A microseismicity study of the Queen Charlotte Islands region

Abstract

Nineteen land and three ocean-bottom seismographs were operated in the Queen Charlotte Islands region for periods of up to 9 weeks and 5 days, respectively, during the summer of 1983. Three hundred and seventeen seismic events were detected. One hundred and nine earthquakes ranging in size from magnitude −0.5 to 5.1 were well recorded at three or more stations and could be accurately located. Of these, 84 lie on or close to the Queen Charlotte Fault, most within the rupture zone of the great 1949 earthquake (MS = 8.1). The seismic gap, between the rupture zones of the 1949 event and the 1970 earthquake (MS = 7.4) that occurred just south of the Queen Charlotte Islands, exhibited little activity. Eighteen earthquakes, the largest with ML = 3.8, were located east of the fault on northern Graham Island or in adjacent Hecate Strait. Focal depths were generally less than 20 km, and none could be associated with known faults. Composite focal mechanism solutions were obtained for four suites of earthquakes along the Queen Charlotte Fault and for a group east of the fault zone on northern Graham Island. In all cases the solutions indicate thrust mechanisms with the predominant orientation of pressure axes northeast–southwest. The presence of thrust faulting close to the Queen Charlotte Fault suggests that the microseismicity is not occurring on the main transcurrent fault but on subsidiary faults that are moving due to the regional stress regime. Thrust faulting on northern Graham Island can best be interpreted as reflecting the stress field from a locked Pacific and North American boundary.