John F. Cassidy

Evidence for both crustal and mantle earthquakes in the subducting Juan de Fuca plate

[1] We investigate the relationship between high-precision hypocentres in the subducting Juan de Fuca plate beneath southwest British Columbia, and the velocity structure from receiver function analysis and reflection seismic data. 108 earthquakes at depths of 40 to 65 km were relocated using a double-difference algorithm to obtain precise earthquake locations. Correlation of the relocated seismicity with structural information shows a concentration of earthquakes near the top of the subducting oceanic crust, and a deeper layer of seismicity in the uppermost mantle of the subducting Juan de Fuca plate. The strong correlation of the seismicity with the structure of the subducting plate suggests that seismic failure is caused by changes in mechanical strength within the plate, supporting the hypotheses that phase transformations and thermal-petrological conditions play an important role in the seismogenesis of double-seismic zones. INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; 7220 Seismology: Oceanic crust. Citation: Cassidy, J. F., and F. Waldhauser, Evidence for both crustal and mantle earthquakes in the subducting Juan de Fuca plate, Geophys. Res. Lett., 30(2), 1095, doi:10.1029/2002GL015511, 2003.

A regional study of shear wave splitting above the Cascadia Subduction Zone: Margin‐parallel crustal stress

Recordings of local earthquakes from 16 three-component broadband seismic stations in southwestern British Columbia, Washington, and northern Oregon are used to study regional variations of shear wave anisotropy in the North American plate above the subducting Juan de Fuca plate. There is evidence for shear wave splitting at all sites, with good agreement of fast polarization directions and travel time delays at adjacent stations. Most stations exhibit fast directions parallel to the strike of the margin, with anisotropy of 1–2%. These fast polarization directions are consistent with earthquake focal mechanisms and borehole stress studies, indicating that the observed anisotropy is likely due to crustal stresses (i.e., extensive dilatancy anisotropy theory). The margin-parallel stresses may be due to oblique subduction of the Juan de Fuca plate. However, at the station closest to the coast (OZB), the fast direction shows a more margin-normal orientation that may be associated with the proximity of the locked portion of the underlying subduction thrust fault.

Structure, seismicity, and thermal regime of the Queen Charlotte Transform Margin

[1]xa0This study examines structural, seismicity, thermal, and current deformation constraints on the tectonic regime of the Queen Charlotte Margin located off the northwest coast of Canada. The margin is primarily a transcurrent boundary between the oceanic Pacific Plate and the continental North America Plate, but plate motion models indicate ∼20° of current oblique convergence that initiated ∼5 Myr. The estimated 120 km of convergence has been variously explained as accommodated by underthrusting or by crustal shortening. The near-vertical Queen Charlotte transcurrent fault and lack of Wadati-Benioff earthquakes argue against underthrusting. However, structural models for the margin, from receiver function and refraction studies, show little evidence of crustal thickening. The pattern of landward decreasing heat flow, gravity anomalies, an accretionary prism landward of the trench or trough, down-bowing ocean crust and sediments horizons, and a variety of other geophysical evidence support underthrusting. Receiver function analysis on teleseismic data indicates a 10 km thick low-velocity layer at 35 km depth, dipping 20° to the east, which is interpreted as underthrusting oceanic crust. The heat flow across the fault zone rapidly decreases landward, in agreement with predictions from a finite element thermal model of underthrusting.

Mapping crustal stress and strain in southwest British Columbia

[1]xa0This paper investigates the orientation and sources of stress in the forearc of the Cascadia subduction zone in southwest British Columbia, using Bayesian inversion results from focal mechanism data and comparing results with GPS derived short-term strain rates. The subduction margin in this region includes a change in orientation from N-S in Washington State to NW-SE in British Columbia. Over 1000 focal mechanisms from North American crustal earthquakes have been calculated to identify the dominant style of faulting, and ∼600 were inverted to estimate the principal stress orientations and the stress ratio. Our results indicate the maximum horizontal compressive stress orientation changes with distance to the trench, from approximately margin-normal along the coast to approximately margin-parallel 100–150 km inland from the coast. Comparing stress orientations with GPS data, we relate the margin-normal stress direction to subduction-related strain rates due to the locked interface between the North American and Juan de Fuca plates just west of Vancouver Island. Further from the margin the plates are coupled less strongly, and the margin-parallel maximum horizontal compressive stress in the North American Plate relates to the northward push of the Oregon Block, which is also observed in the horizontal shortening direction of the residual strain rates, after the subduction component is removed.

