A COMPLEX DOUBLE-COUPLE SOURCE MECHANISM FOR THE M(S)-7.2 1929 GRAND-BANKS EARTHQUAKE

The M(s) 7.2 Grand Banks earthquake of 1929 was one of the largest and the most fatal earthquakes to have occurred in Canada, with most of t he death and destruction having been caused by a tsunami and submarine landslide associated with the earthquake. It has been suggested (Hase gawa and Kanamori, 1987) that a single-force (landslide) mechanism was more consistent with the data than was a double-couple source and tha t therefore the event was not an earthquake. However, that particular study considered only four double-couple solutions and left many unans wered questions, in particular with respect to the source time functio n and sediment volume involved. Here, a larger number of seismograms a re used to examine the full range of double-couple solutions to determ ine more definitively the nature of the event. Waveform modeling using both forward and inverse methods indicates that this event was an ear thquake, with a complex source mechanism. The first and largest subeve nt was a strike-slip double-couple event occurring on a northwest-stri king plane. Two later subevents were probably strike-slip double coupl es on northeast-striking planes, but other mechanisms cannot be comple tely ruled out. The first subevent has a well-constrained focal depth of 20 +/- 2 km. The second and third subevents also appear to have occ urred at 20 km, but are constrained only to within +/- 5 km. These dep ths provide further evidence that the event was not a landslide. The s um of the subevent moments corresponds to an M(W) of 7.2 +/- 0.3, whic h is in close agreement with the M(W) of 7.1 +/- 0.1 obtained by the C MT method using long-period data. These M(W)'s are also consistent wit h the M(s) of 7.2 (+/-0.3) and m(B) of 7.1 (+/- 0.2) calculated direct ly from the seismograms. Modeling of the seafloor displacement for thi s mechanism indicates that the tsunami was generated by the landslide and not directly by the earthquake.