Source mechanism of the magnitude 7.2 Grand Banks earthquake of November 1929: Double couple or submarine landslide?

Abstract

We have examined P, S, and surface waves derived from seismograms that we collected for the 1929 Grand Banks, Canada, earthquake. This event is noteworthy for the sediment slide and turbidity current that broke the trans-Atlantic cables and for its destructive tsunami. Both the surface-wave magnitude, M_S, and the body-wave magnitude, m_B, calculated from these seismograms are 7.2. Fault mechanisms previously suggested for this event include a NW-SE-striking strike-slip mechanism and an approximately E-W-striking thrust mechanism. In addition, because of the presence of an extensive area of slump and turbidity current, there exists the possibility that sediment slumping could also be a primary causative factor of this event. We tested these fault models and a horizontal single-force (oriented N5°W) model representing a sediment slide against our data. Among these models, only the single-force model is consistent with the P-, S-, and surface-wave data. Our data, however, do not preclude fault models which were not tested. From the spectral data of Love waves at a 50-sec period, we estimated the magnitude of the single force to be about 1.4 × 10^(20) dynes. From this value, we estimated the total volume of sedimentary slumping to be about 5.5 × 10^(11) m^3, which is approximately 5 times larger than a recent estimate of volume from in situ measurements. The difference in estimates of overall volume is likely due to a combination of the inherent difficulty in estimating accurately the displaced sediments from in situ measurements, and of inadequacy of the seismic model; or perhaps because not only the slump but also a tectonic earthquake could have been the cause of this event and contributed significantly to the waveforms studied.