Seismic hazard in western Canada from GPS strain rates versus earthquake catalog

[1]xa0Probabilistic seismic hazard analyses (PSHA) are commonly based on frequency - magnitude statistics from 50–100 yearlong earthquake catalogs, assuming that these statistics are representative of the longer-term frequency of large earthquakes. We test an alternative PSHA approach in continental western Canada, including adjacent areas of northwestern U.S.A., using regional strain rates derived from 179 Global Positioning System (GPS) horizontal velocities. GPS strain rates are converted to earthquake statistics, seismic moment rates, and ground shaking probabilities in seismic source zones using a logic-tree method for uncertainty propagation. Median GPS-based moment rates and shaking estimates agree well with those derived from earthquake catalogs in only two zones (Puget Sound and mid-Vancouver Island). In most other zones, median GPS-based moment rates are 6–150 times larger than those derived from earthquake catalogs (shaking estimates 2–5 times larger), although the GPS-based and catalog estimates commonly agree within their 67% uncertainties. This discrepancy may represent an under-sampling of long-term moment rates and shaking by earthquake catalogs in some zones; however a systematic under-sampling is unlikely over our entire study area. Although not demonstrated with a high confidence level, long-term regional aseismic deformation may account for a significant part of the GPS/catalog discrepancy and, in some areas, represent as much as 90% of the total deformation budget. In order to integrate GPS strain rates in PSHA models, seismic versus aseismic partitioning of long-term deformation needs to be quantified and understood in terms of the underlying mechanical processes.

Effects of metamorphic crustal densification on earthquake size in warm slabs

[1]xa0Some recent damaging earthquakes occurred in the lower crust or mantle of warm subducting slabs. They are consistent with a theoretical prediction that larger events tend to be deeper inside the slab as a result of mechanical damage to the crust caused by metamorphic rock densification. The densification begins in a thin layer along the slab surface, inducing a stretching force in it. Fracture spacing scales with layer thickness, resulting in a “shattered” upper crust in which earthquake ruptures have limited propagation distance. In contrast, the more uniform untransformed substrata can host larger ruptures. Often, the lack of compression in warm-slab mantle is also consistent with a shattered crust.

Upper Crustal Investigation of the Gulf of Saint Lawrence Region, Eastern Canada Using Ambient Noise Tomography

We studied the 3-D shear-wave velocity (Vs) structure in the Gulf of St. Lawrence (GSL) and adjacent onshore areas to 20 km depth by inverting Rayleigh-wave dispersion extracted from the vertical components of continuous ambient seismic noise waveforms. The region is divided into three broad zones based on their Vs characteristics. In the northwest, the Grenville Province (i.e., the exposed edge of predominantly Middle-Proterozoic Laurentian crust) is dominated by high-Vs, except for well-known anorthosite sites, which are characterized by relatively lower-Vs. In contrast, the central segment of the GSL region corresponds to a belt with generally low-Vs at upper-crustal levels. In the southeastern part of the GSL, prominent low-Vs in the uppermost-crust are found to coincide with locations of subsidiary basins of the Canadian Maritime Basin, while higher-Vs characterize the accreted Appalachian terranes where they are exposed on land. The Grenville Province is wedged out at depth by the Red Indian Line, which is the suture between composite-Laurentia and peri-Godwanan Ganderia in the Canadian Appalachians. The geometry and Vs characteristics of the south-easternmost peri-Gondwanan terranes of Avalonia and Meguma, suggest that they may be fully or partially structurally overlying a basement with distinct seismic characteristics, which could be a vestige of the West-African craton that was under-thrust beneath composite-Laurentia during the terminal Alleghenian continent-continent collision. In the middle of the GSL, the 3-D geometry of the Canadian Maritimes sedimentary basins overlying the Appalachian terranes shows that the depth to the top of basement is in excess of 8 km.