Relationship between the Cascadia fore-arc mantle wedge, nonvolcanic tremor, and the downdip limit of seismogenic rupture
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Abstract:
Great earthquakes anticipated on the Cascadia subduction fault can potentially rupture beyond the geodetically and thermally inferred locked zone to the depths of episodic tremor and slip (ETS) or to the even deeper fore-arc mantle corner (FMC). To evaluate these extreme rupture limits, we map the FMC from southern Vancouver Island to central Oregon by combining published seismic velocity structures with a model of the Juan de Fuca plate. These data indicate that the FMC is somewhat shallower beneath Vancouver Island (36–38 km) and Oregon (35–40 km) and deeper beneath Washington (41–43 km). The updip edge of tremor follows the same general pattern, overlying a slightly shallower Juan de Fuca plate beneath Vancouver Island and Oregon (∼30 km) and a deeper plate beneath Washington (∼35 km). Similar to the Nankai subduction zone, the best constrained FMC depths correlate with the center of the tremor band suggesting that ETS is controlled by conditions near the FMC rather than directly by temperature or pressure. Unlike Nankai, a gap as wide as 70 km exists between the downdip limit of the inferred locked zone and the FMC. This gap also encompasses a ∼50 km wide gap between the inferred locked zones and the updip limit of tremor. The separation of these features offers a natural laboratory for determining the key controls on downdip rupture limits.Keywords:
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Mexico, located in one of the world's most seismically active regions, lies on three large tectonic plates: the North American plate, Pacific plate, and Cocos plate. The relative motion of these tectonic plates causes frequent earthquakes and active volcanism and mountain building. Mexico's most seismically active region is in southern Mexico where the Cocos plate is subducting northwestward beneath Mexico creating the deep Middle America trench. The Gulf of California, which extends from approximately the northern terminus of the Middle America trench to the U.S.-Mexico border, overlies the plate boundary between the Pacific and North American plates where the Pacific plate is moving northwestward relative to the North American plate. This region of transform faulting is the southern extension of the well-known San Andreas Fault system.
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Recent structural and geophysical studies conducted along the northern Caribbean plate boundary, at sea and on land, have led to a precise description of the geometry and the tectonic regimes along this major transcurrent zone which separates the Caribbean and North American plates. In order to interpret its tectonic features in terms of plate motion, we use a simple numerical model of strike‐slip faulting to test previously proposed kinematic models and to compute new motion parameters. We show that none of the previously proposed models correctly accounts for the observed deformation along the whole plate boundary. On the basis of the deformation pattern obtained from geological data we compute a motion parameters set that integrate rigid plate rotation and “a plate boundary zone deformation component.” Our results show that the deformation along the northern Caribbean plate boundary zone is controled by regional kinematics (i.e., the Caribbean/North America relative motion) rather than by local effects (e.g., small block rotation, intraplate deformation).
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The tectonics of the Pacific margin of North America between Vancouver Island and south-central Alaska are dominated by the northwest motion of the Pacific plate with respect to the North America plate at a velocity of approximately 50 mm/yr. In the south of this mapped region, convergence between the northern extent of the Juan de Fuca plate (also known as the Explorer microplate) and North America plate dominate. North from the Explorer, Pacific, and North America plate triple junction, Pacific:North America motion is accommodated along the ~650-km-long Queen Charlotte fault system. Offshore of Haida Gwaii and to the southwest, the obliquity of the Pacific:North America plate motion vector creates a transpressional regime, and a complex mixture of strike-slip and convergent (underthrusting) tectonics. North of the Haida Gwaii islands, plate motion is roughly parallel to the plate boundary, resulting in almost pure dextral strike-slip motion along the Queen Charlotte fault. To the north, the Queen Charlotte fault splits into multiple structures, continuing offshore of southwestern Alaska as the Fairweather fault, and branching east into the Chatham Strait and Denali faults through the interior of Alaska. The plate boundary north and west of the Fairweather fault ultimately continues as the Alaska-Aleutians subduction zone, where Pacific plate lithosphere subducts beneath the North America plate at the Aleutians Trench. The transition is complex, and involves intraplate structures such as the Transition fault. The Pacific margin offshore British Columbia is one of the most active seismic zones in North America and has hosted a number of large earthquakes historically.
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The geology and tectonism of California have been influenced greatly by the collision and interaction between the Pacific plate and the North American plate. The forces generated by this interaction caused substantial horizontal movement along the San Andreas fault system and created the Gulf of California rift zone. This article summarizes the unique features of the gulf, describes the theory of plate tectonics, explains how tectonism may have affected the geologic evolution and physiography of the gulf, and illustrates the process by which the Colorado River became linked to the gulf.
