Anomalous Rayleigh Waves from Presumed Explosions in East Kazakh

Abstract

Long-period vertical seismograph recordings at SRO stations from events in the Shagan River province of eastern Kazakhstan between 1976 and 1979 show an interesting pattern of Rayleigh wave anomalies. While Rayleigh waves from some of the events are normal at all stations, those from other events exhibit anomalous behaviour, in the form of amplitude perturbations accompanied by phase reversals, in a NE-SW direction, with normal Rayleigh waves in the other directions. For one event the Rayleigh waves are phase-reversed in all directions. A remarkable aspect of the records is that the shapes of the anomalous Rayleigh wave trains are almost perfectly consistent from event to event, but their arrival times are delayed by a few seconds compared to the normal Rayleigh waves. The azimuthal pattern seems to rule out explanations such as depth of burial and spallation, while the consistency in wave shape and the associated time delay are difficult to model by combining the radiation patterns of an explosion and an accompanying earthquake. A reasonable interpretation of the anomalous signals seems to be that pre-existing strain effectively modifies the source functions of the explosions in preferred azimuths.