Research Directed toward the Use of Long and Intermediate Period Seismic Waves for the Identification of Seismic Sources

Seismological research at the Lamont-Doherty Geological Observatory is summarized for the period 1 July to 31 December, 1972. Work has continued on the nature and occurrence of intra-plate earthquakes - the isolated, and often high stress drop events which occur in the relatively seismically inactive interior of lithospheric plates - to reduce the chance of their misidentification as explosive events. Complimenting this study is the work on the Blue Mountain Lake earthquake swarm where a close look at the source region of some intra-plate events has revealed premonitory velocity changes. The nature of the earthquake source is also being investigated experimentally using strong-motion records and theoretically by modelling the kinematics and dynamics of shear failure. The effect of the propagation path on seismic signals is being extended to include lateral inhomogeneities and the effects of anelastic attenuation. The horizontal refraction of surface waves has been treated theoretically, and observations of mantle surface waves have defined usefully distinct velocity models for different tectonic provinces.