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The problem of understanding the deformation occurring along the Pacific-North American plate boundary in the western United States depends upon understanding the forces which drive the plates in this region. One of the primary sources of our knowledge concerning these forces lies in their manifestation as relative displacements which occur throughout the broad zone of deformation surrounding the San Andreas fault system. It is information concerning the spatial and temporal distribution of these motions which will be of greatest benefit in helping to determine which of several possible mechanisms is responsible for driving contemporary plate motions in this region.
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Based on 13 new fault plane solutions and published seismological, geological, and geophysical data, we interpret the deformation along the Pacific‐North American plate margin in the eastern Gulf of Alaska. Three major tectonic units can be distinguished: (1) the North American plate, (2) the Pacific plate, and (3) a belt of mobile borderland terranes. The Pacific plate moves in a NNW direction at rates of about 6 cm/yr in relation to the North American plate. That motion results in mostly right‐lateral strike slip at the Queen Charlotte‐Fairweather fault system, a well‐known observation. A new finding,however, is that a small component (∼1 cm/yr) of convergence may also be present which results in minor subduction of the oceanic plate beneath portions of the continental margin. Heretofore the Queen Charlotte‐Fairweather fault zone and associated continental margin was interpreted as a classical, pure transform boundary. The Yakutat block, a borderland terrane about 400 km long and 100 to 200 km wide, is carried passively by the Pacific plate except that the block slowly overrides this plate at about 1 cm/yr. This motion is taken up by almost pure thrust faulting in a southwesterly direction along a 400‐km long SE striking shelf edge structure. At its NW edge the Yakutat block is in turn being thrust beneath the North American plate along the Pamplona zone‐Icy Bay lineament. The underthrusting of the Yakutat block results in a major orogeny, crustal shortening and uplift of the Chugach‐St. Elias range. The effects of this collision may extend as far as 500 km inland and cause some deformation at the Denali fault in the central Alaska Range. Subduction of the Pacific plate beneath the colliding margin appears responsible for development of an active volcanic arc up to 300 km inland which trends SE from the Wrangell Mountains to Yukon Territory, Canada, and perhaps to Mt. Edgecumbe volcano in southeast Alaska. The tectonic model proposed implies a high seismic hazard for the Queen Charlotte, Fairweather, and Chugach‐St. Elias fault systems. At these fault zones we estimate recurrence times for great events of about 100 years, but they may vary between 50 and 200 years. A temporarily very high potential for a great earthquake has been determined for the ‘Yakataga seismic gap’ located between Icy Bay and Kayak Island. Large or great thrust earthquakes on the detachment fault underlying the entire Yakutat wedge also appear possible but may only occur infrequently. Their recurrence times are estimated to be several
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Global Positioning System (GPS) measurements provide the first direct measurement of plate motion and crustal deformation across the Scotia‐South America transform plate boundary in Tierra del Fuego. This plate boundary accommodates a part of the overall motion between South America and Antarctica. The subaerial section of the plate boundary in Tierra del Fuego, about 160 km in length, is modeled as a two dimensional, strike‐slip plate boundary with east‐west strike. Along the Magallanes‐Fagnano fault system, the principal fault of this portion of the plate boundary, relative plate motion is left‐lateral strike‐slip on a vertical fault at 6.6 ± 1.3 mm/year based on an assumed locking depth of 15 km. The site velocities on the Scotia Plate side are faster than the relative velocity by an additional 1–2 mm/yr, suggesting there may be a wider region of diffuse left‐lateral deformation in southern Patagonia. The north‐south components of the velocities, however, do not support the existence of active, large‐scale transpression or transtension between the South America and Scotia plates along this section of the plate boundary.
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We analyze data spanning up to 5 years from 18 continuous GPS stations in Iceland, computing daily positions of the stations with three different high‐level geodetic processing software packages. We observe large‐scale crustal deformation due to plate spreading across Iceland. The observed plate divergence between the North American and the Eurasian plates is in general agreement with existing models of plate motion. Spreading is taken up within a ∼100–150 km wide plate boundary zone that runs through the island. Of the two parallel branches of the plate boundary in south Iceland, the eastern volcanic zone is currently taking up the majority of the spreading and little is left for the western volcanic zone. The plate boundary deformation field has been locally and temporarily affected in south Iceland by two M w = 6.5 earthquakes in June 2000, inflation at Katla volcano during 2000 to 2004, and an eruption of Hekla volcano in February 2000. All stations with significant vertical velocities are moving up relative to the reference station REYK, with the highest velocity exceeding 20 mm/yr in the center of the island.
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