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S-Wave Velocity Structure beneath the High Lava Plains, Oregon, from Rayleigh-Wave Dispersion Inversion Linda M. Warren (Saint Louis University), J. Arthur Snoke (Virginia Tech), David E. James (Carnegie Institution of Washington) The High Lava Plains (HLP) "hotspot" track is a prominent volcanic lineament that extends from the southeast corner of Oregon in the northern Great Basin to Newberry volcano in the eastern Cascades. With the age of silicic volcanism decreasing along track to the northwest, the HLP and Newberry volcano are a rough mirror image to the Eastern Snake River Plain and Yellowstone but, in the case of the HLP, at an orientation strongly oblique to North American plate motion. Since this orientation is incompatible with plate motion over a fixed hotspot, other proposed origins for the HLP, such as asthenospheric inflow around a steepening slab, residual effects of a Columbia River/Steens plume, backarc spreading, and Basin and Range extension, relate it to various tectonic features of the Pacific Northwest. To begin distinguishing between these hypotheses, we image upper-mantle structure beneath the HLP and adjacent tectonic provinces with fundamental-mode Rayleigh-waves recorded by stations of the USArray Transportable Array, the recently-initiated HLP seismic experiment, the United States National Seismograph Network, and the Berkeley seismic network [Warren et al., 2008]. We estimate phase velocities along nearly 300 two-station propagation paths that lie within and adjacent to the HLP and cross the region along two azimuths, parallel to and perpendicular to the HLP track. The dispersion curves, which typically give robust results over the period range 16-171 seconds, are grouped by tectonic region, and the composite curves are inverted for S-wave velocity as a function of depth. The resulting variations in upper-mantle structure correlate with variations in surface volcanism and tectonics. The lowest velocities (~4.1 km/s) occur at ~50 km depth in the SE corner of Oregon, where there has been extensive basaltic volcanism in the past 2-5 Kyr, and suggest uppermost mantle temperatures sufficient to produce basaltic partial melting. While the seismic velocities of the uppermost mantle beneath the volcanic High Lava Plains are low relative to the standard Tectonic North America (TNA) model [Grand and Helmberger, 1984], they are only slightly lower than those found for the adjacent northern Great Basin and they appear to be significantly higher than upper mantle velocities beneath the Eastern Snake River Plain. Our results provide no evidence for a residual plume signature beneath the HLP region, leaving open questions as to the origin of the HLP volcanic track itself. References Warren, L.M., J.A. Snoke, and D.E. James, S-wave velocity structure beneath the High Lava Plains, Oregon, from Rayleigh-wave dispersion inversion, Earth Planet. Sci. Lett., 274, 121-131, 2008. Grand, S.P., and D.V. Helmberger, Upper mantle shear structure of North America, Geophys. J. R. Astron. Soc., 76, 399-438, 1984. Acknowledgements: This work was supported by the National Science Foundation through grant EAR-0506914 and by the Carnegie Institution of Washington. The tectonic setting of the High Lava Plains. Black triangles mark locations of Cascade volcanoes. Black lines beginning at Newberry Volcano and extending to southeast show increasing age of rhyolitic volcanism [Jordan et al., 2004]. McDermitt Caldera indicates onset of volcanism 16.1 Ma. Colored gray regions show rhyolite calderas along the Snake River Plain [Priest and Morgan, 1992]. Gray line outlines the Great Basin [Wernicke, 1992]. The dashed Sr87/Sr86 = 0.706 line approximates the western boundary of Proterozoic North America [Ernst, 1988]. Lower Sr87/Sr86 values lie to the west. The blue lines labeled A-A' and B-B' indicate the locations of the cross-sections in Figure 2. The inset shows the geographic setting of the study area and the great circle paths to that area from the nine analyzed earthquakes (black circles). Cross-Sections interpreting S-wave velocity with depth along lines A-A' and B-B' of Figure 1. Velocity profiles for the eight regionalized paths are plotted at the center of the appropriate region and dashed lines with question marks interpolate structure between the regions. The large question marks in two of the transition regions indicate possible discontinuities in structure for which we have no constraints. S-wave velocity structure shallower than 50 km is not interpreted because it trades off with crustal thickness. (c) The velocity-depth profile for the modified TNA model is shown for comparison. II-230 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics
