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The Cascadia Subduction Zone and Related Seismicity.Pptx THE CASCADIA SUBDUCTION ZONE AND The Cascadia RELATED SUBDUCTIONSubducon SYSTEMS Zone And Related Seismicity Seismic Structure, Intraslab Earthquakes and Processes, and Earthquake Hazards Editors: Stephen Kirby, Kelin Wang and Susan Dunlop Cheng Cheng 13/04/2012 U.S. Geological Survey Open-File Report 02–328 Geological Survey of Canada Open File 4350 August 2002 Tectonic Framework “warm-slab” Explorer plate Juan de fuca Plate Gorda plate Klamath Mountains Siletzia Terrane (43°N) Crescent Formaon Olympic Peninsula (47°N) PorriV et al., 2011 18 | Wells, Blakely and Weaver: Cascadia microplate models and within-slab earthquakes Hyndman and Wang, 1995], and the inferred narrow locked zone is entirely offshore, apparently following the western limit of the accreted Siletzia mafic terrane. Out- board of the Siletz terrane, sediments of the Eocene to Quaternary accretionary wedge and marginal basin com- plex comprise the geodetically and thermally defined locked zone along the plate boundary megathrust [Hyndman and Wang, 1995]. McNeill et al. [1998; 2000] and Wells et al. [2000] have suggested that the accre- tionary-marginal basin complex is subdivided by a vari- ety of oblique, upper plate folds and faults that may in part be related to large-scale seismic segmentation of the subduction zone. The Oregon segment, which is characterized by a narrow locked zone and our calcu- lated slower convergence rates, coincides with the re- gion of low crustal and slab seismicity and may indicate lower strains inboard of the narrow locked zone. Slower OC-SN inferred convergence rates (~30 mm/yr) in central pole Cascadia are still consistent with 500 year great earth- quake recurrence intervals, assuming 15 m average slip per event is typical, as was calculated from the 1700 AD tsunami waveforms [Satake and Wang, 2000]. Contemporary seismicity in Cascadia is concentrated in western Washington, Vancouver Island and northern California (Figure 2). The crustal earthquakes and active faults outline a western Oregon region that has very low FIGURE 1: Velocity field for Oregon forearcWells et al., 2002 microplate calculated rates of seismicity and internal deformation. Although from OC–NA pole (from Wells and Simpson, [2001], modified from internal deformation rates are low, paleomagnetic rota- (leY) mo.ons relave to NA Wells et al., [1998]).Clockwise Rotaon Oregon block (OC) rotating at Neogene tion rates are high, consistent with its rotation as a large, paleomagnetic rate is linked to Sierra Nevada (SN) block moving semi-rigid block, as discussed above. Focal mechanisms at vlbi rate by Rollback Euler pole (OC–SN) in Klamath Mountains. Exten- in the forearc indicate north–south compression and are sional arc forms along trailing edge of Oregon forearc block which consistent with north–south shortening against the Ca- absorbs Sierra Nevada displacement by rotating over trench. North end of Oregon block deforms Washington forearc against nadian Coast Mountains re-entrant. Canadian re-entrant in the margin, causing north–south compres- On the trailing edge of the rotating Oregon forearc sion, uplift, thrust faulting and forearc earthquakes. Rates from block, the extensional Cascade arc accommodates some very long baseline interferometry (vlbi), paleoseismology (ps), and of the westward motion through magmatism and nor- magmatic spreading (m). mal faulting (Figure 2). The extensional arc ends near the latitude of the calculated pole of rotation and the sparse arc volcanoes to the north rest on a folded and the maximum convergence along the northern and south- uplifted basement (Figure 2c). Compared to some other ern Cascadia subduction zone. These variations in con- circum-Pacific arcs, Cascade volcanic production rates vergence could affect earthquake magnitude-recurrence are modest [Sherrod and Smith, 1990]. The arc is pro- intervals along the subduction zone and within the slab. ducing two major end member primitive basalts which reflect variable input of slab volatile component and Seismicity and volcanism mantle sources: 1) Low potassium olivine tholeiite Great subduction zone earthquakes have occurred re- (LKOT), which indicates hot dry melting of depleted peatedly along the plate boundary in the recent geo- upper mantle; and 2) more fluid-rich calc-alkaline ba- logic past [e.g., Atwater and Hemphill-Haley, 1997], but salt (CAB; Bacon et al., [1997]; Conrey et al., [1997]). the subduction zone is presently seismically quiet. Paleo- LKOT dry melts are associated with rifting and are com- seismic evidence indicates that the entire plate bound- mon