Fault plane solutions of 67 earthquakes along the Middle America arc were determined. These focal mechanisms delineate tectonic activity characteristics of a subduction zone. Focal mechanisms of shallow events beneath the trench axis indicate normal faulting resulting from initial flexure of the downgoing plate. Shallow focus earthquakes along the inclined seismic zone yield focal mechanisms which indicate underthrusting on fault planes dipping beneath the coast. The tensional axes of the focal mechanism solutions of the intermediate focus earthquakes are aligned down the dip of the seismic zone and indicate that the subducted portion of the plate is under extensional stress. The strike-slip solutions of nine events along the arc possess one nodal plane which strikes approximately perpendicular to the trend of the arc. The locations of these nine events correlate very well with transverse features mapped by Stoiber and Carr (1973) from discontinuities and offsets in the volcanic chain and dip changes in t...