Note on an aftershock study, south central Illinois earthquake of November 9, 1968

In an effort to monitor aftershock activity following the southern Illinois earthquake of November 9, 1968, a network of ten high gain seismic stations was established in the vicinity of the epicenter of the earthquake. The network began recording on the fourth day after the earthquake and operated for a period of ten days. During this time only one very small aftershock was recorded. The hypocenter of this shock was about 10 km south and at the same depth as that of the main shock. Permanent stations of the region recorded two other aftershocks within two days of the main shock, and one more three and a half months later. The exceptionally low level of aftershock activity is an anomalous feature of the Southern Illinois-Southeast Missouri seismic zone.