Karen Fischer - Academia.edu (original) (raw)

Papers by Karen Fischer

Geophysical Research Letters, 2008

An expression for the cross correlation of noise between seismic stations and the 2D Green's func... more An expression for the cross correlation of noise between seismic stations and the 2D Green's function is derived assuming that noise travels as 2D surface waves. The phase velocity is obtained directly from the noise correlation function with a phase shift of +p/4. Mean phase velocity dispersion curves are calculated for the TUCAN seismic array in Costa Rica and Nicaragua from ambient seismic noise using two independent methods, noise cross correlation and beamforming. The noise cross correlation and beamforming methods are compared and contrasted by evaluating results from the TUCAN array. The results of the two methods as applied to the TUCAN array agree within 1%, giving good confidence in the phase velocities extracted from noise.

Science, 2001

Shear-wave splitting analysis of local events recorded on land and on the ocean floor in the Tong... more Shear-wave splitting analysis of local events recorded on land and on the ocean floor in the Tonga arc and Lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate. These observations suggest that the direction of mantle flow rotates from convergence-parallel in the Fiji plateau to north-south beneath the Lau basin and arc-parallel beneath the Tonga arc. These results correlate with helium isotopes that map mantle flow of the Samoan plume into the Lau basin through an opening tear in the Pacific plate.

International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1996

We employed ambient-noise measurements to assess the potential for seismic site response in sedim... more We employed ambient-noise measurements to assess the potential for seismic site response in sediment-filled valleys that intersect beneath downtown Providence, Rhode Island. At eight valley stations and at two sites on an adjacent bedrock highland, we recorded ground motion from two types of sources: pile drivers at a local construction site and ambient microtremors. At all valley sites where sediment thicknesses exceed 10 m, spectral ratios contain amplitude peaks at frequencies of 1.5 to 3.0 Hz. In contrast, spectral ratios from the two sites on the bedrock highland where sediment cover is less than 4-m thick are relatively flat within this frequency range. A variety of borehole logs identified two fundamental sediment types (soft sediment and a consolidated glacial till) and were used to map layer thicknesses over the entire study region. Refraction data constrained P-wave velocity in the bedrock to be 3680 _ 160 m/sec and indicated two soft-sediment layers with P-wave velocities of 300 _ 50 and 1580 __+ 120 m/sec. Using a one-dimensional reflection matrix technique, we matched the spectral-ratio peak observed at each valley site with the frequency of fundamental resonance predicted for local layer thicknesses and velocities. A positive correlation between the best-fitting soft-sediment velocities and bedrock depth may reflect greater compaction in the deepest sediments or a locally twodimensional sediment resonance at the deepest sediment sites. We conclude that unconsolidated sediment layers under downtown Providence have the potential to amplify earthquake ground motion at frequencies damaging to engineered structures.

Geochemistry, Geophysics, Geosystems, 2014

Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laborato... more Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry meltfree rocks, but require 1022103extrapolationsingrainsizetomantleconditions;also,theeffectsofwaterandmeltarenotwellunderstood.Atthesametime,bodywaveattenuationmeasuredfromdensebroadbandarraysprovidesreliableestimatesofshearwaveattenuation(Q21S),affordinganopportunityforcalibration.Wereanalyzeseismicdatasetsthatsamplearcandback−arcmantleinCentralAmerica,theMarianas,andtheLauBasin,confirmingveryhighattenuation(QS10 2 210 3 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation (Q 21 S), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (Q S 1022103extrapolationsingrainsizetomantleconditions;also,theeffectsofwaterandmeltarenotwellunderstood.Atthesametime,bodywaveattenuationmeasuredfromdensebroadbandarraysprovidesreliableestimatesofshearwaveattenuation(Q21S),affordinganopportunityforcalibration.Wereanalyzeseismicdatasetsthatsamplearcandback−arcmantleinCentralAmerica,theMarianas,andtheLauBasin,confirmingveryhighattenuation(QS 25-80) at 1 Hz and depths of 50-100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300-1450 C. The Q S measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict Q S too high by a factor of 1.5-5. Modifying models to include effects of H 2 O and rheologydependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H 2 O effects are modest. Therefore, high H 2 O in the arc source region cannot explain the low Q S , nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave Q S can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs.

