General.…”

Estado de esfuerzos estáticos en la interface de placas Coco-Caribe en Costa Rica: Interpretación mecánica para el origen de asperezas en la zona sismogénica

“…For central Costa Rica, where the seamount segment of the Cocos plate subducts, local earthquake seismic tomography data by Husen et al (2003), Arroyo et al (2009) and Dinc et al (2010) show a shallower subduction angle. The deep structure of the subduction zone for the Cocos Ridge segment is less constrained with receiver function data by Dzierma et al (2011) and earthquake hypocenter data by Arroyo (2001) and Arroyo et al (2003), showing the presence of a steeply subducting slab down to a depth of 70 km.…”