Herb Dragert

A wide depth distribution of seismic tremors along the northern Cascadia margin

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as Mw = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements—in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface—are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.

Geodetic and seismic signatures of episodic tremor and slip in the northern Cascadia subduction zone

Slip events with an average duration of about 10 days and effective total slip displacements of severalc entimetres have been detected on the deeper (25 to 45 km) part of the northern Cascadia subduction zone interface by observing transient surface deformation on a network of continuously recording Global Positioning System (GPS) sites. The slip events occur down-dip from the currently locked, seismogenic portion of the subduction zone, and, for the geographic region around Victoria, British Columbia, repeat at 13 to 16 month intervals. These episodes of slip are accompanied by distinct, low-frequency tremors, similar to those reported in the forearc region of southern Japan. Although the processes which generate this phenomenon of episodic tremor and slip (ETS) are not well understood, it is possible that the ETS zone may constrain the landward extent of megathrust rupture, and conceivable that an ETS event could precede the next great thrust earthquake.

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 con- strain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of mag- nitude longer than the decades-long history of seismic moni- toring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station veloci- ties along the coast. After correction for a simple elastic dis- location 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), con- sistent 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.

Current deformation and the width of the seismogenic zone of the northern Cascadia subduction thrust

Evidence has been obtained for the accumulation of elastic strain across the northern Cascadia subduction zone that may be released in a future very large subduction thrust earthquake. Vertical and horizontal strain rates across the southern Vancouver Island region have been determined through (1) long-term trends in tide gauge data, (2) changes in repeated accurate leveling surveys, (3) changes in repeated high-accuracy gravity profiles, and (4) horizontal shortening observed in repeated precise positioning surveys. The outer coast is uplifting at a rate of a few millimeters per year decreasing landward, and shortening is occurring across the 100-km-wide coastal region at a rate of about 0.1 microstrain per year (mm km−1yr−1). The results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault. The location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data. Most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern Vancouver Island and northern Washington, provided the locked portion is restricted to a 60-km-wide band offshore beneath the continental shelf and slope. This conclusion also results from modeling the coseismic subsidence on the outer coast of Vancouver Island about 300 years ago deduced from paleoseismicity data. The unusually narrow downdip extent of the subduction thrust seismogenic zone, that extends little if at all beneath the coast, is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments. The high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km. The distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of Vancouver and Victoria from this source. The narrow width may also limit the earthquake size; however, events of magnitude well over 8 are possible.

Spatial‐temporal patterns of seismic tremors in northern Cascadia

[1] We study in detail the two consecutive episodic tremor-and-slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source-Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30-km and 50-km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50-55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high-frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

Three-dimensional viscoelastic interseismic deformation model for the Cascadia subduction zone

Contemporary deformation of the Cascadia forearc consists of an elastic interseismic strain build-up as part of the subduction earthquake deformation “cycle” anda secular deformation primarily in the form ofarc-parallel translation and clockwise rotationofforearc blocks. Athree-dimensional (3-D) elastic dislocation model, constrainedby vertical deformation data, was developed previously to study the interseismic deformation. In this study, we develop a 3-D viscoelastic finite element model for the Cascadia subduction zone to study the temporal and spatial variations of interseismic deformation, and we compare the model results primarily with horizontal geodetic deformation observations. The model has an elastic lithosphere/slab and a viscoelastic mantle which has a viscosity of 1019 Pa s as constrained by recent postglacial rebound analyses. For comparison, we adopt a seismogenic zone geometry that was used in the previous elastic dislocation model, and we test the effects of different estimates of relative plate motion on the model predictions. Interseismic deformation is simulated by assigning a backslip rate to the locked zone of the subduction fault, preceded by an earthquake rupture of the same zone. Based on preliminary model results, we draw the following conclusions: (1) The deformation rate decreases through the interseismic period. A seaward motion is predicted for inland sites early in the interseismic period, an effect of postseismic creep of the mantle. (2) Model strain rates 300 years after the earthquake are consistent with the observed values, regardless of the plate motion models used. The horizontal velocities in northern Cascadia decrease landward at a slower rate than predicted by the elastic dislocation model, providing a better fit to observations. (3) Oblique subduction causes strain partitioning. As a result, the direction of local maximum contraction is much less oblique than plate convergence. The northerly direction of the GPS velocities in southern Cascadia represent a northward translation of the forearc. The secular deformation of the forearc may be partially accommodated through earthquake deformation cycles, but it may be better modeled as a process independent of the earthquake cycle.

