Source parameters for the September 7, 1988 northeastern Kentucky earthquake have been estimated from the analysis of surface-wave amplitude spectra. The source that best fits the observed data had a seismic moment of 2.0 × 1022 dyne-cm, a mechanism with strike = 198° ± 10°, dip = 51° ± 11°, and slip = −178° ± 17°, (T) trend = 160°, plunge = 25°, (P) trend = 55°, plunge = 28°, and source depth of 4 to 7 km. Thirty-two aftershocks were recorded during 2 weeks of monitoring following the mainshock; 23 of the aftershocks were locatable and fall on a roughly NW-SE linear trend. This trend is subparallel with the NW-SE nodal plane of the mainshock. Our analysis shows the 1988 event to be different from the July 27, 1980 mb,Lg = 5.3 earthquake located 11 km to the northwest. First, the 1988 event is considerably shallower (4 to 7 km) than the 1980 event (14 to 22 km). Second, data from the 1988 event suggest the motion is on a conjugate fault and is in contrast with the 1980 event, which had right-lateral strike-slip on a southeast-dipping plane.

The series of earthquakes that occurred in Meade County, Kentucky, in January and March of 1990, are the first events in an otherwise aseismic area. First motions recorded for the three largest events in January suggest a predominantly strike-slip mechanism with a pressure axis of trend = 275° and plunge = 14° , and a tension axis of trend = 178° and plunge = 31°. A comparison between the mechanisms for the Meade county events and those shown in Taylor et al. (1989) for the southeastern Illinois area, indicates a continuity in the regional deviatoric stress from southeastern Illinois into north-central Kentucky. This finding conflicts with that previously suggested by Ault et al. (1985) who proposed that the two areas were in regions of differing stress regimes.

Four microearthquakes occurred within a dense network of seismic stations in southeastern New York on September 13, 1983. The events occurred during a 75-minute period, had duration magnitudes (m c) ranging from less than 0 to about 2 and were located at depths of about 9 to 10 km. A composite fault plane solution for the sequence indicates thrust or oblique thrust faulting on high-angle faults with northwest to north-northwest strikes. The direction of maximum compressive stress inferred from the P-axis is northeast to east-northeast. The composite solution for these events is similar to composite solutions of Pomeroy et al. (1976) and Seborowski et al. (1982) for microearthquakes located about 40 and 7 km, respectively, from the site of this sequence. The solution is at least locally inconsistent, however, with the proposal of Zoback and Zoback (1980) and Yang and Aggarwal (1981), that a distinct stress domain exists along the Atlantic coast with a west-northwest direction of maximum compressive stress. A re-examination of original data for one of the earthquakes on which Yang and Aggarwal (1981) base their interpretation, shows that a fault plane solution similar to the one for the recent sequence also fits the first motion data for that event. A hypothesis is proposed in which microseismicity in southeastern New York is the result of slip on northwesterly to north-northwesterly striking faults in response to a tectonic stress field characterized by a northeast to east-northeast direction of maximum compressive stress. Such a stress field is similar to that observed in northern and western New York.

Free-field particle velocities for the June 10, 1987 southeastern Illinois earthquake, demonstrate the importante of site and source effects in the prediction of ground motion values for central United States earthquakes. The 54 particle velocity recordings of the 5.0 mbLg magnitude event, ranging in epicentral distances of 56 to 443 kilometers and azimuths of 19° to 282° about the epicenter, exhibit appreciable variability that can be attributed to both site and source effects.

The January 31, 1986 earthquake in northeastern Ohio triggered eight biast monitors in the coal fields of southeastern Ohio and one in western Pennsylvania at epicentral distances of 128 to 297 kilometers from the earthquake. Recordings of the free-field ground motions exhibit predominant frequencies of 1 to 10 Hz, and are consistent with the ground motion model proposed by Nuttli and Herrmann (1984) for the propagation of Lg waves.