During the Carbondale, Colorado, earthquake swarm (Mi <3.2) of April -May 1984, a total of 34 earthquakes were located using regional data or local data from a temporary network installed in the epicentral area. These earthquakes were relocated with the method of Joint Hypocenter Determination (JHD) in order to better delineate the source zone of the swarm. The majority of the earthquakes occurred about 7 km south-southwest of Carbondale, at the northern terminus of the Elk Mountain anticline. The most precisely determined hypocenters of the swarm have focal depths in the 2 to 7 km range. Two composite focal mechanisms were constructed from local first-motion data. One of these mechanisms corresponds to normal faulting and the second suggests reverse faulting.

No abstract

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.

No abstract