Seismic waveforms from the Portable Array for Numerical Data Acquisition (PANDA) network were used to relocate 45 events, occurring in four spatial clusters in the northern New Madrid Seismic Zone (NMSZ). Source time functions of the larger (ML≥2.5) events were obtained and yield estimates of rupture radii between 130 and 175 m and static stress drops at, or below, the megapascal level. There is a tendency for closely located events, within ~200 m from one another, to occur within short time intervals of minutes to days. Available focal mechanism solutions reveal that some of the clustered events have diverse slip directions, indicating that the ruptures occur on adjacent fault patches, rather than being repetitive over the same patch. On the basis of this and a previous study of events farther south, we infer that the properties of the faulting system in the NMSZ are laterally heterogeneous and are subject to effects of some temporally and spatially localized processes that drive the faults toward seismic failure. Comparison of this observation with that in the San Andreas Fault near Parkfield reveals that event clustering occurs in both fault systems on similarly short spatial and temporal scales. The inferred localized processes may occur in both fault systems, despite their different tectonic environment, far-field loading rate, and geometric style of deformation. On the other hand, in the NMSZ we have not observed any quasiperiodic and repetitive ruptures that have been observed near Parkfield. This is consistent with a slower or absent aseismic creeping in the NMSZ. ¿ 2001 American Geophysical Union