SURFACE-WAVE FOCAL MECHANISMS FOR EASTERN NORTH-AMERICAN EARTHQUAKES WITH TECTONIC IMPLICATIONS

Using P wave first motions together with amplitude and phase data from long-period Love and Rayleigh waves, the focal mechanisms of 22 earthquakes occurring in eastern North America between 1962 and 1976 are determined. The techniques used yield good results for events with body wave magnitude as small as 4.0. Eighteen of the events have depths between 5 and 16km; 15 have significant components of strike slip motion; and for focal mechanisms having nearly horizonatl pressure axes, 70% of the pressure axes trend northeast or east. Though the earthquake sample may be biased toward the central US, it is significant that no focal depths greater than 25km have been encountered. A confidence factor is assigned to each focal mechanism for use in future tectonic synthesis.-Author