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 31 in a N^S direction, in good agreement with inference from crustal earthquake statistics. On the longterm, the shortening rate is 5^6 mm yr 31 , 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 (V 7m m yr 31 ), 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. Published by Elsevier Science B.V.

Continuous GPS monitoring of elastic strain in the Northern Cascadia Subduction Zone

Previous monitoring and modeling of crustal deformation across the northern Cascadia margin at Vancouver Is. has provided strong evidence that the subduction thrust fault is locked and may generate future great earthquakes. The recent establishment of the Western Canada Deformation Array (WCDA), a network of continuous GPS trackers in southwestern British Columbia, provides a new tool for monitoring crustal strain and thereby helps to assess the earthquake hazard in this region. 17 months of continuous data from the three longest running WCDA sites (Penticton, Victoria, Holberg) indicate: 1.) A 7 mm/yr easterly motion of Victoria relative to Penticton. Victoria is located in the forearc at the southeastern end of Vancouver Is., 230 km from trench axis, whereas Penticton is located behind the arc, 520 km from the trench axis, and assumed fixed on the stable North American plate; 2.) A 3 mm/yr northwesterly motion and 10 mm/yr uplift (with a large uncertainty) of Holberg relative to Penticton. Holberg is located on the northern-most part of Vancouver Is., 50 km from the trench and near the plate triple junction; and 3.) The presence of probably seasonal variations in the apparent relative positions of the two coastal sites with respect to Penticton. The landward motion of Victoria agrees with the deformation predicted by elastic dislocation models of the interseismic period for a great subduction-thrust earthquake as constrained by previous deformation data. Although not as well resolved, the motion of Holberg, roughly parallel to the margin, is significantly different and is inconsistent with simple subduction models. The non-linear variations in measured position are dominated by an annual period probably resulting from seasonal biases in the precise orbit estimates or in the modeling of tropospheric delays or tidal effects.

Tectonic deformation in western Washington from continuous GPS measurements

The analysis of 3 years of continuous data from 7 permanent GPS stations along the western Washington section of the Cascadia Subduction Zone indicates that the direction of the observed horizontal velocities (with respect to station DRAO, nominally representing the stable North American continent) is roughly parallel to the relative plate convergence direction of the Juan de Fuca and North America plates and that their magnitude decreases away from the trench. Most of this deformation can be attributed to inter-seismic strain accumulation due to the locking of the thrust interface. When the dislocation model predictions are subtracted from the observed velocities, there is evidence for an additional N-S oriented contraction at a rate of ∼4 mm/yr over a distance of 250 km. This signal presumably represents a more long-term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and is most likely related to the occurrence of shallow earthquakes in western Washington that are characterized by predominantly N-S oriented maximum principal stress.

Crustal uplift and sea level rise in northern Cascadia from GPS, absolute gravity, and tide gauge data

We combine data from nine GPS, absolute gravity, and tide gauge stations to estimate the relation between sea-level rise, vertical motion, and solid Earth processes in the Pacific Northwest. GPS vertical velocities (in ITRF2000) and absolute gravity rates are well correlated, with a gradient of 0.2 ±0.1 μGal mm -1 , but show a significant offset of 0.53 ± 0.30 μGal yr -1 (2.2 ± 1.3 mm yr -1 ) (95% confidence). Tide gauge and GPS data indicate a northeast Pacific regional sea-level rise of 1.7 ± 0.5 mm yr -1 , aligned to ITRF2000, or an unlikely regional sea-level fall of -0.5 ± 0.5 mm yr -1 , aligned to absolute gravity. Although we cannot rule out a bias in the GPS reference-frame alignment, our results suggest a possible absolute gravity bias by a long-period mass increase from an unknown near-surface or deep-seated source. The impact of such a mass increase on gravity, vertical motion, and sea level remains to be defined.