The amount of seismicity along the Queen Charlotte Fault varies considerably, but the overall pattern has been stable since the early 1980's. Focal mechanisms in the region are a mixture of primarily strike-slip and thrust. These are consistent with a small element of convergence across the dominantly strike-slip Queen Charlotte Fault, and with the major pressure axis associated with the relative plate motion.
Figure 1: The Queen Charlotte Islands, between Vancouver Island and southeast Alaska, lie adjacent to a major plate boundary. The arrow is an indication of relative plate motion according to the NUVEL-1A model of DeMets et al. [1994].
The resultant movement along the Queen Charlotte Fault is primarily strike-slip, like that of the San Andreas Fault, but with a slight component of convergence.
Small diamonds represent the short period seismograph stations of the Queen Charlotte seismic network.
Three large earthquakes have occured along the Queen Charlotte Fault in this century: a magnitude 7 event in 1929, a magnitude 8.1 occurred in 1949 (Canada's largest recorded earthquake) and a magnitude 7.4 in 1970. The P nodal focal mechanism for the 1949 earthquake indicates virtually pure strike-slip movement with a northwest striking nodal plane corresponding to the strike of the fault, whilst the 1970 earthquake shows a similar strike-slip movement with a small but significant thrust component, consistent with relative plate motion [Rogers 1982].
Rupture zones for the 1949 and 1970 events were defined by their aftershock distributions. The 1949 earthquake rupture zone found to be approximately 490 km long, running from the southern west coast of Moresby Island to southeast Alaska [Bostwick 1984]. A compilation of the rupture zones suggests a seismic gap remains between the 1949 and 1970 earthquakes in the southernmost portion of the Queen Charlotte Fault, just off the southern tip of Moresby Island [Rogers 1986].
The seismicity along the margin is in a broad band, approximately 50 km in width [Figure 2]. The location of events west of the islands requires a fixed depth, due to the absence of seismometers west of the fault to constrain depth. To determine whether the breadth of the fault zone is an artefact of this location procedure, a number of earthquakes were located at fixed depths varying between 0 and 25 km. The result indicated that this could account for no more that 10 km of longitudinal spread in the seismicity along the Queen Charlotte Fault zone. The width of epicentre locations along the margin is attributed to earthquakes occurring on a number of faults subsidiary to the main transform fault. Multi-channel reflection data show numerous faults up to 50 km west of the Queen Charlotte Fault [Rohr 1996]. Although it is not possible to determine on which faults events occur, those with thrust mechanisms clearly occur on subsidiary faults [Figure 3].
Figure 3: Focal mechanisms (lower hemisphere projection) for activity along the plate boundary.
There is a lateral variability in the intensity of earthquake activity along the margin. A region of low activity lies in the vicinity of the epicentre of the 1949 earthquake, off the west coast of Graham Island (at approximately 53.8 degrees N). There is also a drop in activity in the region of the aforementioned seismic gap towards the southern tip of Moresby Island.
Significant activity was found to occur east of the Queen Charlotte Fault zone, on northern Graham Island and in Hecate Strait, including a magnitude 5.3 in 1990, the largest recorded in the Hecate Strait area. Focal mechanisms were calculated for larger off-fault earthquakes and composite solutions for clusters of smaller events [Figure 4]. These are consistent with strike-slip and thrust mechanisms as seen along the plate boundary, and their average pressure axes concur with that expected for the relative plate motion and fault strike along the Pacific - North American boundary. Rohr and Dietrich [1992] have analyzed seismic reflection data in the Queen Charlotte Basin and discovered areas of complex faulting. The seismicity in northern Hecate Strait is in the vicinity of faulted basement, but an extensive area of faulting in southern Hecate Strait has no recorded seismicity associated with it.
Figure 4: Graham Island focal mecanism solutions, with shading an indication of reliability. The mechanisms shown here suggest a north-northeast trending pressure axis, consistent with the motion of the Pacific plate with respect to the North American plate. For Hecate Strait, the fault plane solutions show a slightly less random pattern of fault orientation and movement. Again, the pressure axis is consistent with plate motion.
The work presented here is a portion of a project involving detailed analysis of data collected by the Queen Charlotte seismic network between 1982 and 1996. Thus far, work has concentrated on providing accurate location of all earthquakes detected by the network, analysis of event distribution and investigation into the correspondence between earthquake mechanism and stress regime. References