Local earthquake tomography at the Central Pacific margin of Costa Rica
The Central Pacific area of Costa Rica is a convergent erosional margin, characterized by a high seismicity rate, coincident with the subduction of rough-relief ocean floor. It has generated earthquakes with magnitude up to seven in the past along the subduction megathrust and the inner-forearc faulting. This study uses the records from a temporal seismological network, consisting of on- and offshore stations, installed for a period of six months in this segment of the Middle American margin. The aim was to obtain a model of the 3-D velocity structure of the margin and to characterize the seismicity of the shallow part (< 70 km depth) of the subduction zone, with the focus on the seismogenic zone. A subset of well locatable events was selected to calculate a minimum 1-D model for the P-wave velocity. This model served as initial reference model for the subsequent 3-D inversion in a Local Earthquake Tomography, performed by inverting P- and S-wave traveltimes from 595 selected earthquakes. Several tools for resolution assessment were applied. Additionally, first-motion, double-couple focal mechanisms were determined for earthquakes originated at the plate interface, the subducting slab and the overriding plate. The results reflect the complexity associated to subduction of ocean-floor morphology and the transition from normal to thickened subducting oceanic crust. The subducting slab is imaged as a high-velocity perturbation with a conspicuous band of low velocities (LVB) on top, encompassing the intraslab seismicity deeper than ~30 km. The LVB is locally thickened beneath the margin slope by the presence of at least two subducted seamounts. There is a general eastward widening of the LVB over a relatively short distance, accentuated by a low-velocity anomaly beneath the continental shelf, interpreted as a relative big seamount or plateau. The thickening of the subducting crust closely coincides with the onset of an inverted forearc basin onshore and the appearance of an aseismic low-velocity anomaly beneath the inner forearc. The latter is located in a sector where differential uplift of blocks of the subaerial forearc has been described, suggesting tectonic underplating of eroded material against the base of the upper plate crust. Alternatively, the low velocities could be induced by an accumulation of upward migrating fluids. Other observed velocity perturbations are attributed to various processes taking place at different depths, such as slab hydration through bend faulting at the trench and outer rise, tectonic erosion and slab dehydration. In the west part of the studied area, the interplate seismicity is clearly distinguished as tightly-packed clustering, dipping ~16-19 between 15 and 25 km depth, beneath the continental shelf. The updip limit of the interplate seismicity closely coincides with temperatures of 150C and an increase in the P-wave velocities along the plate boundary, and it closely follows the border of the continental shelf. These observations fit within a recently proposed hydrogeological model for erosional margins, which attributes the onset of stick-slip behaviour in the plate interface to a decrease in the abundance of the fluids released by early dehydration reactions in the subducted sediments; escape of overpressured fluids is propitiated by upward percolation through a fractured upper plate. The thickness and mechanical coherence of the upper plate also influence the start of the seismic behavior. Locally, the presence of subducted seamounts and the associated margin disruption updip of the seismogenic zone could shift the onset of seismicity trenchward. As commonly observed in regions with relatively weak plate coupling, higher seismicity rates concur with the presence of seamounts in the seismogenic zone: in this case, the seamount that originated a Mw-7.0 earthquake in 1990 and the above mentioned seamount or plateau revealed by this study. Furthermore, seismic swarms known as “burst”-type were detected by the experiment in these two subareas as well. They are possibly originated by repetitive rupture of the same small relief patches at the seamounts. If interpretation of the anomaly as a big seamount or plateau is correct, it could act as an asperity in the seismogenic zone, able to generate large earthquakes. The downdip limit of the interplate seismicity seems to be thermally controlled, but the available data is not conclusive. In the east part of the studied area, the interplate seismicity becomes scarce and distributes in clusters. Intraslab earthquakes were detected from the outer rise down to depths greater than 100 km. The highest rates coincide with subduction of thickened crust, where they distribute all over the width of the LVB. If slab dehydration enables seismicity, as widely accepted, and hydration of thickened oceanic crust at the outer rise-trench area is much reduced due to impaired bending ability, then hydration at alternative tectonic scenarios seems to play an important role for seismogenesis. The upper plate seismicity is limited trenchward by the margin wedge. Burst-type seismic swarms were also detected in the overriding plate. The focal mechanisms are in good agreement with previous models of upper plate internal deformation. The low-velocity anomaly at the base of the crust in the inner forearc, seems to promote seismicity in its surroundings by high-friction with the overriding plate, and could be linked to a Mw-6.4 earthquake that caused extensive civil damage along the coast in 2004.