The Joint Hypocenter Determination (JHD) method was used to relocate 1604 earthquakes which occurred during 1988-1993 in the Wadati-Benioff zone (WBZ) beneath the Cook Inlet in southcentral Alaska. The study utilizes both P- and S-wave arrivals recorded by a recently improved regional seismic network, and provides an improvement over previous studies. The JHD locations of the events allowed improved delineation of the geometry of the subducted slab and the detection of a double seismic zone within the WBZ in the 40-100 km depth range. The JHD station corrections obtained correlate with the major surface geological features in the study area (slow velocity sedimentary basins) as well as with crustal and mantle structure (high velocity subducting slab and low velocity anomalies beneath the active volcanoes). To investigate the dependence of the JHD relocation procedure on factors such as the hypocentral depths of the events used, the velocity model, the vp/vs ratio, and random and systematic errors in travel time readings, synthetic travel times were calculated using an exact ray tracing technique, a three-dimensional tomographic P-wave velocity model, the present network configuration, and the actual seismicity distribution. Then, the synthetic events were located with the JHD algorithm. The synthetic experiments showed that the JHD procedure is rather insensitive to the choice of plane-layer velocity model and vp/vs ratio, preserves relative hypocenter locations well, and can handle systematic and large random reading errors effectively.