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We operated an eleven-station network of digital instruments in the Wabash Valley region from November 1995 through June 1996 in order to investigate seismic activity in the Wabash Valley seismic zone. One station of the network was a ten-element, three-component, high-frequency, phased array The array was primarily responsible for lowering the detection threshold by approximately 1.5 magnitude units below that achieved previously, to magnitudes of 1.2 to 1.5. We observe a significant excess of events in the region from that expected by extrapolation of the historical and earlier instrumental catalogs. We show that the excess is related to a cluster of earthquakes near New Harmony, Indiana. We argue that their shallow depth, similarity of waveform characteristics, and proximity to producing oil wells and underground coal mines suggest that these small-magnitude events may be artificially induced. We find that discarding the events from this cluster leads to seismicity rates more consistent with previous data. |
