The nature and distribution of bottom simulating reflectors at the Costa Rican convergent margin

Summary Bottom simulating reflectors (BSRs) at the base of the hydrate stability zone are often observed in marine seismic reflection data. The compressional (P-)wave velocity structure of a BSR offshore Costa Rica was analysed by applying a full-waveform inversion technique. The resulting velocity profile indicates that the BSR is caused by a thin low-velocity layer, suggesting the presence of at least a small amount of free gas in the sediment pore space. In undisturbed sediment sections, BSRs offshore Costa Rica are observed over much of the continental margin. They frequently occur at shallow depths beneath the seafloor, usually ~ 100–400 m; at some locations BSRs appear to intersect the seafloor. This was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards. In the study area, the vertical movement of the hydrate stability zone relative to a given point in the sediment column, one of the potential factors leading to BSR formation or suppression, is controlled by both sedimentation and vertical tectonism. Both processes may hence play a role in controlling BSR distribution. BSRs are absent in areas which have been affected by slumping, except where the sedimentary section above the BSR remained intact during slumping. This indicates that slope failure can cause the destruction of BSRs.