Three-Dimensional Electrical Conductivity Structure of the Pacific Northwest Gary D. Egbert (Oregon State University), Prasanta K. Patro (NGRI, Hyderabad, India) In conjunction with the USArray component of EarthScope, long period magnetotelluric (MT) data are being acquired in a series of arrays across the continental US. Initial deployments in 2006 and 2007 acquired data at 110 sites covering the US Pacific Northwest. The MT sites, distributed with the same nominal spacing as the USArray seismic transportable array (~70 km), produced data in the period range 10-10,000s of very good (2007) to excellent (2006) quality. Three-dimensional inversion of this dataset (Patro and Egbert, 2008) reveals extensive areas of high conductivity in the lower crust beneath the Northwest Basin and Range (C1), and beneath the Cascade Mountains (C2-C3), contrasting with very resistive crust in Siletzia (R1), the accreted thick ocean crust which forms the basement rocks in the Cascadia forearc and the Columbia Embayment). The conductive lower crust beneath the southeastern part of the array is inferred to result from fluids (including possibly partial melt at depth) associated with magmatic underplating. Beneath the Cascades high conductivities probably result from fluids released by the subducting Juan de Fuca slab. Resistive Siletzia represents a stronger crustal block, accommodating deformation in the surrounding crust by rigid rotation. Significant variations in upper mantle conductivity are also revealed by the inversions, with the most conductive mantle beneath the Northeastern part of the array, and the most resistive corresponding to subducting oceanic mantle beneath the Pacific plate (R2). References Patro P. K., G. D. Egbert, 2008. Regional conductivity structure of Cascadia: Preliminary results from 3D inversion of USArray transportable array magnetotelluric data, Geophys. Res. Lett., 35, L20311. Acknowledgements: This work was partially supported by grants from the National Science Foundation (NSF-EAR0345438) and the U.S. Department of Energy 212 (DE-FG02-06ER15819). 3D resistivity image of Pacific Northwest derived from the 3D inversion A: conductive zones in the forearc; C1: conductive lower crust in the Basin and Range, High lave plains and Blue Mountains; C2-C3: conductive features beneath the Cascades; R1: resistive Siletz terrane; R2: resistive oceanic mantle. Dashed white line marks contact interpreted as the southern boundary of Siletzia. 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-231
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volume ii - accomplishments Facilit
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volume ii - accomplishments Facilit
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CONTENTS Introduction..............
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Non-Volcanic Tremor along the Oaxac
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Detecting the Limit of Slab Break-o
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Lower Mantle, Core-Mantle Boundary
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• Non-Earthquake Seismic Sources
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acoustics community by permitting a
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Near-Surface Environments - Hazards
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Why Do Faults Slip? Emily Brodsky (
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Another probe of fault evolution is
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to provide a best match with the Gl
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tectonic disruption and mass accumu
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W3 ∆ 5 . A number of intriguing r
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thermal and compositional effects r
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Towards a Global School Seismic Net
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On-Line Seismology Curriculum for U
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Teachers on the Leading Edge: Earth
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USArray Education and Outreach in S
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From an IRIS Lecture Tour to a Gene
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Explorer Series Robert Bartolotta (
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Workshop “Earth System Science fo
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The IRIS Internship Cycle: From Int
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Community-Outreach Efforts in Data
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Site Reconnaissance for Earthscope
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jAmaseis: Seismology Software Meeti
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Near Real-Time Simulations of Globa
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FuncLab: A MATLAB Interactive Toolb
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Five Years of Distributing the Seis
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IRIS DMS Data Products, Beyond Raw
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Local Earthquakes in the Dallas-Ft.
