Subduction

Earth, Planets and Space, 2001

At the Cascadia margin the Juan de Fuca plate is subducting beneath the North America plate, causing active seismicity within both plates. Earthquakes occur down to a maximum depth of 80 km within the descending oceanic plate and to about 30 km in the overriding continental plate. We use a method of seismic tomography to invert 28,230 P wave arrival times from 2666 local earthquakes that occurred in and around Vancouver Island from 1970 to 1990. The tomography model uses about 30 km horizontal and 12-19 km vertical grid spacing and assumes that the seismic velocity perturbations vary continuously between grid points. Velocity structures can be obtained to a depth of 65 km. The obtained tomographic image shows an extensive low velocity zone above the subducted slab at about 45 km depth and patches of low velocities at shallower depths just seaward of the volcanic front. The deeper extensive low velocity zone may indicate the presence of partially hydrated mantle, most likely serpentinite, as a result of slab dehydration associated with the transformation of metabasalt to eclogite. One of the shallow low velocity patches coincides with an abrupt increase in surface heat flow and may reflect the presence of partial melts or water in the crust.

This paper presents the first regional three-dimensional P wave velocity model for the Northern Cascadia Subduction Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and the fore-arc crust and upper mantle. Beneath southern Vancouver Island the megathrust above the Juan de Fuca plate is characterized by a broad zone (25–35 km depth) having relatively low velocities of 6.4–6.6 km/s. This relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath Vancouver Island, and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks. The rocks of the Olympic Subduction Complex are inferred to deform aseismically as evidenced by the lack of earthquakes within the low-velocity rocks. The fore-arc upper mantle beneath the Strait of Georgia and Puget Sound is characterized by velocities of 7.2–7.6 km/s. Such low velocities represent regional serpentinization of the upper fore-arc mantle and provide evidence for slab dewatering and densification. Tertiary sedimentary basins in the Strait of Georgia and Puget Lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking. In contrast, sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent Terrane. The correlation of in-slab earthquake hypocenters M > 4 with P wave velocities greater than 7.8 km/s at the hypocenters suggests that they originate near the oceanic Moho of the subducting Juan de Fuca plate.

Giant earthquakes have repeatedly ruptured the Cascadia subduction zone, and similar earthquakes will likely also occur there in the near future. We employ a 3-D time-dependent thermomechanical model that incorporates an up-to-date description of the slab geometry to study the Cascadia subduction thrust. Results show a distinct band of 3-D slab dehydration that extends from Vancouver Island to the Seattle Basin and farther southward to the Klamath Mountains in northern California, where episodic tremors cluster. This distribution appears to include a region of increased dehydration in northern Cascadia. The phenomenon of heterogeneous megathrust seismicity associated with oblique subduction suggests that the presence of fluid-rich interfaces generated by slab dehydration favors megathrust seismogenesis in the northern part of this zone. The thin, relatively weakly metamorphosed Explorer, Juan de Fuca, and Gorda Plates are associated with an anomalous lack of thrust earthquakes, and metamorphism that occurs at temperatures of 500-700°C near the Moho discontinuity may represent a key factor in explaining the presence of the associated episodic tremor and slip (ETS), which requires a young oceanic plate to subduct at a small dip angle, as is the case in Cascadia and southwestern Japan. The 3-D intraslab dehydration distribution suggests that the metamorphosed plate environment is more complex than had previously been believed, despite the existence of channeling vein networks. Slab amphibolization and eclogitization near the continental Moho depth is thus inferred to account for the resultant overpressurization at the interface, facilitating the generation of ETS and the occurrence of small to medium thrust earthquakes beneath Cascadia.

Geoscience Letters, 2021

Several tectonic processes combine to produce the crustal deformation observed across the Cascadia margin: (1) Cascadia subduction, (2) the northward propagation of the Mendocino Triple Junction (MTJ), (3) the translation of the Sierra Nevada-Great Valley (SNGV) block along the Eastern California Shear Zone-Walker Lane and, (3) extension in the northwestern Basin and Range, east of the Cascade Arc. The superposition of deformation associated with these processes produces the present-day GPS velocity field. North of ~ 45° N observed crustal displacements are consistent with inter-seismic subduction coupling. South of ~ 45° N, NNW-directed crustal shortening produced by the Mendocino crustal conveyor (MCC) and deformation associated with SNGV-block motion overprint the NE-directed Cascadia subduction coupling signal. Embedded in this overall pattern of crustal deformation is the rigid translation of the Klamath terrane, bounded on its north and west by localized zones of deformation. Since the MCC and SNGV processes migrate northward, their impact on the crustal deformation in southern Cascadia is a relatively recent phenomenon, since ~ 2 -3 Ma.

Earth and Planetary Science Letters, 2010

Mantle flow associated with the Cascadia subduction zone and the Mendocino Triple Junction is poorly characterized due to a lack of shear wave splitting studies compared to other subduction zones. To fill this gap data was obtained from the Mendocino and FACES seismic networks that cover the region with dense station spacing. Over a period of 11-18 months, 50 suitable events were identified from which shear wave splitting parameters were calculated. Here we present stacked splitting results at 63 of the stations. The splitting pattern is uniform trench normal (N67°E) throughout Cascadia with an average delay time of 1.25 s. This is consistent with subduction and our preferred interpretation is entrained mantle flow beneath the slab. The observed pattern and interpretation have implications for mantle dynamics that are unique to Cascadia compared to other subduction zones worldwide. The uniform splitting pattern seen throughout Cascadia ends at the triple junction where the fast directions rotate almost 90°. Immediately south of the triple junction the fast direction rotates from NW-SE near the coast to NE-SW in northeastern California. This rotation beneath northern California is consistent with flow around the southern edge of the subducting Gorda slab.

