Abstract
SUMMARY 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°, 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° trending fault surface, dipping 50 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 (4min 20s 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 a, axis oriented N344. 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.
Highlights
The Armenian earthquake of 1988 December 7 (40.987"N, 44. 18S0E, 5 km depth, origin time: 07:41:24.2 UTM, M, = 6.9 after National Earthquake Information Center (NEIC)) produced widespread destruction in the region around the cities of Spitak, Leninakan and Kirovakan
The joint occurrence of impressive surface faulting and intense aftershock activity, constituted a natural laboratory to study reverse faulting in a tectonic environment different from that of the San Fernando (1971), the El Asnam (1980), the Coalinga (1983) or the Whittier Narrows (1987) earthquakes
In addition to this we observe that the azimuth range for reverse mechanisms varies from 60" to 110°, this is about 40" less than the values obtained from the hypocentres for the general structural directions even though most dips range between 40" and 60°, namely about the same value as that defined by the cross-sections
Summary
18S0E, 5 km depth, origin time: 07:41:24.2 UTM, M, = 6.9 after NEIC) produced widespread destruction in the region around the cities of Spitak, Leninakan and Kirovakan. 12 days after the earthquake, a French-Soviet seismic network was installed in the-epicentral area, and preliminary results from this expedition have been published elsewhere (Cisternas et al 1989; Dufumier 1989; Jimenez, Cara & Rouland 1989). Another field team from the USA obtained similar results (Pacheco et al 1989). The network, during the period under consideration, consisted of three different types of recorders (Fig. 1 and Table 1)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
