Top: Example of data collected by the UTIG OBS. The data shown were collected off the Nicoya Peninsula of Costa Rica.

Bottom: Simplified structure of the Costa Rica margin. OBS 20 was located at a depth of ~60 m; the seafloor deepens offshore to ~4 km at the Middle America Trench. In the example shown, energy from an airgun shot at 50 km distance travels to the base of the subducting slab and then to the OBS. The observed energy at a distance of 50 km in the top figure probably followed a similar path. For shots closer to the OBS, the first arriving energy travels through the accretionary wedge.