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A seismicity study of the Queen Charlotte Islands/Hecate Strait Region Bérubé, Joane


The Queen Charlotte Islands are located east of the seismically active Queen Charlotte transform fault zone which separates the Pacific and North American plates. The fault zone is the locus of significant seismic activity and is distinguished bathymetrically by two steep scarps bounding a 15 to 25 km wide terrace. To better define regional seismicity characteristics, 16 portable seismographs and 6 ocean bottom seismographs were operated for 9 weeks and 5 days, respectively, during June to August 1983. Three hundred and seventeen events were detected; 130 events that were recorded on 3 or more stations have been located. Twenty' events were identified as possible blasts. Ninety-two of the located earthquakes lie along the Queen Charlotte transform fault zone, most within the 1949, Ms = 8.1, earthquake rupture zone along the inner scarp of the terrace. However, several earthquakes are located on the terrace and a few are aligned with the outer scarp where no activity has previously been observed. Most of the activity is well constrained to be less than 15 km in depth. Two areas of low seismicity were observed along the fault zone. Only two earthquakes occurred in the documented seismic gap bound on the north by the 1949 rupture zone and on the south by the 1970 M = 7.0 earthquake. They were both at -the southern tip of the gap. A similar region of low activity was observed for the fault along Graham Island. No major seismicity (M > 4.0) has been located in this region since the 1949 earthquake. Composite fault plane mechanism solutions were determined for five clusters of events along the fault zone. Events to the northwest of Graham Island are consistent with strike-slip motion along a fault in the direction of the Queen Charlotte transform fault. The four remaining clusters were located along Moresby Island. The mechanisms for these are dominated by thrust faulting with a component of compressional stress trending north-south. These events are interpreted as the result of oblique convergence between the Pacific and North American plate. Significant seismicity was located east of the main Queen Charlotte transform fault zone. Eighteen earthquakes, the largest ML = 3.8, were located in northeastern Graham Island and adjacent Hecate Strait - Dixon Entrance area. None could be associated with known faults. The focal depth of these events is well constrained within the crust so they could not be associated with a subducted plate. A composite fault plane mechanism solution determined for some of these earthquakes indicates a thrusting mechanism with north-south trending compressional stress. One event with a well constrained solution at a shallow focal depth occurred in southeastern Hecate Strait. This event could be associated with crustally pervasive faults identified in Hecate Strait. A magnitude scale based on the coda length of the earthquake signal was determined. Magnitudes were calculated for 265 of the events recorded during the study. For the complete data set a b-value of 0.55 ± 0.05 was determined. This value is significantly lower than values from other studies in the Canadian Cordillera, indicating that a greater percentage of the total number of earthquakes occurs at the higher magnitudes. However, the short period of recording and large magnitude seismic activity (4 earthquakes with ML > 3.8 in 9 weeks) might have biased the estimate toward a low value.

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