Precise measurements help gauge Pacific Northwest\u27s Earthquake potential

Except for the recent rumblings of a few moderate earthquakes and the eruption of Mt. St. Helen\u27s, all has been relatively quiet on the Pacific Northwestern front. The Cascades region in the Pacific Northwest, a sporadically active earthquake and volcanic zone, still has great seismic potential [Atwater, 1987], as comparisons with other subduction zones around the world have shown [Heaton and Kanamori, 1984]. Recent tsunami propagation models [Satake, 1996] and tree ring studies suggest that the last great Cascadia earthquake occurred in the winter of 1700 A.D. and had a magnitude of −8.9. The North Cascades or Wenatchee earthquake followed in 1872. With an estimated magnitude greater than 7, it was the largest earthquake in the written history of Washington and Oregon

GPS-determination of along-strike variation in Cascadia margin kinematics: Implications for relative plate motion, Subduction zone coupling, and permanent deformation

High‐precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along‐strike variation in Cascadia margin kinematics. These results constrain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of magnitude longer than the decades‐long history of seismic monitoring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station velocities along the coast. After correction for a simple elastic dislocation model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3–7 mm/yr) to northern and central Oregon (∼9 mm/yr), consistent with oblique convergence and geologic constraints on permanent deformation. The margin‐parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north‐westward velocities reflect the influence of Pacific‐North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin

The October 2012 magnitude (Mw) 7.8 earthquake offshore Haida Gwaii, Canada

Alison L. Bird et al. report on the Mw 7.8 earthquake offshore Haida Gwaii, Canada, from 2012 for the Summary of the Bulletin of the International Seismological Centre

Geomagnetic depth-sounding profile across central British Columbia

Geomagnetic depth-sounding was carried out in a large-spaced profile across central British Columbia in order to map the conductivity structure of the crust and upper mantle in the central Canadian Cordillera. Geomagnetic variographs were set up from east of Jasper to Prince Rupert in two successive east-west profiles during the summer of 1969. Numerical analysis of geomagnetic storm activity indicates that the discontinuity in the attenuation of the vertical magnetic field, as first reported for south-eastern British Columbia by Hyndman (1963), is located in the area of the Rocky Mountain Trench. All stations to the west exhibit typical 'low Z' characteristics and no or little anomalous induction; stations to the east of the trench display a strong, high-frequency Z-variation content as well as anomalous field enhancement. Power spectral and polarization analyses show a first order agreement with the two-dimensional conductivity structure model proposed by Caner (1970) for south-western Canada. Second-order effects suggest a more complex model consisting of two conductivity discontinuities: One shallow structure strikes roughly NW-SE at a depth of 10 to 15 km. and may be associated with the 'edge' of a hydrated layer located at the western front of the Rocky Mountains; a second much deeper structure, trending approximately E-W, is located south of Kootenay Lake and is possibly associated with a strike-slip feature in the upper mantle (Lajoie and Caner, 1970).Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat

GPS deformation in a region of high crustal seismicity: N. Cascadia forearc

We estimate the rate of crustal deformation in the central and northern Cascadia forearc based on a combination of existing global positioning system (GPS) velocity data along the Cascadia subduction zone. GPS strain rates and velocities show that the northwestern Washington–southwestern British Columbia region is currently shortening at 3–3.5 mm yr−1 in a N–S direction, in good agreement with inference from crustal earthquake statistics. On the long-term, the shortening rate is 5–6 mm yr−1, providing that the subduction-related interseismic loading of the margin is purely elastic. Compared to the velocity of the Oregon forearc with respect to North America (∼7 mm yr−1), this indicates that most of the forearc motion is accommodated in the Puget–Georgia basin area, corresponding to the main concentration of crustal seismicity. The difference between the current and long-term shortening rates may be taken up during subduction megathrust earthquakes. Thus, these events could produce a sudden increase of N–S compression in the Puget sound region and could trigger major Seattle-fault-type crustal earthquakes

Field Studies Target 2012 Haida Gwaii Earthquake

At 8:04 P.M. Pacific daylight time (PDT) on 27 October 2012 (03:04 universal time (UT), 28 October), Canada's second largest instrumentally recorded earthquake rocked Haida Gwaii (formerly Queen Charlotte Islands) and the mainland coast of British Columbia. The M 7.7 event off the west coast of Moresby Island caused a tsunami with local runup of more than 7 meters and amplitudes up to 0.8 meter on tide gauges 4000 kilometers away in Hawaii. Shaking was felt as far away as the Yukon, Alberta, Washington, and Montana, up to 1500 kilometers away. Little damage was caused, as the immediate region is an uninhabited National Park Reserve. The closest point of the rupture zone, as defined by aftershocks (Figures 1a and 1c), was 50 kilometers from the nearest community, Queen Charlotte, where damage was confined to a few chimneys and slumped roads