Seismicity of western Canada

1978 ◽  
Vol 15 (7) ◽  
pp. 1170-1193 ◽  
Author(s):  
W. G. Milne ◽  
G. C. Rogers ◽  
R. P. Riddihough ◽  
G. A. McMechan ◽  
R. D. Hyndman
Keyword(s):  
Puget Sound ◽  
Seismic Energy ◽  
Vancouver Island ◽  
Fault System ◽  
Western Canada ◽  
Juan De Fuca ◽  
Lithospheric Plates ◽  
Strain Release ◽  
Tectonic Features

The seismicity of western Canada has been studied for the period 1899–1975. The quality of the data collected improved through this period as the number of recording stations increased and the location and analysis methods developed, but significant uncertainties and biases remain. Although these restrictions limit detailed correlation of seismic events with specific tectonic features, in general the most active earthquake areas correspond to the boundaries between the major lithospheric plates. These are the Queen Charlotte – Fairweather fault system (Pacific–America plates), the offshore ridge-fracture zone system (Pacific – Juan de Fuca plates), and the Vancouver Island – Puget Sound region (Juan de Fuca – America plates). Strain release calculations show that most seismic energy is released along the Queen Charlotte – Fairweather fault system and that at present a significant accumulation of strain may be available for release as earthquakes in the Vancouver Island – Puget Sound area. Except for the absence of thrust earthquakes along the apparently converging margin, focal mechanisms are in good agreement with the postulated plate motions. The b values in the frequency–magnitude recurrence relation for different areas within the region range from 0.65 to 0.82.

A Review of Geophysical Studies in the Canadian Cordillera

1971 ◽  
Vol 8 (7) ◽  
pp. 788-801 ◽  
Author(s):  
M. J. Berry ◽  
W. R. Jacoby ◽  
E. R. Niblett ◽  
R. A. Stacey
Keyword(s):  
British Columbia ◽  
Heat Flow ◽  
Rocky Mountain ◽  
The United States ◽  
Vancouver Island ◽  
Geophysical Data ◽  
Fault System ◽  
Canadian Cordillera ◽  
Crust And Upper Mantle ◽  
Juan De Fuca

Geophysical studies of the crust and upper mantle have been conducted in the Canadian Cordillera for over two decades, but only recently have sufficient data been collected to permit a synthesis and a correlation with the major geological units. The studies have included gravity, heat flow, and magnetotelluric observations, geomagnetic depth sounding, and high level aeromagnetics as well as both small and large scale refraction and reflection seismic surveys.It now appears that major crustal units may be recognized geophysically:(i) Seismic and gravity data suggest that the Plains and Rocky Mountains are underlain by two units of the North American craton with a crustal section 45–50 km thick. The northern unit appears to terminate at the Rocky Mountain Trench while the southern unit may extend to the Omineca Geanticline.(ii) The combined geological and geophysical data suggest that the Rocky Mountain Trench and possibly the Kootenay Arc near the 49th parallel mark the edge of the Precambrian continental margin and that the western Cordillera was formed by a complex succession of plate interactions with repeated reactivation of block boundaries.(iii) A combination of magnetic and heat flow data suggest that the region between the Rocky Mountain Trench and the Fraser Lineament is part of the Cordilleran Thermal Anomaly Zone recognized by Blackwell in the United States.(iv) Seismic data in Central British Columbia suggest that the Pinchi Fault system is a boundary between two crustal blocks.(v) The crustal thickness of the Coast Geanticline appears to increase gradually to the west to approximately 40 km and, at least in southern British Columbia, does not have a root zone below the mountains.(vi) The crustal section beneath Vancouver Island is abnormally thick and there is some paleomagnetic data which suggest that the Island may not have been formed in its present position, contiguous to the Cordillera. The crustal section for the northern part of the Insular Trough is significantly thinner.(vii) The active spreading of the Juan de Fuca Rise – Explorer Trench is now well documented. The geophysical data suggest active subduction of the Juan de Fuca plate beneath Oregon, Washing-ton, and southern Vancouver Island. However, further north there is no evidence for subduction.