Geophysical Monograph Series, 2005

We inverted Rayleigh wave data recorded in the Rocky Mountain Front Broadband Seismic Experiment ... more We inverted Rayleigh wave data recorded in the Rocky Mountain Front Broadband Seismic Experiment for shear-wave velocity structure and azimuthal anisotropy. Distinctive structures are imaged beneath the southern Rocky Mountains, the western Great Plains, and the eastern Colorado Plateau. Beneath the southern Rockies, shear velocities are anomalously low from the Moho to depths of 150 km or more, suggesting replacement or delamination of the mantle lithosphere. The lowest velocities are beneath the extension of the Rio Grande rift into southern Colorado and are probably associated with partial melt. Beneath the Colorado Plateau, a thin, high-velocity lid is underlain by a low velocity layer to a depth of at least 160 km. Under the high plains, the velocities are above average down to ~150 km depth, but not as fast as beneath the cratonic core of the continent. A crustal, low-velocity anomaly is observed beneath the high elevations of central Colorado. Elsewhere, inferred crustal thickness correlates with elevation, with the thickest crust beneath the San Juan Mountains in southwestern Colorado. These crustal anomalies suggest that much of the isostatic compensation for the high topography takes place within the crust. We observe a simple pattern of azimuthal anisotropy in the Rocky Mountain region with fast directions rotated slightly counterclockwise from the absolute plate motion of the North America plate and strength increasing with period. The observed anisotropy can be explained by deep asthenospheric flow dominated by current plate motion and shallower and perhaps laterally variable anisotropy in the upper lithosphere.

Eos, Transactions American Geophysical Union, 2004

This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for unifo... more This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for uniform worldwide coverage of the Earth.A total of 136 GSN stations are now sited from the South Pole to Siberia, and from the Amazon Basin to the sea floor of the northeast Pacific Ocean-in cooperation with over 100 host organizations and seismic networks in 59 countries worldwide (Figure 1). Established in 1986 by the Incorporated Research Institutions for Seismology (IRIS) to replace the obsolete, analog Worldwide Standardized Seismograph Network (WWSSN), the GSN continues a tradition in global seismology that dates back more than a century to the network of Milne seismographs that initially spanned the globe.The GSN is a permanent network of state-of-the-art seismological and geophysical sensors connected by available telecommunications to serve as a multi-use scientific facility and societal resource for scientific research, environmental monitoring, and education for our national and international community. All GSN data are freely and openly available via the Internet both in real-time and from archival storage at the IRIS Data Management System (www. iris.edu). GSN instrumentation is capable of measuring and recording with high fidelity all of Earth's vibrations, from high-frequency, strong ground motions near an earthquake, to the slowest free oscillations of the Earth (Figure 2). GSN seismometers have recorded both the greatest earthquakes on scale (for example, the 1994 Mw-8.2 Bolivia earthquake at 660 km depth; Wallace [1995]),as well as the nano-earthquakes (M < 0) near the sea floor at the Hawaii-2 Observatory [Butler, 2003]. GSN sensors are accurately calibrated, and timing is based on GPS clocks. The primary focus in creating the GSN has been seismology. However, the power, telemetry, site, and logistical infrastructure at GSN stations