Tectonophysics, 1998

In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was conducted aboard the German R=V Sonne. This cooperative experiment by GEOMAR and the USGS acquired wide-angle reflection and refraction seismic data, using ocean-bottom seismometers (OBS) and hydrophones (OBH), and multichannel seismic reflection (MCS) data. The main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the Cascadia subduction zone and to image the downgoing plate. Coincident MCS and wide-angle profiles along two tracks are presented here. The plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at ca. 13 km depth. Thus, the downgoing plate dips more shallowly than previously assumed. The dip of the plate changes from 2º to 4º at the eastern boundary of the wedge on the northern profile, where approximately 3 km of sediment is entering the subduction zone. On the southern profile, where the incoming sedimentary section is about 2.2 km thick, the plate dips about 0.5º to 1.5º near the deformation front and increases to 3.5º further landwards. On both profiles, the deformation of the accretionary wedge has produced six ridges on the seafloor, three of which represent active faulting, as indicated by growth folding. The ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement. Thus the entire incoming sediment package is being accreted. At least two phases of accretion are evident, and the rocks of the older accretionary phase(s) forms the backstop for the younger phase, which started around 1.5 Ma ago. This documents that the 30 to 50 km wide frontal part of the accretionary wedge, which is characterized by landward vergent thrusts, is a Pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods. Velocities increase quite rapidly within the wedge, both landward and downward. At the toe of the deformation front, velocities are higher than 4.0 km=s, indicating extensive dewatering of deep, oceanic sediment. Further landward, considerable velocity variation is found, which indicates major breaks throughout the accretionary history.

Journal of Geophysical Research, 1992

The 1982 MAGMA seismic refraction experiment yielded a large set of accurate P wave travel times corrected for bathymetry and anisotropy which sample the structure of the East Pacific Rise (EPR) at 12ø50'N. The arrivals were recorded using ocean bottom seismographs at three sites: on axis, and at 7 km and 16 km east of the axis. We invert 2320 travel times for the twodimensional crustal seismic velocity structure across the EPR axis, assuming that the velocity structure is invariant along strike. The travel times provide strong evidence for compressional velocity auisotropy in the upper crust corresponding to ~10% faster velocities for propagation parallel to the axis than perpendicular to the axis; the travel times used for the tomography are corrected for the effects of this azimuthal anisotropy. Our preferred model contains only the structure dearly required by the data (structure which is stable under excessive smoothing) and achieves a variance reduction of 81% relative to the laterally homogeneous starting model. We resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0.4 to 0.7 krn/s; this low-velocity zone (LVZ) is about 7 km wide and extends from a depth of about 1.5-2.0 km down to Moho at a depth of 5.5 kin. The LVZ is slightly asymmetric, extending 1 km further to the east than to the west of the axis. In the shallow (<1.0 km depth) crust, a pattern of velocity variations is imaged in which velocities are high at the spreading axis, decrease between 3 krn and 7 km east of the axis, and then increase again between 10 and 15 km east of the axis. We investigate the resolution of the inversion using an impulse response method; the LVZ and off-axis upper crustal variations are well resolved. In addition, the travel time data indicate that an axial high-velocity anomaly less than 2 krn wide exists in the upper crust but is not resolved by the inversion. The small velocity reductions of the LVZ are consistent with hot rock containing only a small quantity of melt. These results, combined with the multichannel seismic reflection lines and expanded spread profiles from the northern EPR, suggest that the zone of high melt fraction under the spreading center is confined to a narrow, thin lens capping a broad zone of hot plutonic rocks. The upper crustal velocity reduction within 1 km of the axis reflects near-axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near-axial tectonism; the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration.

2021

The Cascadia subduction zone (CSZ) has a high potential for an inevitable and devastating megathrust earthquake. This margin is characterized by a complex seismicity pattern. Particularly in Oregon, there is a seismically quiescent zone bounded by high seismicity regions to the north and south. To comprehend these variations in seismicity, it is important to study the differences in crustal architectures and physical properties (densities and magnetic susceptibilities) along the CSZ. The primary objectives are to develop two plate-scale 2D integrated models through different seismicity zones and to map major tectonic structures from filtered potential fields. The Juan de Fuca oceanic crust requires a number of lower density zones with respect to adjacent oceanic crust to fit gravity data. These zones correlate to previously identified propagator wakes that are formed during spreading ridge propagation and mapped from disturbances of seafloor magnetic stripes. However, this correlati...

Geophysical Research Letters, 1984

We determined the variations in seismic structure beneath southern California by using a tomographic method of inversion on teleseismic P delays recorded with the Southern California Array. The algorithm employed was a modified form of an Algebraic Reconstruction Technique (ART) used in medical X-ray imaging. Deconvolution with an empirically estimated point spread function was also used to help in focusing the image.

Earth and Planetary Science Letters

In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40-70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the "plate boundary" as a characteristic "velocity-jump" at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.