Relocation of earthquakes west of Vancouver Island, British Columbia, 1965–1983

1992 ◽  
Vol 29 (5) ◽  
pp. 953-961 ◽  
Author(s):  
Rutger Wahlström ◽  
Garry C. Rogers
Keyword(s):  
Shear Zones ◽  
Vancouver Island ◽  
Fault System ◽  
Transform Fault ◽  
Arrival Times ◽  
Magnitude Range ◽  
Juan De Fuca ◽  
Internal Deformation ◽  
Seismograph Network ◽  
Hypocentre Locations

In the tectonically complex region of young plate interaction west of Vancouver Island, 360 earthquakes have been relocated. The earthquakes occurred in the years 1965 – 1983, when the Canadian seismograph network in the region did not significantly change configuration, and are in the magnitude range 3–5. A traveltime model was derived and applied to arrival times for a selected, limited set of station–phase combinations. Time corrections for these combinations were derived from joint-hypocentre locations of earthquakes in specific regions using independently located reference events. An algorithm for routine location of offshore earthquakes in this region is suggested.The correlation between seismicity and mapped bathymetrical features is strong along the Revere–Dellwood transform fault and the northern segments of the Explorer ridge – transform fault system. Considerable seismicity occurs inside the Explorer Plate, indicating internal deformation. The Sovanco and Nootka shear zones, the southern borders of the Explorer Plate, are characterized by broad belts of seismicity and evidently are not simple transform margins. The Explorer and northern Juan de Fuca ridges are aseismic in the investigated magnitude range.

scholarly journals Fault locking, block rotation and crustal deformation in the Pacific Northwest

2007 ◽  
Vol 169 (3) ◽  
pp. 1315-1340 ◽  
Author(s):  
Robert McCaffrey ◽  
Anthony I. Qamar ◽  
Robert W. King ◽  
Ray Wells ◽  
Giorgi Khazaradze ◽  
...  
Keyword(s):  
North America ◽  
Shear Deformation ◽  
Pacific Northwest ◽  
Puget Sound ◽  
Vancouver Island ◽  
Block Rotation ◽  
The Pacific ◽  
Juan De Fuca ◽  
Gps Velocities ◽  
Relative Motions

Summary We interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr−1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr−1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

A model for recent plate interactions off Canada's west coast

1977 ◽  
Vol 14 (3) ◽  
pp. 384-396 ◽  
Author(s):  
R. P. Riddihough
Keyword(s):  
Fine Structure ◽  
Continental Margin ◽  
Vancouver Island ◽  
Geometrical Model ◽  
Crustal Shortening ◽  
Juan De Fuca ◽  
Lithospheric Plates ◽  
History Of ◽  
Northern Edge ◽  
Spreading Rates

Detailed re-examination of existing magnetic anomaly data reveals the fine structure of variations in spreading rates and directions at the Juan de Fuca and Explorer Ridges during the last 10 million years. A geometrical model using these variations delineates the theoretical history of the interactions between the lithospheric plates involved. These interactions demonstrate the independent movement of the Juan de Fuca and Explorer plates and the development of the Sovanco Fracture Zone. The latter was apparently initiated E–W at 7 Ma, rotated clockwise to 120° and may have been the site of up to 50 km of crustal shortening. The model demonstrates that subduction rates at the Canadian continental margin declined from 5 cm/yr to a present 1.5 cm/yr and that recent relative movements are compatible with the N–S compression observed from earthquakes. It also suggests that the existence of both E–W and NE trending faults in the downgoing lithosphere beneath Vancouver Island, shows that a triple junction remained static near the northern end of Vancouver Island from 10–4 Ma, and predicts a buried northern edge of subducted material striking NE in this area.

Harbor porpoise (Phocoena phocoena) recovery in the inland waters of Washington: estimates of density and abundance from aerial surveys, 2013–2015

2016 ◽  
Vol 94 (7) ◽  
pp. 505-515 ◽  
Author(s):  
Thomas A. Jefferson ◽  
Mari A. Smultea ◽  
Sarah S. Courbis ◽  
Gregory S. Campbell
Keyword(s):  
Puget Sound ◽  
Distance Sampling ◽  
Line Transect ◽  
Inland Waters ◽  
San Juan ◽  
Phocoena Phocoena ◽  
Harbor Porpoise ◽  
San Juan Islands ◽  
Juan De Fuca ◽  
Harbor Porpoises

