COMPLEXITY OF INTRACONTINENTAL SEISMIC FAULTINGS - THE GAZLI, UZBEKISTAN, SEQUENCE

Reconnaissance of seismic activity in the area of the three M(S) simil ar to 7 Gazli earthquakes (8 April and 17 May 1976 and 19 March 1984) was carried out during May through June 1991 as part of a collaborativ e program between the University of Grenoble and the Academy of Scienc es of the Uzbek Republic. More than 300 events (-0.3 < ML < 4.0) recor ded by at least six stations were located using an array of 16 seismic stations. We study this 70 X 50 km area using (1) seismic activity re corded for the 15 yr following the 1976, M(S) similar to 7 sequence, a nd 7 yr after the last mainshock; (2) the rupture planes of the three mainshocks deduced from geodetic data; (3) fault patterns deduced from satellite images; and (4) location by the Uzbek network of the main a ftershock activity and the available focal mechanisms. The seismicity recorded in 1991 is located mainly at shallow depth, but both 1991 hyp ocenter locations and geodetic fault planes appear to be bounded by a critical 20- to 25-km depth, i.e., the thickness we propose for the lo cal brittle crust. Most of the earthquakes located by the 1991 dense s eismic survey were located southward of the 1984 geodetically determin ed rupture plane, and this seismic activity does not exhibit any link with the 1976 mainshocks. Since the onset of the sequence, on April 19 76, the seismicity has unilaterally migrated 90 km along a N240 degree s E direction. There are no simple mechanisms to explain this overall migration, even taking into account the potential role of the exploita tion of a local gas field. The direction defined by the epicenters of the largest events of the 1976 to 1993 period is correlated with the d irection of one of the main structural populations. We propose that th is seismic fault population represents a structurally immature fault z one that could explain both the occurrence of three major earthquakes and the variety of seismic fault plane orientations within the immatur e fault network. Within such an immature fault network, energy partiti oning for three M = 7 earthquakes appears as a more efficient system t han the energy release by a single M greater than or equal to 7.3 main shock.