[A study of seismic storms occurred from October 1989 to June 1990, in the neighborhood of Pico El Aguila and Piñango, Venezuelan Andes].
Between October 1989 and June 1990 there were a large number of microearthquakes in the neighborhood of Pico El Aguila and Piñango, Mérida State, Venezuela. The epicenters form two clearly separated groups. The first one comprises events which occurred between October 1989 and March 1990. Their epicenters were located between 8.75 degrees N and 8.88 degrees N and between 70.75 degrees W and 70.88 degrees W. They are lined up in a northwest-southeast trend. The zone is crossed in its center by the Boconó fault zone, and the Valera fault is the northwest limit of the epicentral distribution. The second group of microearthquakes was recorded during April 1990, and its epicenters were located between 8.85 degrees N and 9.00 degrees N, and between 70.75 degrees W and 70.90 degrees W. They are lined up in a trend parallel to the first group. The Valera fault forms its southwest border, and the Piñango fault its northwest one. Based on the polarities, six composite focal mechanisms were constructed, three for the first group of events and three for the second one. In the first group, two mechanisms, one in each side of the Boconó fault zone, correspond to reverse faulting. A third mechanism, obtained from events close to the Boconó fault zone, correspond to a right-lateral strike-slip displacement. In the second group, two mechanisms correspond to reverse faulting and the third, located in the southwestern end of the lineation, close to the Valera fault, shows a left-lateral strike-slip mechanism. The Angelier graphical method was applied to the six mechanisms, which produced a range of acceptability for the orientation of the maximum stress axes between N 24 degrees W and N 51 degrees W. These characteristics can be explained by a mechanical model, postulated in this work for the study area, which explain the coexistence of reverse, right lateral strike slip and left lateral strike slip faulting under the same stress regime, assuming that this faulting occurs on preexisting faults. Some 25 Km northeast of the study area, the azimuth of the Boconó fault zone is approximately N 49 degrees E. In case that the orientation of the compression axis remains constant, this would mean, from the mechanics point of view, that the Boconó fault zone is a "weak" one, just as other strike-slip faults such as San Andreas Fault and the Big Sumatra Fault.
Palme
C
C
Departamento de Física y Matemáticas, Núcleo Universitario Rafael Rangel. cpalme@ing.ula.ve
Choy
J E
JE
D004190:Disasters / Q000706:statistics & numerical data*; D012189:Retrospective Studies; D014687:Venezuela