in the basin and range and in the extensional arc as ary has ruptured in the past, although geodetic evidence far north as Mount St. Helens [Conrey et al., 1997]. These suggests that the width of the plate boundary locked zone magma types and modest volumes are consistent with may be much narrower off Oregon. The thick mafic core subduction of a young, warm slab [Kirby et al., 1996], of the rotating Oregon block [Tréhu et al., 1994] has the but it is worth noting that the largest slab earthquakes lowest uplift rate along the coast [Mitchell et al., 1994; occur north of Mount St. Helens and the LKOT-bearing 20 | Wells, Blakely and Weaver: Cascadia microplate models and within-slab earthquakes 20 | Wells, Blakely and Weaver: Cascadia microplate models and within-slab earthquakes Convergence Rate.. FIGURE 3: Cascadia convergence velocities with respect to the forearc. North of 47°N, calculated from the JDF–NA pole of DeMets and Dixon [1999]; south of 47°N, from the JDF–OC pole of Wells and Simpson [2001]. Note that convergence rate is low offshore of Oregon. TABLE 1: Convergence velocities of the Juan de Fuca plate (JDF) with respect to North America (NA) and Oregon Coastal block (OC). Site JDF-NA (DD99) JDF-OC (WS00) (Lat ° N/Lon ° W) Az ° E rate mm/yr Az ° E rate mm/yr 40/125 72.7 35.0 41/125 45.6 28.7 72.1 34.0 42/125 47.6 29.8 71.6 33.0 43/125 49.4 30.9 71.0 32.0 44/125 51.1 32 70.3 31.0 45/125 52.7 33.1 69.6 29.9 46/125.5 52.1 34.9 69.2 28.8 FIGURE 3: Cascadia convergence velocities with respect to the forearc. North of 47°N, calculated from the JDF–NA47/126 pole of DeMets and 53.5 36 68.7 27.6 Dixon [1999]; south of 47°N, from the JDF–OC pole of Wells and Simpson [2001]. Note that convergence rate is low offshore of Oregon. 48/126 54.8 37.2 67.9 26.6 Rotaon of the Oregon 49/127 54.2 38.9 67.7 25.2 TABLE 1: Convergence velocities of the Juan de Fuca plate (JDF) with respect to North America (NA) and Oregon Coastal block (OC). forearc significantly 50/127 53.5 40.5 66.7 24.2 Site JDF-NA (DD99) JDF-OC (WS00) Note: DD 99 is pole of DeMets and Dixon (1999); WS00 is pole of Wells and Simpson, [2001]; (Lat N/Lon W) Az E rate mm/yr Az E rate mm/yr ° affects Juan de Fuca plate ° ° ° bold rates are used in this paper. 40/125(JDF) convergence rates 72.7 35.0 41/125 45.6 28.7 72.1 34.0 Wells et al., 2002 42/125 47.6 29.8 71.6 33.0 43/125 49.4 30.9 71.0 32.0 44/125 51.1 32 70.3 31.0 45/125 52.7 33.1 69.6 29.9 46/125.5 52.1 34.9 69.2 28.8 47/126 53.5 36 68.7 27.6 48/126 54.8 37.2 67.9 26.6 49/127 54.2 38.9 67.7 25.2 50/127 53.5 40.5 66.7 24.2 Note: DD 99 is pole of DeMets and Dixon (1999); WS00 is pole of Wells and Simpson, [2001]; bold rates are used in this paper. Seismicity The Cascadia Subduction Zone and Related Subduction Systems | 19 Wells et al., 2002 FIGURE 2: Cascadia earthquakes, faults, volcanoes and Neogene forearc rotation. A) lower plate seismicity; B) upper plate seismicity, recent focal mechanisms (Mw>5), and late Cenozoic faults; C) Quaternary arc volcanism (white), major volcanoes (open triangles), post-fivePeople argue that the co-locaon of upper and lower plate Ma volcanic vents (filled triangles) and post-15 Ma forearc rotations with uncertainties (arrows). earthquakes is the result of fluid flow into the upper plate from dehydraon reac.ons in the subducng plate extensional arc segment. On the other hand, the Cali- Mendocino and JDF/Explorer/NA triple junctions. fornia rifted arc segment is associated with abundant Wadati–Benioff zone seismicity. Why are there few in-slab earthquakes beneath Oregon? The distribution of in-slab earthquakes generally fol- The paucity of intermediate-depth earthquakes beneath lows that of upper plate earthquakes in the forearc; they southwest Washington and western Oregon is puzzling are concentrated in the Puget Lowland, are rare beneath (Figure 4). If dehydration reactions and phase changes Oregon and become most abundant in the Gorda “plate” are major constituents in triggering intermediate-depth near the Mendocino triple junction (MTJ) (Figure 2). Kirby earthquakes, one might expect earthquakes beneath et al. [1996] and Rogers [this volume] argue that the Oregon as well. However, our calculated 20% south- co-location of upper and lower plate earthquakes is the ward decrease in convergence between 47° and 46°N result of fluid flow into the upper plate from dehydra- coincides with the drop off in within-slab seismicity (Fig- tion reactions in the subducting plate. Some of the earth- ure 4) but it is not clear if this modest decrease could quakes in the lower plate beneath Washington appear affect the seismic behavior of the slab.
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