Abstract

The 2001 Enola earthquake sequence started with an M 4.4 mainshock and was followed by 2,500 earthquakes in two months, confined in a crustal volume of 8 km3, at depths between 4 and 6 km. We have computed precise absolute and relative hypocentral locations using relative travel times based on waveform cross-correlation and a double-difference relative hypocentral location method. Focal mechanisms were computed using P-, SV-, and SH-wave polarities. Highly similar strike-slip focal mechanisms are consistent with the stress regime of the mid-continent province. The swarm volume produced two clusters of earthquakes at different depths and times. We speculate that these clusters represent a system of conduits possibly filled by fluids. The focal mechanisms of the first, seismically deeper sequence show a strong thrust component, while the later and shallower cluster solutions show predominant strike-slip motion. Daily seismicity rates reveal short and sudden increases in activity rather than typical aftershock decay. A b-value of 0.9 is observed for the entire swarm. The sequence appears to recapitulate aspects of a more intense earthquake swarm in the same area in 1981.

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