GEOMETRY, SEGMENTATION AND STRESS REGIME OF THE SPITAK (ARMENIA) EARTHQUAKE FROM THE ANALYSIS OF THE AFTERSHOCK SEQUENCE

The aftershock sequence of the Spitak earthquake, as recorded by a dense portable network deployed around the source region, is analysed in order to obtain a precise description of the mechanics of the rupture. A collection of 708 well-recorded events corresponding to a period of about two weeks is studied, their hypocentres are accurately located, and their focal mechanisms are calculated individually as well as by a joint procedure that permits us to estimate the stress regime. The epicentral distribution of these aftershocks indicates that the fault at depth extends well beyond the surface rupture towards the west and northwest. Neotectonic and seismic observations permit us to identify five segments broken during the earthquake. The southeastern one, oriented N140-degrees, corresponds to the surface ruptures along the Alavar right lateral shear fault. The main surface ruptures, between Spitak and Gekhasar, correlate well with aftershocks showing a N120-degrees trending fault surface, dipping 50-degrees to the NE and acting as a thrust with a right lateral component. Two similar segments towards the west, are offset and hidden under active folds. A fifth segment, towards the NW, corresponds to a right lateral blind shear fault buried in depth, in the vicinity of the large Pambak-Sevan fault. 14 vertical sections across the fault, including focal mechanisms, illustrate these features. A relocation of the main shock and aftershocks for the period before the installation of the portable network, confirms the spatial extent of the seismicity. The foreshock, the main shock and the strongest aftershock (4 min 20 s after the main shock) have approximately the same epicentre. Thus the rupture started at the crossing of the Pambak-Sevan and Alavar faults and propagated bilateraly from there, although the seismic moment associated to the Alavar branch is only about one sixth of the total moment. The stress regime is one of triaxial compression with a sigma-1 axis oriented N344-degrees. The same orientation is given by the microtectonic observations made on the central segment of the surface ruptures, a result that agrees with previous estimates for the Georgian Caucasus.