Geometry, segmentation and stress regime of the Spitak (Armenia) earthquake from the analysis of the aftershock sequence

Abstract

S U M M A R YThe 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 o1 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.

Source complexity of the 1988 Armenian Earthquake: Evidence for a slow after‐slip event

“…The contrast of the moment release rates between subevents 1 and 3 is clearly seen in the moment rate function shown in Figure 9a. As shown in Figure 9c, our subevent 3 probably occurred in the area characterized by active folds [Dorbath et al, 1992] it may have occurred on one of the blind faults mentioned by Dorbath et al [1992]. The mechanism, the delay from the main sequence, and the slow source process suggest that this event is a slow after-slip event which released, after the main rupture was completed, the north-south tectonic stress in a complex, possibly weak, structure beneath the geologically young fold.…”

“…(c) Simplified map showing major tectonic features and early aftershocks shown by circles. The solid circles indicate the mainshock and the largest aftershock [Dorbath et al, 1992).…”

“…Dorbath et al [1992] identified five segments of aftershock activity and found that the main surface ruptures correlate well with the aftershocks on an N120°E trend, dipping 50° to the NE, and with thrust and rightlateral components. Field observations support that both right-lateral and thrust faulting occurred during the Spitak earthquake.…”

The rupture process of the Armenian earthquake from broad-band teleseismic body wave records

“…Results obtained from aftershocks recorded by a local temporary seismic network (Dorbath et al 1992), and observations of surface rupture and coseismic deformation (Philip et al 1992), indicate a complex source formed by at least five segments. These results allow to constrain an otherwise loosely determined inverse problem.…”

Accuracy of teleseismic event locations in the Middle East and North Africa

“…An aftershock study of the December 1988 Spitak, Arrneni~MS6.9 earthquake was reported by Dorbath et al (1992). As was the case with the El Asnam earthquake, this earthquake sequence consisted of shallow thrust fault events with depths less than 10 km.…”