Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; April 2005; v. 95; no. 2; p. 431-445; DOI: 10.1785/0120040052
© 2005 Seismological Society of America
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The 6 June 2003 Bardwell, Kentucky, Earthquake Sequence: Evidence for a Locally Perturbed Stress Field in the Mississippi Embayment

Stephen P. Horton1, Won-Young Kim2 and Mitch Withers1

1 Center for Earthquake Research and Information
University of Memphis
3876 Central Avenue
Memphis, Tennessee 38152
shorton{at}memphis.edu
 (S.H., M.W.)

2 Lamont-Doherty Earth Observatory of Columbia University
61 Route 9W
Palisades, New York 10964
wykim{at}ldeo.columbia.edu
 (W.-Y.K.)

This article describes an unusually well-behaved, unusually well-documented central and eastern United States (CEUS) earthquake sequence. Detailed analysis of regional and local waveform data from the 6 June 2003 Bardwell, Kentucky, earthquake indicates that the mainshock has the seismic moment of M0 1.3 (±0.5) x 1015 N m (Mw 4.0) and occurred at a depth of about 2 (±1) km on a near-vertical fault plane. A temporary seismic network recorded 85 aftershocks that delineate an east-trending fault approximately 1 km in length. The hypocenters illuminate a vertical plane between 2.0 and 2.7 km depth. The centroid of the aftershock distribution is at 36.875° N, 89.010° W and a depth of 2.4 km. The aftershock cluster is interpreted as a circular fault area with a radius of 0.44 (±0.03) km. This source radius yields a static stress drop, {Delta}{sigma} = 67 (±14) bars for the mainshock. The focal mechanism for the mainshock has strike = 90°, dip = 89°, and rake = –165° with a subhorizontal P axis trending 135°. A formal stress inversion based on the focal mechanisms of the mainshock and ten aftershocks indicates the maximum compressive stress trends 104° with a plunge of 5°. The local stress field near Bardwell is therefore rotated about 40° clockwise relative to 65° for eastern North America as a whole. The Bardwell earthquakes have the opposite sense of slip to earthquakes with east-trending nodal planes that occur near New Madrid, Missouri. This requires a significant local rotation of the stress field over a distance of 60 km.




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