Seismic image and the interpretative velocity structure of the central Costa Rica subduction zone. (a) Prestack depth-migrated SO81-Line 4 (Ranero & von Huene 2000). Basement extensional fractures are indicated by normal faults. The OBH(S) stations are marked on the seafloor. The location of Mound 11 (mud volcano) is projected onto the Line 4 (see Fig. 4 for a detailed location). Plate-boundary temperatures are from Ranero et al. (2008). (b) Superimposed image of MCS Line 4 and the seismic velocity field of Profile 15. (c) The interpretative model of the basal erosion structure. Fluids are generated by mineral dehydration and transported upward to the mound site (Hensen et al. 2004; Ranero et al. 2008). Red dots indicate local seismicity (Dinc et al. 2007).