Queen Charlotte Islands Region Research Articles

Stress in western Canada from regional moment tensor analysis

More than 180 regional moment tensor (RMT) solutions for moderate-sized earthquakes (M ≥ 4) are used to examine the contemporary stress regime of western Canada and provide valuable information relating to earthquake hazard analysis. The overall regional stress pattern shows mainly NE–SW-oriented P axes for most of western Canada with local variations. In the northern cordillera, the maximum compressive stress direction (σ1) varies from east–west to north–south to NE–SW from south to north. The stress direction σ1 is consistent with the P axis direction for the largest earthquakes, except in the central and northern Mackenzie Mountains where there is a 16° difference. The Yakutat collision zone shows a steady change in σ1 from east–west in the east to north–south in the west. In the Canada – United States border region, RMT solutions suggest a north–south compressional regime may extend through southern British Columbia and northern Washington to the eastern Cordillera. In the Vancouver Island – Puget Sound region, RMT solutions do not show any obvious pattern in faulting style. However, the stress results are consistent with margin-parallel compression in the crust and downdip tension in the subducting slab. Along the Queen Charlotte fault σ1 is oriented ~45° to the strike of the northern section of the fault, which is dominated by strike-slip faulting, and ~60° to the strike of the southern section, which is dominated by high-angle thrust faults. The amount of thrust faulting infers a significant amount of convergence between the Pacific and North America plates in the southern Queen Charlotte Islands region.

Stress Distribution along the Fairweather-Queen Charlotte Transform Fault System

Tectonic loading and Coulomb stress transfer are modeled along the right-lateral Fairweather–Queen Charlotte transform fault system using a three-dimensional boundary element program. The loading model includes slip below 12 km along the transform as well as motion of the Pacific plate, and it is consistent with most available Global Positioning System (gps) displacement rate data. Coulomb stress transfer is shown to have been a weak contributing factor in the failure of the southeastern (Sitka) segment of the Fairweather fault in 1972, hastening the occurrence of the earthquake by only about 8 months. Failure of the Sitka segment was enhanced by a combination of cumulative loading from below (95%) by slip of about 5 cm/yr since 1848, by stress transfer (about 1%) from major earthquakes on straddling segments of the Queen Charlotte fault ( M 8.1 in 1949) and the Fairweather fault ( M 7.8 in 1958), and by viscoelastic relaxation (about 4%) following the great 1964 Alaska earthquake, modeled by Pollitz et al. (1998). Cumulative stress increases in excess of 7 MPa at a depth of 8 km are projected prior to the M 7.6 earthquake. Coulomb stress transferred by the rupture of the great M 9.2 Alaska earthquake in 1964 (Bufe, 2004a) also hastened the occurrence of the 1972 event, but only by a month or two. Continued tectonic loading over the last half century and stress transfer from the M 7.6 Sitka event has resulted in restressing of the adjacent segments by about 3 MPa at 8 km depth. The occurrence of a M 6.8 earthquake on the northwestern part of the Queen Charlotte fault on 28 June 2004, the largest since 1949, also suggests increased stress. The Cape St. James segment of the fault immediately southeast of the 1949 Queen Charlotte rupture has accumulated about 6 MPa at 8 km through loading since 1900 and stress transfer in 1949. A continued rise in earthquake hazard is indicated for the Alaska panhandle and Queen Charlotte Islands region in the decades ahead as the potential for damaging earthquakes increases.

Moment Magnitude-Local Magnitude Calibration for Earthquakes off Canada's West Coast

Local magnitude ( M L ) values of earthquakes off Canada9s west coast are known to be underestimated by at least 0.5 magnitude units compared with other magnitude scales. Moment magnitude ( M w ), derived from moment tensor analysis, provides the most robust estimate of the magnitude of earthquakes. Moment tensor analysis of regional seismic data in western Canada is now possible due to the installation of more than 40 three-component broadband stations in western Canada, the U.S. Pacific Northwest, and southeast Alaska. Moment tensor solutions are now possible down to M ∼4.0. More than 230 regional moment tensor solutions have been calculated off Canada9s west coast at the Geological Survey of Canada for 1995–2002. These solutions, along with 14 previous solutions by Oregon State University in 1994–1995 and 13 Harvard solutions for 1984–1993, allow a systematic M w – M L calibration for earthquakes in this region. The study area extends from the Queen Charlotte Islands region in the north to the area off the west coast of southern Vancouver Island. At the northern end of the study area, where there is little oceanic crust in the source-receiver travel path, M w is systematically larger than M L by 0.28 ± 0.08 magnitude units. At the southern end of the study area, where there is a significant amount of ocean crust in the source-receiver travel path, M w is systematically larger than M L by 0.62 ± 0.08 magnitude units. Calibration of M L with M w will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.

