Occasionally in intraplate areas, isolated swarms of small earthquakes occur with hypocenters only a kilometer or two apart, many kilometers away from any other seismic activity [Crampin, 1991]. The Blue Mountain Lake swarm, N.Y., 1972–1973, and the Enola swarm, Ark., 1982–1983, were two such swarms.
The largest events in isolated swarms, which I shall call “typical events,” are usually smaller than magnitude 4 and typically tend to repeat with similar magnitudes and locations every few days or weeks. These typical events have shown precursory phenomena similar to those seen before much larger earthquakes in regions of higher seismicity. Changes in Vp/Vs ratios were observed at Blue Mountain Lake [Aggarwal, 1973] and at Enola [Chiu et al., 1984], and changes in shear-wave splitting were also observed at Enola [Booth et al., 1990]. Presumably the reason for the sensitivity that allows precursors to be observed before comparatively small events in isolated swarms is that the behavior before small earthquakes can be identified in quiet aseismic areas that would be hidden in the tectonic complexity of more active seismic regions.