THE EARTHQUAKE SEQUENCE OF JUNE 1992 NEAR SABA, WEST-INDIES

During June 1992, a minor earthquake swarm occurred close to the island of Saba, a small composite volcano which is the northernmost member of the inner volcanically active Lesser Antilles are. The initial phase of the seismic activity was recorded at some permanent eastern Caribbean seismic stations, the nearest of which was located 55 km away. On 14 June 1992, a seismograph was installed on Saba to improve the detection and location capacities of the permanent network with regard to this swarm. Sixty events were detected of which only fifteen could be located. The largest earthquake of the sequence occurred on 11 June 1992, had a duration magnitude of M(D) 4.5 and was felt at Modified Mercalli intensities IV-V in Saba and III in St. Kitts. The epicentres exhibit a SW-NE trend and focal depths are mainly clustered in the range 10-16 km. A b-value of 0.73 was obtained for the sequence. Spectral analysis yielded various source parameters: moments in the range 0.38 X 10(13)-23.66 x 10(13) Nm, source radii from 87 to 126 m and stress drops from 1.3 to 79.9 bar. Moment increases with increasing magnitude while stress drop appears to decrease with decreasing seismic moment, similar to many other studies. Although the evidence as to the nature of the origin of this swarm is not conclusive, it seems to suggest a tectonic rather than volcanic origin.