Hypocenters of earthquakes occurring during the period 1950 through 1972 in the Middle America arc-trench system near Managua, Nicaragua, have been relocated by the method of joint hypocenter determination (JHD) or the master event method with the Managua earthquake of December 23, 1972 as the calibration event. The relocated hypocenters show a considerably narrower Benioff zone than previous hypocenters of the same earthquakes and clearly separate shallow-focus volcanic terrane earthquakes of the type which struck Managua from the Benioff zone earthquakes. The shallow-focus volcanic terrane earthquakes are probably caused by tectonic conditions associated with the formation of the Nicaraguan depression or the principal chain of Quaternary volcanoes. The Managua earthquake of December 23, 1972 may have occurred as a consequence of slippage along a transform fault zone which connects offset segments of the chain of Quaternary volcanoes; the volcanic chain is then taken to be the surface expression of a secondary spreading zone which lies above the Benioff zone of the Middle America arc-trench system. Alternatively, the recent Managua earthquake may have occurred as a consequence of regional east-west tension in the Nicaraguan depression.

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