We propose two geodetic and seismic transects across the Middle America Trench and Costa Rica, immediately above the seismogenic interface between subducting Cocos and overriding Caribbean plates. We will use GPS, leveling, and three-component digitally-recording seismometers on land, and Ocean-Bottom Seismometers (OBS) offshore. Our goal is to map the three-dimensional distribution and nature of the seismogenic zone (the locked of partly locked plate interface that generates large or great earthquakes), for comparison to candidate processes (e.g., sedimentary diagenesis, metamorphic reactions) that may control the distribution of seismicity and plate coupling. Geodesy outlines the locked or partly locked portion of the plate interface by measurement of strain accumulation, and are particularly sensitive to the percentage of locked slip and the down-dip extent of the seismogenic zone. Seismology outlines the portion of the plate interface that generates microseismicity (not necessarily the same portion outlined by geodetic data), and given the OBS coverage can also outline the up-dip extent. This ability to "image" the seismogenic zone with combined and closely coordinated geodetic and seismological data is enhanced in regions like the Nicoya and Osa Peninsulas because of the close approach of local coastline to the trench axis (60 km for Nicoya, and only 30 km for Osa!). This unique geography allows us to deploy precision instrumentation immediately above and close to essentially all of the seismogenic zone, giving one of the first complete transects above the most critical part of a subduction zone, namely the part that generates large or great earthquakes and tsunamis.