A Comparison of the Source Mechanisms of the 1975 Laurentian Channel Earthquake and the Tsunamigenic 1929 Grand Banks Event

Abstract

The source mechanisms of the magnitude 7.2 Grand Banks event of 1929 (GB29), as analyzed by Hasegawa and Kanamori (1987) and the magnitude 5.2 Laurentian Channel earthquake of 1975 (LC75) that we have analyzed are quite different. The source mechanism of GB29 that is based on P-wave first motions, S-wave polarities and Love to Rayleigh wave ratios is a single force acting on the continental slope in a N5°W direction, which is in the upslope direction. This force of approximately 1020 dynes is consistent with the sudden release or slump of about 1011m3 of unconsolidated sediments down the continental slope. Available seismograms do not preclude double couple mechanisms but place necessary conditions or restrictions on any acceptable double couple mechanism. In contrast, analysis of surface and body wave data of LC75 indicates a predominantly thrust fault mechanism and a focal depth of 30 km. Because a seismic refraction study in the epicentral region of LC75 indicates a depth to the upper mantle of 25 km, the focus of this earthquake is probably in the upper mantle. From an analysis of the fundamental mode Love and Rayleigh waves, the seismic moment of this earthquake is 5×1023 dyne-cm. The actual plane of failure is uncertain but could be along any one of the weak zones created by the following: sediment loading of the oceanic crust; rift or transform faults created during the opening of the Atlantic Ocean; or along the seaward extension of faults that manifest surface offsets on land. Because 4 of the 5 larger (M>5) events at the mouth of the Laurentian Channel occurred before the deployment of the WWSSN and the expansion of Canadian seismograph networks, there are in general too few seismograms of these events for a reliable determination of source mechanisms. This deficiency precludes a more definitive assessment of the seismotectonic regime in this region.