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Arc-Parallel Flow beneath the TUCAN Broadband Seismic Experiment in Central America David L. Abt (ExxonMobil Exploration Company), Karen M. Fischer (Brown University), Geoffrey A. Abers (Lamont- Doherty Earth Observatory, Columbia University) Resolving the geometry of flow in subduction zones is essential in understanding mantle wedge thermal structure, slab dehydration, melting and melt transport in the wedge, and subduction zone dynamics. The TUCAN Broadband Seismic Experiment deployed 48 broadband IRIS/PASSCAL seismometers in Nicaragua and Costa Rica from 2004-2006. In three-dimensional models of anisotropy obtained by tomographically inverting shear-wave splitting measurements from local events recorded by the TUCAN array, olivine a-axes are predominantly arc-parallel in the mantle wedge beneath the arc and back-arc at depths of 50 to 150 km (except in northern Nicaragua). The arc-parallel a-axes extend into mantle wedge well beyond the cold, shallow wedge corner where B-type olivine fabric may occur. The observed anisotropy cannot be explained by simple two-dimensional arc-normal corner flow, and instead suggests significant arc-parallel flow. This hypothesis is confirmed by the trend of a distinct Pb and Nd isotopic signature in arc lavas associated with subducting seamounts offshore of Costa Rica. The anomalous signature systematically decreases for nearly 400 km from a maximum in central Costa Rica, directly inboard of the seamounts, down to background levels in northwestern Nicaragua. As the timing of the initial input of the isotopic signature beneath Costa Rica can be constrained and its transport distance is known, northwestward flow rates can be estimated (~63–190 mm/y) and are comparable to the magnitude of subducting Cocos plate motion (~85 mm/y). These results indicate flow in the mantle wedge can be significantly three-dimensional. Shear-wave splitting in SK(K)S phases recorded by the TUCAN array shows arc-parallel fast directions but significantly larger splitting times than seen in local S phases, indicating significant anisotropy consistent with arc-parallel flow below the slab. References Abt, D. L., K. M. Fischer, G. A. Abers, J. M. Protti, V. González, and W. Strauch (2010), Constraints on upper mantle anisotropy surrounding the Cocos Slab from SK(K)S splitting, J. Geophys. Res., 115, B06316. Across-Arc Distance (km) [100]-axis Azimuth Arc- Parallel Arc- Normal Abt, D. L., K. M. Fischer, G. A. Abers, W. Strauch, J. M. Protti, and V. González (2009), Shear wave anisotropy beneath Nicaragua and Costa Rica: Implications for flow in the mantle wedge, Geochem. Geophys. Geosyst., 10, Q05S15. Hoernle K., D.L. Abt, K.M. Fischer, H. Nichols, F. Hauff, G. Abers, P. van den Bogaard, G. Alvarado, M. Protti, W. Strauch (2008), Geochemical and geophysical evidence for arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua, Nature, 451, 1094-1098. Acknowledgements: This research was supported by the NSF MARGINS program under awards OCE-0203650 (Boston University), and OCE- 0203607 and EAR-0742282 (Brown University). 100 N 12.5 km 37.5 km 62.5 km 87.5 km 112.5 km 137.5 km 162.5 km 0 CR NIC -100 Surface 200 100 TUCAN Station Permanent Station Pacific Ocean Cocos Plate 0 Trench Trench -100 -200 Arc Volcano Model of anisotropy in the Nicaragua-Costa Rica subduction zone. Vectors represent well-resolved olivine a-axes in an olivine-orthopyroxene model. Vector orientation and color indicate horizontal azimuth, length corresponds to the strength of anisotropy relative to single-crystal values, and thickness corresponds to model parameter resolution. The TUCAN seismic array and volcanic arc are shown at the surface, and the volcanic arc position is plotted on each slice through the model. The slab–wedge interface is shown by grey shading. The mantle wedge is located in front of the slab in the layers spanning 50–175 km depth. Roughly arc-parallel a-axes dominate well-resolved wedge regions beneath the arc and the rear- and back-arc at depths of 50–150 km. II-182 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics
Effect of Prior Petrological Constraints on Global Upper Mantle Models of Radial Anisotropy Caroline Beghein (University of California at Los Angeles) Despite efforts from multiple research groups, large discrepancies remain among global models of seismic anisotropy. Besides the inherent nonuniqueness of inverse tomographic problems, one source of uncertainties can originate from the prior information introduced in the inversion. In the case of inversion of global surface wave phase velocity maps to obtain models of radial anisotropy, prior relationships are often imposed between the different elastic parameters. Inversions are then performed for the best-resolved parameters only, i.e. shear-wave velocity and anisotropy. We tested the robustness of uppermost mantle radial anisotropy models with respect to these prior constraints [Beghein, 2010]. We applied a forward modeling technique [Sambridge, 1999] to fundamental mode Rayleigh and Love wave global phase velocity maps up to spherical harmonic degree 8. This