Seismic data from explosions and local earthquakes are used to determine shallow crustal structure near Managua, Nicaragua, and to derive a complete crustal model for northern Costa Rica. A new method, referred to as the minimum apparent velocity (MAV) method, has been developed and applied in the analysis of local earthquake data. The results indicate that the MAV method of multilayer analysis, when combined with well-determined station corrections, can be quite useful in regions of high seismicity. Based upon the analysis presented here, the total thickness of the crust beneath the central volcanic province of northern Costa Rica is about 43 km. Four crustal layers are identified with compressional-wave velocities of 2.6, 5.1, 6.2, and 6.6 km/sec. The upper mantle velocity is 7.9 km/sec. The major features of this model closely resemble those of other seismically active, marginal zones of the Pacific.