1,721,185 research outputs found
Downgoing plate topography stopped rupture in the A.D. 2005 Sumatra earthquake
No full textEarthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmentation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the Mw = 8.7, A.D. 2005 Simeulue-Nias earthquake, and the northern termination of a major 1797 earthquake that was partly filled by an Mw = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation
Unlocking a global ocean mixing dataset: toward standardization of seismic-derived ocean mixing rates
No full textTurbulent mixing is vital for water transformation in the ocean and sustains the global thermohaline circulation. Despite decades of global observations using different platforms, our understanding of ocean turbulence is still limited. More observations are needed to better characterize the spatiotemporal distribution of mixing to reduce uncertainties in climate models. Marine seismic reflection surveys are an untapped data resource for high-resolution ocean turbulence observation. Turbulent mixing can be extracted from seismic data through horizontal internal wave slope spectra. However, to date, a standardized approach to prepare seismic data for this spectral analysis is still lacking, leading to insufficient consideration of the impact of noise on the resulting diffusivities. To address these issues, we perform a full-wavefield synthetic modeling and processing to reveal noise-induced overestimation of diffusivities. We further propose a widely applicable workflow and apply it to three field seismic surveys with increasing noise levels conducted in regions of different turbulence environments: ocean ridges, open ocean interior, and continental slope. The derived diffusivities are bench-marked against direct measurements around the region to show the fidelity of this seismic method. The extended observation records by seismic data across the Kauai Channel and away from the Mid-Atlantic Ridges reveal the importance of topography in modifying the propagation of internal tides and the distribution of turbulent mixing in both near and far fields. Our proposed workflow marks a key advancement toward standardization of seismic-derived ocean mixing rates and holds the potential to unlock massive marine seismic reflection datasets worldwide for ocean mixing characterization.</p
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The impact of climate and tectonics on sedimentary and deformational processes, Gulf of Alaska
No full texttextCollision of the Yakutat Terrane with North America in southern Alaska has
driven growth of the Chugach-St. Elias orogen. Glaciation of the St. Elias Range has
periodically increased since the Miocene, but began dominating erosion and spurred
enhanced exhumation since the mid-Pleistocene transition at ~1 Ma. Ice associated with this glacial intensification carved cross-shelf sea valleys that connect the St. Elias Range to the deep-sea Surveyor Fan. A newly increased terrigenous sediment flux into the fan triggered the formation and growth of the Surveyor Channel. The change in
geomorphology observed throughout Fan sequences allows us to characterize the
influence that a glaciated orogen can have in shaping margin processes and the sediment
pathways from source to sink. Seismic data also reveal an isolated, large, short runout,
mass-transport deposit (MTD) buried in the Surveyor Fan. The MTD geometry, size and location on a convergent margin lend support to recent studies suggesting seismic
strengthening and infrequent sediment failure on active margins. This study provides
insight into the magnitude and scope of events required to cause submarine mega-slides
and overcome higher than normal sediment shear strength, including the influence of
climate and sea level change. Beneath the Surveyor Fan, integrated geophysical data
reveals massive intraplate shearing, and a lack of oceanic crust magnetic lineaments in
regions of Pacific Plate crust. We argue that stress from the Yakutat-North America
collision transferred outboard to the Pacific Plate is the major driver for the deformation
causing these features. This stress would have resulted in significant strain in the NE
corner of the Pacific Plate, creating pathways for sill formation in the crust and Surveyor
Fan. The collision further intensified as the thickest Yakutat portion began to subduct
during the Pleistocene, possibly providing the impetus for the creation of the Gulf of
Alaska Shear Zone, a >200 km zone of shear extending out into the Pacific Plate. This
study highlights the importance of farfield stress from complex tectonic regimes in
consideration of large-scale oceanic intraplate deformation.Earth and Planetary Science
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Quaternary seismic stratigraphic investigations, Yakutat Bay region, Gulf of Alaska : subglacial drainage mechanics and glacial expanse
No full textQuaternary tectonic and climate interactions have and continue to contribute to a substantial stratigraphic record that preserves the key regional events that result from the interplay of these dynamic processes. Near Yakutat Bay, Alaska, temperate glacial cover continuously manipulates and shapes the southern Alaskan margin and delivers substantial amounts of sediment annually. During cooler climatic periods, these glaciers, like most glaciers in Alaska, experience increases in volume and advance across the shelf. Using an integrated seismic data set, we investigate stratigraphic signatures buried along the shelf, between the present Yakutat and Alsek Sea Valleys, in order to understand the mechanics governing the regional glacial system during advance phases. Our investigations reveal four glacial unconformities, of which, two show preserved retreat sediment packages. The events are indicative of ice advance phases during the Little Ice Age (LIA), the Last Glacial Maxima (LGM), and two pre-LGM events. The events show dominance by ice expanse from the Malaspina and Alsek River districts. The LGM event transgresses the shelf and concentrates erosion in highly overdeepened troughs, somewhat analogous to ice stream behavior. In addition, associated with these regional events, we find a suite of large-scale channel forms, exhibiting widths between 0.6-4.2 km and depths between 49-353 m. The channels are highly eroded vertically relative to lateral extent; widths to thickness ratios of the buried channel deposits are 15.7 on average. The channels show good correlation with the interpreted erosional events and