forward modeling approach enabled us to obtain reliable model uncertainties, and to determine which model features are constrained by the data and which are dominated by the prior. We compared the most likely and mean models obtained with and without prior constraints, and found that the most likely models obtained in both cases are highly correlated. This demonstrates that for the best data-fitting solution, the geometry of uppermost mantle radial anisotropy is not strongly affected by prior petrological constraints. We found, however, significant changes in the amplitude of the anomalies, with stronger amplitudes in the best data-fitting model obtained without petrological constraints. This could become an issue when quantitatively interpreting seismic anisotropy models, and thus emphasizes the importance of accurately accounting for parameter uncertainties and trade-offs, and of understanding whether the seismic data or the prior constraints the model. In addition, we showed that model distributions are not necessarily Gaussian a priori, but that imposing petrological constraints can force the models to follow a Gaussian-like posterior distribution in addition to reducing posterior model uncertainties, in agreement with inverse theory. Finally, we demonstrated that the dependence of seismic wave velocities with the age of the ocean floor is robust and independent of prior constraints. A similar age signal exists for anisotropy, but with larger uncertainties without prior constraints. References 3-D models of S-wave radial anisotropy (ξ) in the uppermost mantle obtained with (A) and without (B) prior petrological constraints. In this convention, a negative ξ corresponds to fast horizontally propagating shearwaves. These models correspond to the mean of the model distributions obtained. Beghein, C., Radial Anisotropy and Prior Petrological Constraints: a Comparative Study, J. Geophys. Res., 115, 2010. Sambridge, M., Geophysical inversion with a Neighbourhood Algorithm - I. searching a parameter space, Geophys. J. Int., 138 (2), 479–494, 1999 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics | II-183
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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
Page 143 and 144: An Integrated Analysis of an Ancien
Page 145 and 146: Crustal Velocity Structure of Turke
Page 147 and 148: Ambient Noise Tomography of the Pam
Page 149 and 150: Ambient Noise Monitoring of Seismic
Page 151 and 152: Mushy Magma beneath Yellowstone Ris
Page 153 and 154: Imaging the Seattle Basin to Improv
Page 155 and 156: Developing a Database of ENA Ground
Page 157 and 158: Lithospheric Layering in the North
Page 159 and 160: eflective zones on depthmigrated se
Page 161 and 162: FischerFig05.pdf 1 2/8/10 12:31 PM
Page 163 and 164: S-Velocity Structure of Cratons, fr
Page 165 and 166: Seismic Structure of the Crust and
Page 167 and 168: Subducted Oceanic Asthenosphere and
Page 169 and 170: Detecting the Limit of Slab Break-o
Page 171 and 172: Pn Tomography of the Western United
Page 173 and 174: 3-D Isotropic Shear Velocity Model
Page 175 and 176: Seismic Anisotropy Associated with
Page 177 and 178: Anisotropy in the Great Basin from
Page 179 and 180: Source-Side Shear Wave Splitting an
Page 181 and 182: S-Velocity Structure of the Upper M
Page 183 and 184: Velocity Structure of the Western U
Page 185 and 186: Segmented African Lithosphere benea
Page 187 and 188: Imaging the Mantle Wedge in the Cen
Page 189 and 190: A Slab Remnant beneath the Gulf of
Page 191 and 192: Imaging the Southern Alaska Subduct
Page 193: Opposing Slabs under Northern South
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 and 242: Imaging Attenuation in the Upper Ma
Page 243 and 244: Three-Dimensional Electrical Conduc
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Core-Mantle Boundary Heat Flow Thor
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Constraints on Lowermost Mantle Ani
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Localized Double-Array Stacking Ana
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Waveform Modeling of D" Discontinui
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Absence of Ultra-Low Velocity Zones
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Moving Seismic Tomography Beyond Fa
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A Three-Dimensional Radially Anisot
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The Importance of Crustal Correctio
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Slabs Do Not Go Gentle Karin Sigloc
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Localized Temporal Change of the Ea
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Core Structure Reexamined Using New
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Large Variations in Travel Times of
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Observations of Antipodal PKIIKP Wa
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Regional Variation of Inner Core An
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Wide-Scale Detection of Earthquake
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