Margin architecture and seismic attenuation in the central Costa Rican forearc

Seismic attenuation across the central Costa Rican margin wedge is determined from amplitude analysis of wide-angle seismic data. Travel time and amplitude modeling are applied to ocean bottom hydrophones along two trench-parallel profiles, located 30 km (P21) and 35 km (P18) landward of the deformation front northeast of Quepos Plateau. Tomographic inversion images a progressively thinning margin wedge from the coast to the lower slope at the trench. A 1-1.5 km thick décollement zone with seismic velocities of 3.5-4.5 km/s is sandwiched between the margin wedge and the subducting Cocos plate. For strike line P21, amplitude modeling indicates a Qp value of 50-150 for the upper margin wedge with seismic velocities ranging from 3.9 km/s to 4.9 km/s. Along strike line P18, Qp values of 50-150 are determined with velocities of 4.3-5.0 km/s in the upper margin wedge, increasing to 5.1-5.4 km/s in the lower margin wedge. Quantitative amplitude decay curves support the observed upper plate Qp values. In conjunction with earlier results from offshore Nicoya Peninsula, our study documents landward decreasing attenuation across the margin wedge, consistent with a change in lithology from the sediment-dominated frontal prism to the igneous composition of the forearc middle prism.