The harbor porpoise (Phocoena phocoena (L., 1758)) used to be common in Puget Sound, Washington, but virtually disappeared from these waters by the 1970s. We conducted systematic aerial line-transect surveys (17 237 km total effort) for harbor porpoises, with the goal of estimating density and abundance in the inland waters of Washington State. Surveys in Puget Sound occurred throughout the year from 2013 to 2015, and in the Strait of Juan de Fuca and the San Juan Islands (and some adjacent Canadian waters) in April 2015. We used a high-wing, twin-engine Partenavia airplane and four observers (one on each side of the plane, one looking through a belly port, and one recording data). A total of 1063 harbor porpoise groups were sighted. Density and abundance were estimated using conventional distance sampling methods. Analyses were limited to 447 harbor porpoise groups observed during 5708 km of effort during good sighting conditions suitable for line-transect analysis. Harbor porpoises occurred in all regions of the study area, with highest densities around the San Juan Islands and in northern Puget Sound. Overall, estimated abundance for the Washington Inland Waters stock was 11 233 porpoises (CV = 37%, 95% CI = 9 616 – 13 120). This project clearly demonstrated that harbor porpoises have reoccupied waters of Puget Sound and are present there in all seasons. However, the specific reasons for their initial decline and subsequent recovery remain uncertain.

The reproducing earthquakes of the Galapagos Islands

1980 ◽  
Vol 70 (5) ◽  
pp. 1759-1770
Author(s):  
Kris Kaufman ◽  
L. J. Burdick
Keyword(s):  
Large Scale ◽  
Fault Plane ◽  
Seismic Energy ◽  
Galapagos Islands ◽  
The Body ◽  
Galápagos Islands ◽  
Plane Solution ◽  
Extensional Faulting ◽  
Strain Release ◽  
Wave Trains

abstract The largest swarm of earthquakes of the last few decades accompanied the collapse of the Fernandina caldera in the Galapagos Islands in June of 1968. Many of the events were relatively large. (The largest 21 had moments ranging from 6 ×1024 to 12 ×1024 dyne-cm.) They produced teleseismic WWSSN records that were spectacularly consistent from event to event. The entire wave trains of the signals were nearly identical on any given component at any given station. This indicates that the mode of strain release in the region was unusually stable and coherent. The body waveforms of the events have been modeled with synthetic seismograms. The best fault plane solution was found to be: strike = 335°, dip = 47°, and rake = 247°. The depths of all the larger shocks were close to 14 km. Previous work had suggested that the seismic energy was radiated by the collapsing caldera block at a depth of about 1 km. The new results indicate that large scale extensional faulting at depth was an important part of the multifaceted event during which the caldera collapsed.

Earthquakes in Southern California with geologic relations*

1947 ◽  
Vol 37 (2) ◽  
pp. 107-157
Author(s):  
Harry O. Wood
Keyword(s):  
High Intensity ◽  
Small Area ◽  
Seismic Energy ◽  
Earthquake Magnitude ◽  
Regional Strain ◽  
Historical Outline ◽  
Energy Equivalence ◽  
Region Data ◽  
Made In

ABSTRACT A brief historical outline concerning the investigation is followed by description of the geologic faulting in the region. Data of the instrumentally recorded earthquakes of recent years, giving shock magnitudes and the location of epicenters, are tabulated, with notes, comments, and discussion of the quality of the location of the shocks and the incompleteness of the information. Summaries of the tabulation are made in different ways. Maps show the geographic interrelationships of the shocks tabulated, and the faults. These relationships are also discussed in the text. Earthquake magnitude is discussed with relation to the method used in mapping the epicenters, and its energy equivalence is considered. A table shows the geographic distribution of the seismic energy and the corresponding variation in the “density” of activity as well as the corresponding number of shocks. The correlation of the earthquake origins with the faulting is discussed. The prevailing depth of the origins is considered and, based upon the findings, a hypothesis is advanced to account for the random, or scatter, distribution of many small shocks away from the mapped faults—i.e., repeated, migrating slips of small area on nearly horizontal planes at or near the depth of origin, with resultant growth of regional strain. As yet this is only a hypothesis, but the importance of continuing consideration of it is emphasized. The possibility of such action elsewhere, with the production of great earthquakes having large areas of high intensity, is suggested. No basis for earthquake prediction has resulted from the investigation. Future needs are discussed briefly.

Notes on Polychaeta from the East Coast of Vancouver Island and from Adjacent Waters, with a Description of a New Species of Aricidea

1956 ◽  
Vol 13 (4) ◽  
pp. 541-546 ◽  
Author(s):  
E. Berkeley ◽  
C. Berkeley
Keyword(s):  
New Species ◽  
North America ◽  
East Coast ◽  
Vancouver Island ◽  
Western Canada ◽  
Western North America ◽  
A New Species

Records are given of two species and a variety new to western Canada and notes on three other species already known from the region. A new species, Aricidea lopezi, and four species new to western North America, are described from the neighbourhood of Friday Harbour, Washington.

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