The Cascadia subduction zone, where the oceanic Juan de Fuca plate descends
beneath the overlying North American plate, extends 1100 km from northern California
to northern Vancouver Island. Sedimentary rocks originally deposited on the
oceanic plate are scraped off and accreted to the western edge of the continent;
the initiation of this process is indicated by the deformation front in Fig.
Figure 1: Map of the northern Cascadia subduction zone showing the distribution of earthquakes in the subducting Juan de Fuca plate and the locations of seismic cross-sections. Green dashed line shows azimuth onto which section in Fig. 3 is projected to create a representative section.
The intersection of the deformation front with the SW-trending Nootka transform fault, which is characterized by high levels of seismicity caused by relative motion between the Juan de Fuca plate and a small oceanic plate to the north, marks the approximate northern limit of the subduction zone.
Seismic hazard along the Cascadia margin is related to three different classes of earthquakes:
Numerous inslab and crustal earthquakes with magnitudes less than 4 occur each year in the Cascadia region. Beneath the western and eastern edges of Vancouver Island, earthquakes within the oceanic plate occur in two well-defined, ~50 km wide, geographic bands that merge further south. These earthquakes have been attributed to geochemical dehydration reactions that generate water, reducing the normal stresses across faults and allowing them to slip more easily.
West of Vancouver Island, seismic reflection surveys show that beneath the continental shelf the top of the subducting oceanic crust lies immediately beneath a seismic reflector that dips landward at between 12 and 25 km depth (JdF in Fig.2). This reflector is correlated with the top of the basaltic crust in the deep ocean basin, which there lies beneath horizontal sedimentary strata.
Figure 2: Migrated seismic section of line 85-01 extending from the deep ocean basin, across the deformation front to the west coast of Vancouver Island. JdF indicates reflections marking top of subducting plate, and is “pushed down” beneath the low velocity water layer.
Beneath southern Vancouver Island and its margins, seismic reflection surveys also identify two regionally extensive structures: a thick zone of reflectors, denoted by the letter E, and a deeper, likely individual, reflector, identified by the letter F. On a composite reflection section across the northern Cascadia forearc (Fig. 3a), the E zone appears as a sequence of anastomosing reflections that extends over 1.0 to 3.2 s (corresponding to a thickness of 3-10 km), dips at 4-15o, and reaches a depth of at least 45 km east of Vancouver Island. The second reflection, F, is observed intermittently, between ~0.5 and 2.0 s (~2-6 km) later than the deepest reflector of the E sequence. The short duration of this reflection implies that it originates either from a thin, <2 km thick, region, or at a single interface, and the F reflection is interpreted to arise at the top of the subducting plate.
The E reflections dip shallowly to the ENE at approximately 4o across southern Vancouver Island. As the reflections reach the eastern edge of the island, their dip increases sharply to ~15o and they continue into the uppermost mantle with this dip. In contrast, the F reflection from the top of the subducting plate dips to the ENE at ~15o near the western edge of Vancouver Island, and then flattens out further to the east. It is not possible to identify unequivocally the shorter F reflection segments near the east coast of Vancouver Island due to elevated levels of coherent noise; however, it seems unlikely that the F reflection would crosscut the E reflections. Therefore the F reflector must also increase in dip as it presumably merges into the deepest of the E reflectors near the eastern edge of Vancouver Island.
Figure 3: Composite seismic cross-section across the Cascadia forearc near southern Vancouver Island. a, Unmigrated section showing the reflection, JdF, from the top of the subducting Juan de Fuca plate west of Vancouver Island, the E reflection zone, and the F reflection which marks the top of the subducting Juan de Fuca plate further east. b, Migrated section superimposed on a display of P wave velocities and relocated earthquakes (Ramachandran, 2001). Oceanic Moho constrained by wide-angle reflections - blue; oceanic Moho assuming subducting crust is 6 km thick – dashed blue; top of subducting plate - yellow; deepest E reflection - green; shallowest E reflection – brown; Leech River fault – red. Vertical exaggeration is 1.5:1.
Thus identified, the E and F reflectors define a ~110 km-wide duplex structure: the F reflection, which marks the top of the subducting plate, is the floor thrust with a ramp-flat-ramp geometry, and the E reflectors form the roof thrust (Fig. 4).
Figure 4: Schematic cross-section of the northern Cascadia convergent margin at southern Vancouver Island. The plate boundary forms a thick duplex structure, within which crustal rocks are imbricated. Vertical exaggeration is 1.5:1.
Great earthquakes occur along the inter-plate boundary where it is locked, a region that probably extends 60 km east of the deformation front. Further east, relative motion between the two plates appears to take place through aseismic slow-slip events that last 2-3 weeks. The E reflectors are probably one location of the slow-slip events. Slow slip is also likely occurring along the floor thrust, which is inferred to be the top of the subducting plate.
Near the west coast of Vancouver Island, numerous earthquakes, with a variety of focal mechanisms, occur beneath the top of the subducting oceanic crust where it dips most steeply. As the top of the plate flattens, the number of earthquakes within the slab decreases markedly even though dehydration of the crust, which has been linked to earthquake occurrence, should be occurring here. As indicated in Fig. 1, there is another belt of inslab earthquakes near the east coast of Vancouver Island, and Fig. 2b shows that this belt corresponds to the location where the inferred dip of the subducting plate increases once more. It thus seems likely that the two belts of seismicity in the subducting slab are linked either to flexure or fragmentation of the oceanic plate as it subducts around the edges of the imbricated inter-plate boundary zone. Nevertheless fluids released by dehydration reactions must also be important in stressing the downgoing plate, and the deformation of the plate beneath Vancouver Island may simply provide the additional stress that triggers seismicity here.
Subduction is usually viewed as occurring along a single, thin interface beneath which an oceanic plate descends into the mantle, and inslab seismicity is considered to be driven by dehydration metamorphism. In the northernmost Cascadia subduction zone, however, the inter-plate boundary is a complex zone up to 16 km in vertical extent, and inslab seismicity is controlled by deformation in the upper part of the subducting plate.
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