The Hecate sub-basin is one of two similar sedimentary structures comprising Queen Charlotte Basin, which is located between the British Columbia mainland and the Queen Charlotte Islands. The Queen Charlotte Basin was the locale of an active but unsuccessful exploration program, including drill holes, in the 1960's. However, recent studies incorporating modern concepts of plate tectonics have indicated a re-evaluation of the resource potential of the area is warranted. The Hecate sub-basin and its southern counterpart, the Charlotte sub-basin, are filled with Tertiary sediments that are underlain by a thick sequence of Tertiary volcanics. Penetration of the latter unit using the reflection method has been difficult. Thus the thickness of the volcanics and the existence or not of more sediments below them has not been established. To address this problem an airgun/ocean bottom seismograph (OBS) refraction survey was carried out across the Hecate sub-basin in 1983. Data from the airgun shots at approximately 160 m spacings were recorded on four OBSs deployed at 20 km intervals to provide a series of reverse profiles extending over 60 km. The principal interpretation procedure involved calculation of theoretical seismograms and travel-time curves for 2-D velocity structure models and comparisons with observed record sections. The interpreted structure model shows significant lateral variations. Low velocity Pleistocene and Pliocene sediments form an upper layer varying between 0.5 and 1.0 km thick. The principal sedimentary unit is the Tertiary Skonun Formation with interpreted velocities of 2.7 km/s and a gradient averaging 0.4 km/s/km, values that are consistent with well log data. These sediments are generally thicker (approximately 2.5 km) on the western side of the sub-basin although they reach their maximum thickness of 3 km in a depression near the central part of the basin. Toward the eastern side of the basin, the Tertiary sediments thin to about 1 km as the underlying Tertiary volcanics rise toward the mainland. The maximum sediment thickness in the basin is about 4 km. The upper surface of the volcanic unit shows a pronounced topography which is consistent with the erosional nature of this surface. Velocities for the volcanics vary between 4.8 and 5.0 km/s; thickness of the unit ranges from about 0.2 km to 1.8 km. Below the Tertiary volcanics on the eastern 20 km of the model, a low velocity zone less than 1 km thick had to be introduced to satisfy the data. This zone is inferred to contain Upper Cretaceous sediments. A unit with a poorly constrained velocity of 5.9 km/s which underlies the Tertiary volcanics and low velocity zone on the eastern side is interpreted to be the Paleozoic Alexander Terrane. Most of the characteristics of this model are similar to those determined from an earlier study in the Charlotte sub-basin. An additional component of this thesis project was the development of an interactive procedure for the inversion of densely spaced seismic refraction data by wavefield continuation to derive a l-D velocity-depth profile, and its application to data derived from 2-D structures. The procedure consists of two steps: a slant stack followed by a downward continuation. The method was found to yield velocity-depth structures which, when compared with an average velocity-depth structure from the 2-D model, have very similiar gradients and velocity increases. In general the velocity depth curve from the inversion had lower velocities at deeper depths than the averaged 2-D structure. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Traditional stock assessment methods do not incorporate remotely sensed ecosystem variables such as seafloor type, relief, and complexity. Incorporation of these and other ecosystem variables allows for targeting of species' optimal habitat during surveys. Recently, acoustic remote sensing methods have allowed us to gain insight into groundfish habitat. In June 2002, a geophysical survey was performed in selected fishing areas of Hecate Strait. While underway, single beam acoustic data were collected along survey lines utilizing a 50 kHz echosounder coupled with Quester-Tangent's QTC VIEW 5; a Huntec seismic system, and a dual frequency sidescan sonar system. Surficial sediment distribution and seabed features were mapped through examination of seismic, sidescan, and bottom grab data and compiled into a GIS. The surficial sediment classes were compared to bottom type classifications obtained from QTC single beam, with results showing the Gravel and Sand class from the surficial sediment data being classified best by the single beam system. Catch data from the groundfish bottom trawl fishery for the areas of interest were made available by the Department of Fisheries and Oceans Canada (DFO). The distribution of groundfish aggregates and individual fish species were compared to surficial sediment classes using correspondence analysis to investigate habitat associations. Results show that the Rock Sole aggregate had a habitat preference of gravel and sand mixture and the Dover Sole and Arrowtooth Flounder aggregates had a habitat preference of sandy mud. Correspondence analysis allows for a 2-dimensional view of multivariate categorical data which are the norm for habitat-based biological studies. Results suggest that the procedures developed in this work can improve stock assessment methodology and indicate that using various acoustic remote sensing techniques can be effective in characterizing seafloor habitats and ecological connections between groundfish species and seafloor types.

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. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate