Rupture characteristics of clustered microearthquakes and variations in fault properties in the New Madrid Seismic Zone

Seismic waveforms from the Portable Array for Numerical Data Acquisition (P ANDA) network were used to relocate 45 events, occurring in four spatial cl usters in the northern New Madrid Seismic Zone (NMSZ). Source time function s of the larger (M-L greater than or equal to 2.5) events were obtained and yield estimates of rupture radii between 130 and 175 in and static stress drops at, or below, the megapascal level. There is a tendency for closely l ocated events, within similar to 200 m from one another, to occur within sh ort time intervals of minutes to days. Available focal mechanism solutions reveal that some of the clustered events have diverse slip directions, indi cating 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 syste m in the NMSZ are laterally heterogeneous and are subject to effects of som e temporally and spatially localized processes that drive the faults toward seismic failure. Comparison of this observation with that in the San Andre as Fault near Parkfield reveals that event clustering occurs in both fault systems on similarly short spatial and temporal scales. The inferred locali zed processes may occur in both fault systems, despite their different tect onic environment, far-field loading rate, and geometric style of deformatio n. On the other hand, in the NMSZ we have not observed any quasiperiodic an d repetitive ruptures that have been observed near Parkfield. This is consi stent with a slower or absent aseismic creeping in the NMSZ.