Earthquakes along the Northwestern Boundary of the Labrador Sea

Abstract

Abstract As in much of northeastern Canada, earthquakes in the Labrador Sea occur predominantly along the passive margin. Geologically and geophysically, this region is complex and consists of an extinct spreading ridge and transform faults, both oceanic and continental crust, a possible hot spot trace, and magnetic and gravity anomalies. We have studied five recent earthquakes in the magnitude 4.5 to 5.5 range to determine their source properties and to better understand how they fit into the seismotectonic framework of the region. A combination of body and surface wave analysis techniques were used to determine the source parameters. LS69, LS89 and PB89 were well recorded teleseismically, and thus their source properties are better constrained than those for LS86 and LS87, for which only a few teleseismic records were available and whose source parameters were determined more from first motions than by modeling. The two events (LS69, LS89) that occurred near the intersection of the extinct spreading ridge (and associated transform) and Mesozoic rifted margin are noteworthy in that the former is of the thrust-fault and the latter of the normal-fault type. Local structure and possibly post-glacial rebound could be the causative factors for the occurrence of normal and thrust faulting in the same area. The two events (LS86, LS87) that occurred along the continent ocean transition zone have fairly similar fault-plane solutions, and are both thrust-faulting events. The fifth event (PB89), which occurred in Payne Bay, is also a thrust-faulting event and could be associated with the Ungava Transform fault to the northeast, or the Cape Smith fold belt to the northwest. The focal depths of all five events lie between 10 and 15 km, and may be dominated by thermal effects. The observation that the deviatoric compression axis of all five earthquakes lies in the northwest (or equivalently in the southeast) quadrant is consistent with recent modeling efforts of the tectonic stress field in this region.