Abstract

The largest earthquake to occur in the central Mississippi seismic region this century took place in south central Illinois on November 9, 1968. The hypocenter and origin time based on observations from twelve regional stations varying in epicentral distance from 171 to 549 km, are 37.95°N, 88.48°W, h = 25 km, 0 = 17h01m42.0s ± 0.2s.

Travel times of P at stations distant less than 2600 km indicate regional mantle variations, corresponding to rays bottoming at depths down to 650 km. Beyond this point travel times show a much smaller dependence, if any, on region. For stations in the central United States P times may be fitted by two straight line branches which intersect at about 600 km. The first branch corresponds to Pn, the second to rays refracted from a surface at depth 97 km with a velocity below it of 8.37 km/sec. At larger distances (48°-100°) there are non-azimuth dependent residuals with respect to the Herrin Tables averaging about −1.5 sec, indicating a source-region correction with respect to these tables.

Body wave magnitude was determined to be mb = 5.54 ± 0.44 for stations for which Δ > 25°, and mb = 5.44 ± 0.29, using Evernden's formula, for Pn in eastern North America. Surface waves give a value Ms = 5.2.

The fault plane solution determines two nodal planes each striking approximately north-south and dipping 45° to the east and to the west, respectively. This corresponds to dip slip, reverse motion, and to a horizontal east-west axis of compressional stress. While there are no mapped faults in the immediate epicentral region, the motion indicated is in conformity to that along the Wabash Valley Fault System 10 miles to the east.

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