S U M M A R YThe structure and seismicity of the subduction zone of central Costa Rica have been investigated with local earthquake tomography down to ca. 50 km depth. Seismic traveltime data sets of three on-and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations, 3-D structure of P-wave velocity and V p /V s ratio using about 2000 highquality events. The seismicity and slab geometry as well as V p and V p /V s show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest, a seamount province in the centre and the subducting Cocos Ridge in the southeast of the investigation area. Three prominent features can be identified in the V p and V p /V s tomograms: a high-velocity zone with a perturbation of 4-10 per cent representing the subducting slab, a low-velocity zone (10-20 per cent) in the forearc crust probably caused by deformation, fluid release and hydration and a low-velocity zone below the volcanic arc related to upwelling fluids and magma. Unlike previously suggested, the dip of the subducting slab does not decrease to the south. Instead, an average steepening of the plate interface from 30 • to 45 • is observed from north to south and a transition from a plane to a step-shaped plate interface. This is connected with a change in the deformation style of the overriding plate where roughly planar, partly conjugated, clusters of seismicity of regionally varying dip are observed. It can be shown that the central Costa Rica Deformation Belt represents a deep crustal transition zone extending from the surface down to 40 km depth. This transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well.

[1] The P wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of Central America covering offshore and onshore parts of southern Nicaragua and northern and central Costa Rica. Our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half-year intervals between 2002 and 2006. The P wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic P wave velocity distribution. In the upper crust beneath Costa Rica, the seismic anisotropy is of the order of 2%-5% and locally strongly variable in orientation corresponding to the complicated tectonic structure. In contrast, the upper mantle beneath both study regions shows stronger anisotropy of the order of 5%-10% and a more coherent orientation of the axis of fast P wave velocity that varies systematically on regional scales: The upper mantle of the incoming oceanic plate is characterized by a trench-normal orientation of the fast P wave velocity axis presumably corresponding to mineral alignment (lattice preferred orientation LPO) in transport direction. This pattern is corroded in the uppermost part of the subduction zone, possibly by the influence of bending-related trench-parallel faults and serpentinization that can overprint or annihilate the LPO anisotropy. The upper mantle of the overriding plate and mantle wedge shows a clear trench-parallel orientation of the fast P wave velocity axis. The anisotropy is stronger in Nicaragua than in southern central Costa Rica (factor of 2 orders of magnitude). The development of a stronger anisotropy in the Nicaragua could be driven by a change in the stress regime from compressional in southern Costa Rica to transpressional in Nicaragua corresponding to a change from near-orthogonal subduction in the SE to oblique subduction and slab retreat in the NW. From a comparison with S/SKS wave, GPS, and geochemical observations follows that the most likely explanation for the observed pattern of P wave anisotropy in the mantle wedge is LPO caused by a trench-parallel shear deformation and/or NW oriented escape flow originating in the compressional zone near the Cocos Ridge collision area.

S U M M A R YThe continental margin of Nicaragua and Costa Rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab, a decreasing magma production and a shift in the volcanic front. To investigate this transition, a joint on-and offshore local earthquake tomography was performed. Low P-wave velocities and high V p /V s ratios, indicative for hydration, were found in the upper-mantle and lowermost crust beneath the Sandino Basin. The mantle wedge hydration can be estimated to 2.5 wt. per cent beneath south Nicaragua. In contrast, the mantle wedge beneath north Costa Rica is weakly or not hydrated. The hydration leads to a local gap in the seismicity in Nicaragua. The lateral transition between the hydrated and non-hydrated areas occurs within a distance of about 10 km. This transition coincides with a change in the crustal thickness in the order of 5-10 km, thickening to the south, and in the tectonic regimes. The change in the tectonic regimes towards a stronger extension along the margin of Nicaragua could be the key for understanding the observations: the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of low V p , high V p /V s and low seismicity.

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