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

Abstract

SUMMARY The destructive earthquake of December 1988 in Armenia (M, = 6.9) was recorded on broad-band and very long-period channels at teleseismic distances by Geoscope and GDSN networks. These records are well distributed in azimuth, and allow a detailed study of the rupture process of this earthquake. The average focal mechanism obtained by P- and SH-wave modelling ( C#I = 300 f lo, 6 = 63 f 5, A = 100 f 20) is compatible with the mechanism obtained from very long-period surface waves and intermediate-period single-station determinations, as well as field observations. The mean depth of the rupture is also fixed by waveform modelling between 5 and 7 km which means that the rupture surface extends from the surface to a depth between 10 and 14 km, in agreement with aftershock depth distribution. The rupture is found to be complex, composed of a weak beginning or a small foreshock and two pulses well separated in time. The time delay between the two events is estimated for each station by waveform modelling and by spectral analysis. The azimuthal variation of this time delay is interpreted in terms of direction and velocity of rupture on the fault plane. A more detailed analysis of the source implies the use of additional information coming from aftershock studies and tectonics. We use forward modelling to investigate several rupture mechanisms. A three-source model gives an acceptable fit to the observed records but the western mechanism is at odds with observed tectonics and, furthermore, rupture propagation is not well simulated. A five-segment model of the source obtained from field seismotectonic data gives a better waveform fit, a time sequence of individual breaks that simulates a rupture propagating away from the hypocentre, and subsource mechanisms that are compatible with surface tectonics.