other stratigraphic signatures indicative of ice occupation, i.e. moraines, grounding lines, etc. The channels exhibit distinct facies and facies assemblages of variable thicknesses and occurrence and show common morphologic characteristics with one another, i.e. cross-cuts. The facies and assemblages appear dominated by glacio-proximal, glacio-fluvial and glacio-lacustrine/marine physical processes. The characteristic morphology of the channels implies they are subglacially derived, and facies suggest subsequent fill by subaerial/submarine processes after ice retreat. Based on morphologic interpretations of the channels and their associations with glacial stratigraphy and erosional events, we interpret them as tunnel valleys. Expanding on their predictable relationship with specific glacial stratigraphy, we conclude that the tunnel valleys are indicators of retreat following glacial advance periods, when significant amounts of ice and melt are available to initiate and generate large tunnel valley deposits. As such, we conclude that the tunnel valleys are useful paleoclimate indicatorsEarth and Planetary Science
An Abrupt Transition in the Mechanical Response of the Upper Crust to Transpression along the Queen Charlotte Fault
Full text linkThe Queen Charlotte Fault (QCF) is a major strike-slip fault that forms the boundary between the Pacific and North American plates from 51 degrees to 58 degrees N. Near 53.2 degrees N, the angle of oblique convergence predicted by the Mid-Ocean Ridge VELocity (MORVEL) interplate pole of rotation decreases from > 15 degrees in the south to < 15 degrees in the north. South of 53.2 degrees N, the convergent component of plate motion results in the formation of a 40 km wide terrace on the Pacific plate west of QCF and earthquakes with thrust mechanisms (including the 2012 Haida Gwaii earthquake sequence) are observed. North of 53.2 degrees N, in the primary rupture zone of the M 8.1 strike-slip earthquake of 1949, the linear terrace disappears, and topography of the continental slope west of the QCF is characterized by a complex pattern of ridges and basins that trend obliquely to the primary trace of the QCF. Deformation within the Pacific plate appears to occur primarily through strike-slip faulting with a minor thrust component on secondary synthetic faults. The orientations of these secondary faults, as determined from seismic reflection and bathymetric data, are consistent with the reactivation of faults originally formed as ridge-parallel normal faults and as thrust faults formed parallel to the QCF south of the bend at 53.2 degrees N and subsequently translated to the north. We suggest that an oblique convergence angle of 15 degrees represents a critical threshold separating distinct crustal responses to transpression. This result is consistent with theoretical and analog strain models of transpressive plate boundaries. The sharpness of this transition along the QCF, in contrast to purely continental transform boundaries, may be facilitated by the relatively simple structure of oceanic crust and the presence of pre-existing, optimally oriented faults in the young Pacific plate.National Science Foundation (NSF) EAR-9219870, EAR-9527011Institute for Geophysic
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Geophysical investigations in the Nankai Trough and Sumatran subduction zones
Full text linktextThe 2004 Sumatra-Andaman and the 2011 Tohoku-Oki earthquakes demonstrate the importance of understanding subduction zone earthquakes and the faults that produce them. Faults that produce earthquakes and/or tsunamis in these systems include plate boundary megathrusts, splay faults (out of sequence thrusts), and strike-slip faults from strain partitioning. Offshore Japan, IODP Exp. 314 collected logging while drilling (LWD) data across several seismically-imaged fault splays in the Nankai Trough accretionary prism. I combine LWD resistivity data with a model of fluid invasion to compare the permeabilities of sands. My results indicate that sands within faulted zones are 2-3 orders of magnitude more permeable than similar undisturbed sands. Therefore fault zones are likely to be fluid conduits within the accretionary wedge. Fluids can affect the physical and chemical properties of the faulted material, increasing pore pressures and effectively lubricating the faults.
Fluids play an important role in fault slip, but hazard analysis also requires an understanding of fault geometry and slip direction. Both Japan and Sumatra exhibit strain partitioning, where oblique convergence between tectonic plates is partitioned between the megathrust and strike-slip faults proximal to the arc. Offshore Sumatra, I combine profiles from a 2D seismic survey (SUMUT) with previous bathymetry and active seismic surveys to characterize the West Andaman Fault adjacent to the Aceh forearc Basin. Along this fault I interpret transpressional flower structures that cut older thrust faults. These flower structures indicate that the modern West Andaman Fault is a right lateral strike-slip fault and thus helps to accommodate the translational component of strain in this highly oblique subduction zone.
Offshore the Kii Peninsula, Japan, I analyze a trench-parallel depression that forms a notch in the seafloor just landward of the megasplay fault system, along the seaward edge of the forearc Kumano Basin. Using a 12 km wide, 3D seismic volume, I observe vertical faults and faults which dip toward the central axis of the depression, forming apparent flower structures. The along-strike geometry of the vertical faults makes predominantly normal or thrust motion unlikely. I conclude, therefore, that this linear depression is the bathymetric expression of a transtensional fault system. While the obliquity of convergence in the Nankai Trough is small (~15 degrees), this Kumano Basin Edge Fault Zone could be due to partitioning of the plate convergent strain. The location of the West Andaman Fault and KBEFZ within the forearc may be controlled by the rheology contrast between active accretionary wedges and the more stable crust beneath forearc basins.Earth and Planetary Science
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Basement and Regional Structure Along Strike of the Queen Charlotte Fault in the Context of Modern and Historical Earthquake Ruptures
Full text linkAbstract The Queen Charlotte fault (QCF) is a dextral transform system locate
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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