The June 10 southeastern Illinois earthquake was the 11th largest earthquake felt in the central U.S. during this century. Source parameters of the main shock were estimated from an analysis of surface-wave amplitude spectra. The source that best fit the observed data has focal depth of 10 ± 1 km; mechanism with strike= 40.6°± 5.9°, dip= 76.2° ± 5.6°, slip= 159.7° ± 6.0°; tension and pressure axes of (T) trend= 357°, plunge= 24°, (P) trend= 89°, plunge= 4°; and a seismic moment of 3.1 * 1023 dyne-cm.

With the combined efforts of six institutions, a 24-station analog microearthquake network was deployed around the main shock epicenter. One hundred eighty-five aftershocks were recorded in the first week of monitoring, providing 144 hypocenter determinations. A subset of 51 well recorded events was used for joint relocation and calculation of station corrections for the stations within 100 km of the main shock epicenter. Joint hypocenter locations differ only slightly from the original locations. The spatial distribution of well located aftershocks indicates that the rupture was confined to a pencil-like zone within the Precambrian basement, extending from 7 to 11 km depth.

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