Aftershocks of the 13 December 1982 North Yemen earthquake: Conjugate normal faulting in an extensional setting

The North Yemen epicentral locale in the southwestern part of the Arabian Peninsula is 200 to 300 km landward from the active rifting of the Red Sea and Sea of Aden. The magnitude 6.0 (MS and mb) main shock of 13 December 1982 locally caused considerable death, injury, and damage and was followed by an extensive aftershock sequence. A 12-day study employing a 10-station portable seismograph network was conducted between 29 December 1982 and 9 January 1983.Hypocentral locations were determined for 230 shocks selected from the thousands of recorded events (duration magnitudes between 1.8 and 4.6). These aftershocks define a source volume that is roughly 20 × 20 × 10 km. From that volume, about half (∼110) of only the best-constrained hypocenters with depths greater than 3 km were selected for detailed analysis. The 110 aftershock data set was divided into subsets according to geographic position (northern and southern) and temporal sequencing (a distinct aftershock sequence late in the monitoring period). A series of composite focal mechanisms show the aftershocks are dip-slip faulting (normal) on planes with north-northwest to northwest strikes and with dips that are variable in amount (∼30° to ∼80°) and direction (southwest and northeast).The strike and extensional nature of these composite focal mechanism solutions are in good agreement with the main shock focal mechanisms, the surficial and bedrock geology of the epicentral area, and the linear surface cracks observed in the field there following the December main shock. We interpret the spatial distribution of our results to describe conjugate faulting episodes associated with north-northwest striking faults.