Microseismic Swarm Activity in the New Madrid Seismic Zone

We analyze event archives and continuous waveform data recorded by the Cooperative New Madrid Seismic Network from 1995 to 2008 in conjunction with waveform cross-correlation techniques to investigate the spatiotemporal distribution of small-magnitude (M-D < 2.4) earthquakes in the New Madrid Seismic Zone (NMSZ). The resulting clusters are divided into two major groups based on the interevent time period: (1) swarm clusters, in which the number of highly similar events recorded in a day is more than the seismic zone maximum daily rate (similar to 3 events/day) and (2) repeating earthquakes clusters, which consist of highly similar events separated by longer time periods. Most swarm clusters occur near Ridgely, Tennessee, and this 4-km x 2-km x 2-km elongated source zone produces swarms every 1-3 years that contain large numbers of strikingly similar events. Other swarms and repeating earthquake clusters occur at proposed fault intersections in the crystalline basement or along strong velocity contrasts. Focal mechanism solutions for NMSZ clusters are consistent with previously reported solutions for each major fault. We suggest that anomalously high pore-fluid pressure, inferred from artesian wells, porous intrusions, and faulted, fractured crustal rocks, is the most likely cause of swarm activity. Repeating earthquake ruptures are interpreted as reactivation of small asperities.