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Epicentral Location Based on Raylei
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Analysis of Spatial and Temporal Se
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Exceptional Ground Motions from the
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The 2010 Mw7.2 El Mayor-Cucapah Ear
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Effects of Kinematic Constraints on
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Imaging of the Source Properties of
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Teleseismic Inversion for Rupture P
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The Global Aftershocks of the 2004
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The 17 July 2006 Java Tsunami Earth
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Seismic Cycles on Oceanic Transform
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Migration of Early Aftershocks Foll
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Apparent Stress Variations at the O
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Automated Identification of Telesei
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Evaluating Ground Motion Prediction
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Tremor Monitoring Aaron Wech (Unive
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Slow Slip and Tremor in the Norther
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0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2
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Intimate Details of Tremor Observed
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Global Search of Triggered Tremor a
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Slab Morphology in the Cascadia For
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Source Analysis of the Memorial Day
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Iceberg Tremor and Ocean Signals Ob
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Elucidating the Stick-Slip Nature o
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Probing the Atmosphere and Atmosphe
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Volcanic Plume Height Measured by S
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Eruption Dynamics at Mount St. Hele
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A Search for the Lunar Core Using A
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Seismic Imaging of the Mt. Rose Fau
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Structure of the California Coast R
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Temporal Variations in Crustal Scat
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High-Resolution Locations of Trigge
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Adjoint Tomography of the Southern
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Crustal Structure of the High Lava
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Controlled Source Seismic Experimen
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Structural Interpretations Based on
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Optimized Velocities and Prestack D
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40 Crustal Structure beneath the Hi
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Controlled-Source Seismic Investiga
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Northward Thinning of Tibetan Crust
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An Integrated Analysis of an Ancien
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Crustal Velocity Structure of Turke
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Ambient Noise Tomography of the Pam
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Ambient Noise Monitoring of Seismic
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Mushy Magma beneath Yellowstone Ris
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Imaging the Seattle Basin to Improv
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Developing a Database of ENA Ground
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Lithospheric Layering in the North
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eflective zones on depthmigrated se
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FischerFig05.pdf 1 2/8/10 12:31 PM
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S-Velocity Structure of Cratons, fr
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Seismic Structure of the Crust and
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Subducted Oceanic Asthenosphere and
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Detecting the Limit of Slab Break-o
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Pn Tomography of the Western United
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3-D Isotropic Shear Velocity Model
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Seismic Anisotropy Associated with
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Anisotropy in the Great Basin from
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Source-Side Shear Wave Splitting an
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S-Velocity Structure of the Upper M
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Velocity Structure of the Western U
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Segmented African Lithosphere benea
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Imaging the Mantle Wedge in the Cen
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A Slab Remnant beneath the Gulf of
Page 191 and 192: Imaging the Southern Alaska Subduct
Page 193 and 194: Opposing Slabs under Northern South
Page 195 and 196: Effect of Prior Petrological Constr
Page 197 and 198: Global Azimuthal Seismic Anisotropy
Page 199 and 200: The Stratification of Seismic Azimu
Page 201 and 202: Upper Mantle Anisotropy beneath the
Page 203 and 204: The Teleseismic Signature of Fossil
Page 205 and 206: Seismic Anisotropy under Central Al
Page 207 and 208: Stress-Induced Upper Crustal Anisot
Page 209 and 210: Tau-p Depropagation of Five Regiona
Page 211 and 212: Mapping the Upper Mantle with the S
Page 213 and 214: Upper Mantle Structure of Southern
Page 215 and 216: The Africa-Europe Plate Boundary in
Page 217 and 218: The Isabella Anomaly Imaged by Eart
Page 219 and 220: Tomographic Image of the Crust and
Page 221 and 222: Small-Scale Mantle Heterogeneity an
Page 223 and 224: Mantle Heterogeneity West and East
Page 225 and 226: New Geophysical Insight into the Or
Page 227 and 228: The Plume-slab Interaction beneath
Page 229 and 230: Imaging the Shear Wave Velocity "Pl
Page 231 and 232: Slab Fragmentation and Edge Flow: I
Page 233 and 234: Mantle Shear-Wave Velocity Structur
Page 235 and 236: Discordant Contrasts of P- and S-Wa
Page 237 and 238: Upper Mantle Discontinuity Topograp
Page 239 and 240: Edge-Driven Convection beneath the
Page 241: Imaging Attenuation in the Upper Ma
Page 245 and 246: Core-Mantle Boundary Heat Flow Thor
Page 247 and 248: Constraints on Lowermost Mantle Ani
Page 249 and 250: Localized Double-Array Stacking Ana
Page 251 and 252: Waveform Modeling of D" Discontinui
Page 253 and 254: Absence of Ultra-Low Velocity Zones
Page 255 and 256: Moving Seismic Tomography Beyond Fa
Page 257 and 258: A Three-Dimensional Radially Anisot
Page 259 and 260: The Importance of Crustal Correctio
Page 261 and 262: Slabs Do Not Go Gentle Karin Sigloc
Page 263 and 264: Localized Temporal Change of the Ea
Page 265 and 266: Core Structure Reexamined Using New
Page 267 and 268: Large Variations in Travel Times of
Page 269 and 270: Observations of Antipodal PKIIKP Wa
Page 271 and 272: Regional Variation of Inner Core An
Page 273: Wide-Scale Detection of Earthquake
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