The rupture process of a large back arc thrusting earthquake, the April 22, 1991, Valle de la Estrella, Costa Rica (Mw=7.7), earthquake, is investigated using broadband body waves and long-period surface waves. We find the source process to be relatively simple, with the source models separately obtained from body and surface waves being very consistent. The event occurred on a shallow, southwest dipping rupture plane on which most energy is released updip of the hypocentral location (10--20 km deep). High-frequency radiation appears to have been released over a relatively small source area. Our preferred model has a focal mechanism with strike 102¿10¿, dip 17¿14¿ and rake 63¿17¿, a seismic moment of 3.8¿1.5¿1020 N m, and a total rupture duration of 40¿6 s. The earthquake appears to be associated with the North Panama Deformed Belt (NPDB), a thrust and fold complex that has accommodated the oroclinal deformation of Panama. This event, along with previous large events north of Panama in 1882 and 1916, indicates that there is substantial convergence along the NPDB, marking the NPDB as a probable emerging plate boundary. It remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity. ¿ American Geophysical Union 1993