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USArray Observations of Quasi-Love Surface-Wave Scattering: Orienting Anisotropy in the Cascadia Plate Boundary Duayne M. Rieger (Yale University), Jeffrey Park (Yale University) Love surface waves scatter to elliptically-polarized Rayleigh waves when they encounter regional concentrations of anisotropy. These scattered waves, called Quasi-Love, or QL waves, are useful for detecting shear near plate boundaries, where the strong lithosphere deforms mantle asthenosphere surrounding the plates as lithosphere sinks at oceanic trenches, spreads at rift zones, or crumples where continents collide. QL waves can be observed on individual seismograms, and complement other analyses of seismic data. We found QL scattering in and around the Cascadia subduction zone in seismic data from the land-based USArray, a component of Earthscope. The dense sampling and broadband response of the USArray transportable array allows us to detect and correlate the scattering of 100-sec surface waves at closely-spaced locations. We demonstrate that a pattern of Love-to-Rayleigh scattering seen across the western USA (Figure 1) can be related to an anisotropic gradient that scatters energy from a location offshore the Juan de Fuca trench. The azimuthal variation of scattering (Figure 2) indicates an alignment of the anisotropic symmetry axis with present-day plate motion. We reject slab rollback as the causative process and suggest the entrainment of asthenosphere with the overriding Juan de Fuca and Gorda plates. The strong coherence of scattered waveforms between neighboring stations in USArray suggests that such long-period surface wave motion can detect lateral gradients of anisotropy beneath Cascadia and, potentially, the rest of the western USA. Detailed reconstruction of anisotropy could be tried by grouping great-circle paths that intersect at a distribution of possible scattering points throughout the subduction system in an effort to further constrain its mantle dynamics. References Rieger, D. M., and J. Park, USArray observations of quasi‐Love surface wave scattering: Orienting anisotropy in the Cascadia plate boundary, J. Geophys. Res., 115, B05306, doi:10.1029/2009JB006754, 2010. Acknowledgements: This work was supported by NSF Grant EAR-0208652. Figure 1. USArray record sweeps along the great-circle path for the Loyalty Islands 4/9/2008 (Ms=7.3) earthquake. The Love wave on the transverse components and the QL wave on the vertical-components are highlighted. The station each record was recorded at is along the left Y-axis and the distance from the great-circle path intersection at 42.5°N 127.5°W is along the right Y-axis. The variation in amplitude of the QL wave with distance appears to be accompanied by the gradual modulation of the waveform. This can be characteristic of an interference pattern that would suggest a more complex anisotropic structure than a single point scatterer with a horizontal axis of symmetry. Figure 2. Spatial plot of the QL/Love amplitude ratio at 100s-lowpass, normalized by the Love-wave amplitude along each great-circle path. The black circles are station locations. The red line is the zero-scattering azimuth, parallel to our proposed anisotropic axis of symmetry. This orientation correlates well with the hotspot referenced Juan de Fuca plate motion, illustrated by the black arrows, and regional shear-wave splitting measurements. The orange line represents an oblique orientation to the anisotropic axis of symmetry suggested by the data. The largest QL/Love amplitude ratios are measured close to this orientation which is encouraging. The green line is the azimuth normal to the zeroscattering azimuth. II-196 | 2010 IRIS Core Programs Proposal | Volume II | Upper Mantle Structure and Dynamics
Tau-p Depropagation of Five Regional Earthquakes Recorded by the Earthscope Usarray to Constrain the 410-km Discontinuity Velocity Gradient Josh Stachnik (Univ. Wyoming), Ken Dueker (Univ. Wyoming) Seismic recordings by the EarthScope USArray from three Queen Charlotte region earthquakes and two southern Baja Mexico earthquakes are transformed to the Tau-P domain and the post-critical P-wave branches are picked. Five one dimensional velocity models are formed via depropagation of the picked Tau-P curves. All five models are in good agreement with the ak135 reference model velocity gradients both above and below the 410 km discontinuity and the observed 410 P-wave velocity step is within 10% of ak135. The 410 velocity gradients for three of the models are sharp (
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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
Page 157 and 158: Lithospheric Layering in the North
Page 159 and 160: eflective zones on depthmigrated se
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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 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: Stress-Induced Upper Crustal Anisot
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
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
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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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