Possible implications of recent microearthquakes in southern New York State

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 (sub 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 the 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.