Abstract

Reconnaissance of scismic activity in the area of the three MS ∼ 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 collaborative program between the University of Grenoble and the Academy of Sciences of the Uzbek Republic. More than 300 events (−0.3 < ML < 4.0) recorded by at least six stations were located using an array of 16 seismic stations. We study this 70 × 50 km area using (1) seismic activity recorded for the 15 yr following the 1976, MS ∼ 7 sequence, and 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 aftershock activity and the available focal mechanisms. The seismicity recorded in 1991 is located mainly at shallow depth, but both 1991 hypocenter 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 local brittle crust. Most of the earthquakes located by the 1991 dense seismic survey were located southward of the 1984 geodetically determined rupture plane, and this seismic activity does not exhibit any link with the 1976 mainshocks. Since the onset of the sequence, on April 1976, the seismicity has unilaterally migrated 90 km along a N240°E direction. There are no simple mechanisms to explain this overall migration, even taking into account the potential role of the exploitation 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 direction of one of the main structural populations. We propose that this seismic fault population represents a structurally immature fault zone that could explain both the occurrence of three major earthquakes and the variety of seismic fault plane orientations within the immature fault network. Within such an immature fault network, energy partitioning for three M = 7 earthquakes appears as a more efficient system than the energy release by a single M ≧ 7.3 mainshock.

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