Precursors to a magnitude 4.8 earthquake in the Virgin Islands: Spatial clustering of small earthquakes, anomalous focal mechanisms, and earthquake doublets

abstract Waveforms of small earthquakes that preceded a magnitude 4.8 earthquake in the Virgin Islands were studied using digital recordings from a local seismic network. During the 10 months prior to the main shock, a spatial cluster of five events (1.4 ≦ M ≦ 2.5) occurred within 3 km of the impending shock. Variations in the P -wave first motions at one station, as well as P/SV amplitude ratios, indicate that the focal mechanisms of the events within the cluster changed 2 months before the main shock. The focal mechanism of the later events and of two foreshocks that occurred the day of the main event differed from the typical focal mechanism observed for the area. The main event and most of its aftershocks exhibited the normal set of first motions recorded for events in this region. The cluster events and the foreshocks occurred in pairs, each pair with very similar waveforms. Cross-correlations of these waveforms reveal that the earthquakes in each pair are located within about 110 m of each other. Each pair can be interpreted as an earthquake doublet, that is, two similarly sized events with adjacent rupture zones that are related in time. The occurrence of doublets is unusual for events in this region and may be indicative of high stress in the vicinity of the impending shock. No significant differences were observed between the spectra of the cluster events, the foreshocks, or the aftershocks.