Structure beneath Queen Charlotte Sound from seismic-refraction and gravity interpretations

A combined multichannel seismic reflection and refraction survey was carried out in July 1988 to study the Tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the Queen Charlotte Islands region. Simultaneously with the collection of the multichannel reflection data, refractions and wide-angle reflections from the airgun array shots were recorded on single-channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this paper a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the nearby subduction zone.Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in Queen Charlotte Sound. On a margin-parallel line, Moho depth was modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 5 km of the Tertiary sediments, the crust in the latter area is only about 18 km thick, suggesting substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust supports an extensional mechanism for the origin of the sedimentary basin. Deep crustal layers with velocities of more than 7 km/s were interpreted in the southern portion of Queen Charlotte Sound and beneath the continental margin. They could represent high-velocity material emplaced in the crust from earlier subduction episodes or mafic intrusion associated with the Tertiary volcanics.Seismic velocities of both sediment and upper crust layers are lower in the southern part of Queen Charlotte Sound than in the region near Moresby Island. Well velocity logs indicate a similar velocity variation. Gravity modelling along the survey line parallel to the margin provides additional constraints on the structure. The data require lower densities in the sediment and upper crust of southern Queen Charlotte Sound. The low-velocity, low-density sediments in the south correspond to high-porosity marine sediments found in wells in that region and contrast with lower porosity nonmarine sediments in wells farther north.

Triassic to Neogene geologic evolution of the Queen Charlotte region

A wealth of new geological and geophysical data from recent studies of the Queen Charlotte region are integrated into a coherent model. In this paper, we summarize these new studies and discuss possible correlations with other areas. Four tectonostratigraphic divisions are distinguished by stratigraphic, structural, and magmatic character, and each is separated by a major unconformity. The oldest division comprises widely distributed, upper Paleozoic through Middle Jurassic strata of Wrangellia that accumulated in volcanic-arc and stable shelf and basinal settings. No significant deformation occurred in the Queen Charlotte Islands region during the accumulation of these rocks. A Middle and Upper Jurassic assemblage comprises two plutonic suites and volcanic and epiclastic rocks. The unconformity below the Middle and Upper Jurassic assemblage marks a regional, southwest-vergent contractional deformation that is the most significant Mesozoic or Cenozoic deformation in the region. Jurassic plutons in the Queen Charlotte Islands are the oldest and most primitive members of an eastwardly migrating and evolving Jura-Cretaceous magmatic front recognized by other workers in the Coast Plutonic Complex. Widespread Late Jurassic block faulting led to differential uplift and erosion of northwest-trending fault blocks. A third assemblage consists of Cretaceous marine sedimentary rocks derived principally from subjacent Jurassic volcanic rocks as well as older strata. The present distribution of Cretaceous strata reflects a gradual eastward transgression, briefly interrupted in the Coniacian by progradation of conglomerate fans from the east. A second regional contractional deformation event in latest Cretaceous time was concentrated along a northwest-trending zone coinciding with Jurassic block faults. The early Tertiary marked another distinct shift in sedimentation style, with the inception of local nonmarine deposition on the present islands and widespread volcanism and plutonism on the southern islands. Syntectonic deposition in offshore extensional basins (Hecate Strait and Queen Charlotte Sound) may have commenced at this time. Later in the Tertiary, extensive deposition occurred in offshore regions, coeval with northward migration of plutonism and volcanism on the islands. Contractional structures in Pliocene sediments in Hecate Strait are the youngest deformational features observed.

A microseismicity study of the Queen Charlotte Islands region

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.

Electromagnetic induction in the Queen Charlotte Islands region: Analogue model and field station results.

A comparison of results from field stations and a laboratory analogue model shows that the electromagnetic response of the Queen Charlotte Islands region is primarily related to the distribution of land and seawater. Almost all of the induction arrows from the simple land-sea model, both in phase and quadrature, have similar directions to those available from the five field sites over the period range of 4 to 120 minutes. The few arrows which differ in direction are influenced by conductive crustal features, not included in the simple model. In contrast to the excellent agreement in direction, the amplitudes of the coastal induction arrows differ between the model and field results: the model amplitudes are too large at short periods (4min) and too small at long periods (120min). Thus a more complex structure is required for the lithosphere and upper asthenosphere for the oceanic and land regions than the uniform layer of the present laboratory model. The effect of deflection of the induced electric currents is observed in the model near all capes and bays along the coastline and is very large near Rose Point at the northeast corner of the islands. Contour plots and three-dimensional views of the magnetic and electric field components clearly demonstrate the difficulty in locating a representative reference station for correcting marine or land magnetic surveys in this type of region.

An analogue model study of electromagnetic induction in the Queen Charlotte Islands region.

In order to ease the interpretation of geomagnetic data for resource charting and tectonic studies of the Queen Charlotte Islands region, the behaviour of the electromagnetic variations over the region is studied using a laboratory analogue model. The results indicate that conductive channelling is important in Hecate Strait for both E and H source field polarizations. Current deflection is observed at Rose Point for E-Polarization and at the northern and southern tips of the Queen Charlotte Islands for H-Polarization. Model results for simulated variations of 4, 40, and 120min show a wide range of field values over the Queen Charlotte Islands, for example, Hz varies by a factor of 4 or more between coastal and inland locations for E-Polarization at short periods and by as much as a factor of 5 between certain coastal locations at long periods. The model results show that in decreasing the depth of the conducting layer in the mantle from 200 to 100km, the Hz, Hx, Ex, and Ey anomalies are attenuated by 20%, and the Hy anomalies by roughly 8% over the deep ocean and by 12% over Hecate Strait for 4min period variations.