The sub-project 'Coupling and mass-transfer between upper and lower plate' (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters is concerned with the seismogenic zones of Costa Rica and Nicaragua. These zones possess fundamental differences wich are poorly understood. Researching the updip limits of these zones will lead to a better understanding of the reasons for these differences. Existing data from seismological networks on land together with detailed velocity information recently obtained will lead to more accurate earthquake localisation enabling us to relate events to crustal structure, metamorphic reactions and mass transfer.