The Focal Mechanisms of a Small Intra Caribbean Plate Earthquake as Determined From Rayleigh Waves

Abstract

The focal mechanism and depth of a small yet tectonically important, intra-Caribbean plate earthquake have been determined using observed Rayleigh wave signals. This earthquake occurred on August 14, 1972 and is believed to be the first known hypocenter in this area of the Caribbean Sea; about 330 kilometers from any previous event. Thus, the source mechanism of this earthquake should yield important information about the stresses in the interior of the Caribbean plate. Only about 5 reliable P-wave first motions were recorded at WWSSN stations, due to the low magnitude (M b = 4.7) of this earthquake. Therefore, the focal mechanism of this event can not be determined from the body wave data alone. Nonetheless, coherent surface wave trains were recorded as far away as 7,000 kilometers. The deduced mechanism is obtained from a comparison of the observed Rayleigh wave phase from this earthquake with that of a master event (September 13, 1971: M b = 5.7) located along the northern boundary of the Caribbean plate. The mechanism which yields the best fit to both the surface wave data and the body wave data is of the strike–slip type, with the compression axis trending approximately NW–SE.