SOURCE COMPLEXITY OF THE 1988 ARMENIAN EARTHQUAKE - EVIDENCE FOR A SLOW AFTER-SLIP EVENT

We analyzed teleseismic P and S waves using a multiple-event deconvolution method to investigate the source process of the Spitak, Armenia, earthquake of December 7, 1988. Teleseismic long-period body waves exhibit complex waveforms, significantly more complex than those normally seen for an event of this size. We identified two groups of subevents. One is a group of strike-slip events during the first 20 s. The other is a dip-slip event initiating at about 30 s after the initial rupture. The tensor sum of all the subevents is essentially strike-slip (strike=302-degrees, dip=59-degrees, rake=143-degrees) with a scalar moment of 1.47 x 10(19) N m. The seismic moment of the dip-slip event (strike=89-degrees, dip=60-degrees, rake=88-degrees) is 5.6 x 10(18) N m, nearly 40% of the total, and the moment release rate is slower than the other subevents with a strike-slip mechanism. We call this event a slow after-slip event. The mechanism of the after-slip event is consistent with the present-day deformation pattern in the Caucasus where north-south compression is predominant due to the continental collision between the Arabian plate and the Russian Platform. This result suggests that in a region where both strike slip motion and crustal shortening are taking place, like southern California, a large strike-slip event may accompany large thrust events and vice versa. In order to determine the total coseismic deformation pattern, it is essential to resolve these multiple events in time, space, and mechanism.