scholarly journals Crustal structure of the northern Juan de Fuca plate and Cascadia subduction zone - new results, old data

2002 ◽  
Author(s):  
R M Clowes
Keyword(s):  
Subduction Zone ◽  
Crustal Structure ◽  
Cascadia Subduction Zone ◽  
Juan De Fuca Plate ◽  
Juan De Fuca

scholarly journals Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone

2019 ◽  
Vol 217 (3) ◽  
pp. 1929-1948 ◽  
Author(s):  
Helen A Janiszewski ◽  
James B Gaherty ◽  
Geoffrey A Abers ◽  
Haiying Gao ◽  
Zachary C Eilon
Keyword(s):  
Phase Velocity ◽  
Subduction Zone ◽  
Upper Mantle ◽  
Seismic Data ◽  
Cascadia Subduction Zone ◽  
Crust And Upper Mantle ◽  
Juan De Fuca Plate ◽  
Wave Phase Velocity ◽  
Juan De Fuca ◽  
S Period

SUMMARY A new amphibious seismic data set from the Cascadia subduction zone is used to characterize the lithosphere structure from the Juan de Fuca ridge to the Cascades backarc. These seismic data are allowing the imaging of an entire tectonic plate from its creation at the ridge through the onset of the subduction to beyond the volcanic arc, along the entire strike of the Cascadia subduction zone. We develop a tilt and compliance correction procedure for ocean-bottom seismometers that employs automated quality control to calculate robust station noise properties. To elucidate crust and upper-mantle structure, we present shoreline-crossing Rayleigh-wave phase-velocity maps for the Cascadia subduction zone, calculated from earthquake data from 20 to 160 s period and from ambient-noise correlations from 9 to 20 s period. We interpret the phase-velocity maps in terms of the tectonics associated with the Juan de Fuca plate history and the Cascadia subduction system. We find that thermal oceanic plate cooling models cannot explain velocity anomalies observed beneath the Juan de Fuca plate. Instead, they may be explained by a ≤1 per cent partial melt region beneath the ridge and are spatially collocated with patches of hydration and increased faulting in the crust and upper mantle near the deformation front. In the forearc, slow velocities appear to be more prevalent in areas that experienced high slip in past Cascadia megathrust earthquakes and generally occur updip of the highest-density tremor regions and locations of intraplate earthquakes. Beneath the volcanic arc, the slowest phase velocities correlate with regions of highest magma production volume.

Upper mantle structure of the northern Cascadia subduction zone

1995 ◽  
Vol 32 (1) ◽  
pp. 1-12 ◽  
Author(s):  
M. G. Bostock ◽  
J. C. Vandecar
Keyword(s):  
British Columbia ◽  
Subduction Zone ◽  
Upper Mantle ◽  
Puget Sound ◽  
Cascadia Subduction Zone ◽  
Mantle Structure ◽  
Data Set ◽  
Juan De Fuca Plate ◽  
Upper Mantle Structure ◽  
Juan De Fuca

Previous knowledge of the structure of the Cascadia subduction zone north of the Canada–United States border has been derived from a variety of geophysical studies that accurately delineated the downgoing Juan de Fuca plate from the offshore deformation front to depths of ~50–60 km beneath south-central Vancouver Island and the Georgia Strait. Little is known, however, of the structure of the Cascadia subduction zone farther westward and to greater depths in the upper mantle. We have assembled a set of some 1100 teleseismic traveltimes from events recorded on the Western Canadian Telemetered Network to augment a previously existing data set recorded on the Washington Regional Seismograph Network. The composite data set is inverted for upper mantle structure below Washington, Oregon, and southwestern British Columbia. We analyze the new northern portion of the model between 48.5–50°N and 118–127°W, which provides the first images of the deep slab structure in this region. The model is parameterized using splines under tension over a dense grid of knots. The nonlinearity of the inverse problem is treated by iteratively performing three-dimensional ray tracing and linear inversion. Resolution tests performed with a synthetic slab model indicate that the deep structure is resolved by the data north to at least 50°N. The inversions are characterized by a quasi-planar, high-velocity body inferred to represent the thermal and compositional anomaly of the subducted Juan de Fuca plate. This body exhibits velocity deviations of up to 3% from the background reference model and extends to depths of at least 400–500 km. The depth contours of the slab in the upper mantle mimic those of the shallow slab by changing strike, in the latitude range 48.0–48.5°N, from north–south in Washington to northwest–southeast in southern British Columbia. This forces the development of two arch-type structures: a main arch observed in previous studies trending east–west over Puget Sound and a possible second arch extending northeasterly from the Georgia Strait into the British Columbia interior. A steepening of the deep slab dip from British Columbia south towards Puget Sound and complexity in the evolution of the arches in depth may be the result of a change in plate motions at 3.5 Ma associated with the detachment of the Explorer plate.