Geophysical Journal International, 2014

We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by a... more We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by applying shear wave splitting analysis and shear wave splitting tomography to local S waves and teleseismic SKS, SKKS and PKS phases. Overall, a very complex pattern of fast directions throughout the southern Puna plateau region and a circular pattern of fast directions around the region of the giant Cerro Galan ignimbrite complex are observed. In general, teleseismic lag times are much greater than those for local events which are interpreted to reflect a significant amount of sub and inner slab anisotropy. The complex pattern observed from shear wave splitting analysis alone is the result of a complex 3-D anisotropic structure under the southern Puna plateau. Our application of shear wave splitting tomography provides a 3-D model of anisotropy in the southern Puna plateau that shows different patterns depending on the driving mechanism of upper-mantle flow and seismic anisotropy. The trench parallel a-axes in the continental lithosphere above the slab east of 68W may be related to deformation of the overriding continental lithosphere since it is under compressive stresses which are orthogonal to the trench. The more complex pattern below the Cerro Galan ignimbrite complex and above the slab is interpreted to reflect delamination of continental lithosphere and upwelling of hot asthenosphere. The a-axes beneath the Cerro Galan, Cerro Blanco and Carachi Pampa volcanic centres at 100 km depth show some weak evidence for vertically orientated fast directions, which could be due to vertical asthenospheric flow around a delaminated block. Additionally, our splitting tomographic model shows that there is a significant amount of seismic anisotropy beneath the slab. The subslab mantle west of 68W shows roughly trench parallel horizontal a-axes that are probably driven by slab roll back and the relatively small coupling between the Nazca slab and the underlying mantle. In contrast, the subslab region (i.e. depths greater than 200 km) east of 68W shows a circular pattern of a-axes centred on a region with small strength of anisotropy (Cerro Galan and its eastern edge) which suggest the dominant mechanism is a combination of slab roll back and flow driven by an overlying abnormally heated slab or possibly a slab gap. There seems to be some evidence for vertical flow below the slab at depths of 200-400 km driven by the abnormally heated slab or slab gap. This cannot be resolved by the tomographic inversion due to the lack of ray crossings in the subslab mantle.

Science, 2011

Seismic imaging reveals the degree to which extensional forces can pull apart the lithosphere.

Pure and Applied Geophysics, 1998

We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath ... more We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520-660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410-520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow.

Physics of the Earth and Planetary Interiors, 2010

We model 56 segments of subduction zones using kinematically defined slabs based on updated geome... more We model 56 segments of subduction zones using kinematically defined slabs based on updated geometries from Syracuse and Abers (2006) to obtain a comprehensive suite of thermal models for the global subduction system. These two-dimensional thermal models provide insight to the dehydration and melting processes that occur in subduction zones. Despite the wide range of slab geometries, ages, convergence velocities and upper plates the predicted thermal structures share many common features. All models feature partial coupling between the slab and the overriding plate directly downdip of the thrust zone, invoked to replicate the cold nose observed in measurements of heat flow and seismic attenuation. We test four separate assumptions about the causes of the partial coupling: (1) the downdip end of the partial coupling is at a constant depth, (2) it is at constant distance trenchward from the arc, (3) it is defined by a critical surface slab temperature, or (4) it is adjusted such that the hottest part of the mantle wedge beneath the arc is at a constant temperature for all subduction zones. In all of these models, slabs reach temperatures where the top of the oceanic crust and sediments dehydrate before they reach subarc depths, and the overlying mantle wedge is too hot for hydrous minerals to be stable at subarc depths. By contrast, the interior of the oceanic crust and underlying mantle within the downgoing plate remains cold enough for hydrous phases to be stable beyond the arc in all but the hottest subduction zones, allowing water to be carried beyond the arc in the slab.

The Journal of the Acoustical Society of America, 2006

We investigate the resolving power and applicability of a recently developed technique for multic... more We investigate the resolving power and applicability of a recently developed technique for multichannel inversion of scattered teleseismic body waves recorded at dense seismic arrays. The problem is posed for forward-and back-scattered wavefields generated at discontinuities in a 2D isotropic medium, with the backprojection operator cast as a generalized Radon transform (GRT). The approach allows for the treatment of incident plane waves from arbitrary backazimuths, and recovers estimates of material property perturbations about a smoothly varying reference model. An investigation of the main factors affecting resolution indicates that: (1) comprehensive source/station coverage is necessary to optimize geometrical resolution and recover accurate material property perturbations; (2) the range in dip resolution diminishes with increasing depth and is inversely proportional to array width (e.g., reaches [−45°,45°] at depths equivalent to~1/2 array width); (3) distortion of the image due to spatial aliasing is only significant at depths ≤2 × [station spacing]; and (4) unaccounted for departures from model assumptions (i.e., isotropy and 2D geometry) result in defocusing and mismapping of structure. Two applications to field data are presented. The first considers data from the Abitibi 1996 broadband array, in which stations were deployed at~20 km intervals. Imaging results show that this level of spatial sampling, which is characteristic of modern broadband arrays, is sufficient to adequately resolve structure below mid-crustal depths. For these data, we introduce a new preprocessing algorithm that uses eigenimage decomposition of seismic sections to suppress wavefield contamination by PcP and PP phases. The second application involves short period data from the Los Angeles Region Seismic Experiment and shows that images obtained from high frequency records are subject to significant contamination by scattered surface waves.

