A portable digital network (the PANDA array) of 40 three-component stations with an aperture of about 35 km was deployed for 4 months in the Arkansas swarm area in 1987. Only 12 swarm events occurred during the deployment, in contrast to the intense seismic activity that characterized this region in 1982 to 1984. These events were relocated using a joint hypocentral determination technique (JHD). The JHD method used here allows for the simultaneous determination of P- and S-wave station corrections while providing information on the uniqueness of the solution based on the singular values of a matrix related to the station corrections. P-wave station corrections, determined when all nonzero singular values were used in the computations (or with the two smallest nonzero singular values deleted), show a circular pattern of positive values surrounded by negative values. The epicentral area is localized slightly displaced from the center of the pattern. Since positive and negative corrections correspond to velocities that are lower and higher, respectively, than the average, our results indicate that the swarm area is characterized by seismic velocities lower than those of its surroundings. Independent information on this region is afforded by reflection seismic lines recorded in the swarm area and its vicinity, which show that the hypocenters are located in a region where strong reflectors completely lose their coherence, indicating that this volume is anomalous when compared to surrounding crust. Additional support for a low-velocity zone comes from the results of a 3-D velocity inversion of the same PANDA data. A selected subset of data recorded digitally by the USGS in 1982 was also relocated. Comparison with the results from the PANDA data shows that the seismic activity did not migrate over a 5-yr period and that it is concentrated within a small volume between about 3 km and 6 km depth. While the results of this study do not determine the ultimate cause of the Arkansas swarm, the discovery of a pronounced localized low velocity zone is consistent with a previously proposed magmatic intrusion or a zone of highly fractured, fluid-filled crust.