Crustal structure of the Cascadia subduction zone, southwestern British Columbia, from potential field and seismic studies

1997 ◽  
Vol 34 (3) ◽  
pp. 317-335 ◽  
Author(s):  
Ron M. Clowes ◽  
David J. Baird ◽  
Sonya A. Dehler
Keyword(s):  
Subduction Zone ◽  
Potential Field ◽  
Vancouver Island ◽  
Strike Slip ◽  
High Density ◽  
Pacific Rim ◽  
Cascadia Subduction Zone ◽  
Crustal Material ◽  
Juan De Fuca Plate ◽  
Juan De Fuca

The northern Cascadia subduction zone is a region of convergence between the oceanic Explorer and northern Juan de Fuca plates and the continental North American plate. Potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density – magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2.5- and 3-dimensional interpretations. The lateral extent and thickness of the accreted wedge vary significantly along the zone. The narrow, metasedimentary Pacific Rim terrane lies immediately west of Wrangellia and extends the length of Vancouver Island, consistent with its emplacement by strike-slip faulting following the accretion of Wrangellia. The ophiolitic Crescent terrane is a narrow slice lying seaward of the Pacific Rim terrane but not extending northward beyond the Juan de Fuca plate. In this region, the Crescent terrane was emplaced in a strike-slip or obliquely convergent style during the latter stages of emplacement of Pacific Rim terrane. Below the accreted terranes, the Explorer plate is shallower than Juan de Fuca plate, resulting in a thinner crust. High-density lower crustal material lies beneath the western edge of Vancouver Island, supporting interpretations of wide-scale underplating of Wrangellia. The shape of the boundary region between Wrangellia and the Coast belt to the east varies along strike and may be controlled by properties of preexisting plutonic rocks. The low-density Coast belt plutons extend throughout most of the crust and are underlain by a lowermost crustal high-density layer, which may be indicative of fractionation accompanying magma generation.

Summary of Coastal Geologic Evidence for past Great Earthquakes at the Cascadia Subduction Zone

1995 ◽  
Vol 11 (1) ◽  
pp. 1-18 ◽  
Author(s):  
Brian F. Atwater ◽  
Alan R. Nelson ◽  
John J. Clague ◽  
Gary A. Carver ◽  
David K. Yamaguchi ◽  
...  
Keyword(s):  
British Columbia ◽  
North America ◽  
Subduction Zone ◽  
Forest Soils ◽  
Pacific Coast ◽  
Cascadia Subduction Zone ◽  
Level Change ◽  
The Past ◽  
The Pacific ◽  
Juan De Fuca

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

An assessment of the megathrust earthquake potential of the Cascadia subduction zone

1988 ◽  
Vol 25 (6) ◽  
pp. 844-852 ◽  
Author(s):  
Garry C. Rogers
Keyword(s):  
United States ◽  
Subduction Zone ◽  
Subduction Zones ◽  
Oceanic Lithosphere ◽  
The United States ◽  
Cascadia Subduction Zone ◽  
The South ◽  
The North ◽  
The Pacific ◽  
Juan De Fuca

The active tectonic setting of the southwest coast of Canada and the Pacific northwest coast of the United states is dominated by the Cascadia subduction zone. The zone can be divided into four segments where oceanic lithosphere is converging independently with the North American plate: the Winona and the Explorer segments in the north, the larger Juan de Fuca segment that extends into both Canada and the United States, and the Gorda segment in the south. The oceanic lithosphere entering the Cascadia subduction zone in all segments is extremely young, less than 10 Ma. Of the other six zones around the Pacific where young (< 20 Ma) lithosphere is being subducted, five have had major thrust earthquakes (megathrust events) on the subduction interface in historic time. An estimation based on potential area of rupture gives maximum possible earthquake magnitudes along the Cascadia subducting margin of 8.2 for the Winona segment, 8.5 for the Explorer segment, 9.1 for the Juan de Fuca segment, and 8.3 for the South Gorda segment. Repeat times for maximum earthquakes, based on the ratios of seismic slip to total slip observed in other subduction zones, are predicted to be up to several hundred years for each segment, well beyond recorded history of the west coast, which began about 1800. Thus the lack of historical seismicity information provides a few constraints on the assessment of the seismic potential of the subduction zone.

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