The Denver earthquake sequence of March–April 1981 was monitored by a network of four permanent and eight portable seismographs. In addition to the main shock (mb = 4.3) on 2 April, six microaftershocks (M < 2) during the subsequent two-week period were recorded and located. Five of those six events had epicenters within the most active area of the 1967–1968 Rocky Mountain Arsenal (RMA) sequence. A composite focal mechanism solution for the main shock and the six aftershocks showed a combination of reverse and strike-slip faulting (14% inconsistency in the 29 P-wave polarities) that is different from the predominantly normal faulting reported for the 1967–1968 RMA sequence. These different focal mechanisms, plus variable water-level response at the RMA well during the earthquake sequence in the 1960’s, may suggest the presence of a multiple fracture system in the source volume.

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