Journal of Geophysical Research, 2010

SKS and SKKS splitting observations are used to constrain the pattern of mantle flow in the Centr... more SKS and SKKS splitting observations are used to constrain the pattern of mantle flow in the Central American subduction zone beneath Costa Rica and Nicaragua. After removing the effects of shallow wedge anisotropy on SK(K)S waveforms, a best-fitting model of anisotropy beneath the Cocos Plate and in the deeper mantle wedge is determined. Fast polarization directions and model symmetry axis orientations in both regions (as well as the shallow wedge) are dominated by roughly arc-parallel azimuths and, therefore, are not consistent with sublithospheric mantle flow entrained by the subducting Cocos Plate or simple two-dimensional corner flow in the wedge. In conjunction with geochemical data and localS splitting tomography, the SK(K)S splitting observations and anisotropy models are consistent with flow to the WNW within the mantle wedge on the Caribbean side of the Cocos Plate, possibly drawn through a slab window beneath Panama and southern Costa Rica. Anisotropy in the Pacific mantle beneath the Cocos Plate is also best explained by flow with a component that is roughly parallel to the strike of the slab, although the absolute direction of this flow is not uniquely constrained.

Journal of Geophysical Research, 2007

S-toP (Sp) scattered energy independently confirms the existence of a seismic velocity discontinu... more S-toP (Sp) scattered energy independently confirms the existence of a seismic velocity discontinuity at the lithosphere-asthenosphere boundary that was previously imaged using P-to-S (Ps) scattered energy in eastern North America. Exploration of the different sensitivities of Ps and Sp scattered energy suggests that the phases contain independent yet complementary high-resolution information regarding velocity contrasts. Combined inversions of Ps and Sp energy have the potential to tightly constrain associated velocity gradients. In eastern North America, inversions of Sp and Ps data require a strong, 5-10% velocity contrast that is also sharp, occurring over less than 11 km at 87-105 km depth. Thermal gradients alone are insufficient to create such a sharp boundary, and therefore another mechanism is required. A boundary in composition, hydration, or a change in anisotropic signature could easily produce a sufficiently localized velocity gradient. Taken separately, the magnitudes of the effects of these mechanisms are too small to match our observed velocity gradients. However, our observations may be explained by a boundary in hydration coupled with a boundary in depletion and/or anisotropy. Alternatively, a small amount of melt in the asthenosphere could explain the velocity gradient. The tight constraints on velocity gradients achieved by combined modeling of Ps and Sp energy offer promise for defining the character of the lithosphere-asthenosphere boundary globally.

Journal of Geophysical Research: Solid Earth, 2003

The Archean Tanzanian craton, nestled between the eastern and western branches of the East Africa... more The Archean Tanzanian craton, nestled between the eastern and western branches of the East African Rift, presents a unique opportunity to study the interaction of active rifting with stable cratonic lithosphere. The high density of Rayleigh wave paths recorded in a regional seismic array yields unusually precise determinations of phase velocity within the Tanzanian craton. Shear velocities in the cratonic lithosphere are higher than a global average to a depth of 150 ± 20 km. Beginning at 140 km, shear velocity decreases sharply, reaching a minimum of 4.20 ± 0.05 km/s at depths of 200-250 km. The base of the lithosphere, identified by the depth to the center of the maximum negative velocity gradient, is similar to that found beneath other Archean lithospheres. Where Cenozoic rifting crosscuts the southern corner of the craton, velocities up to 130 km depth are reduced, indicating recent disruption of the lithosphere. The anomalously low velocities beneath the Tanzanian craton indicate high temperatures and the presence of melt, consistent with the spreading of a mantle plume head beneath the craton. Tests for the possibility of a radial pattern of azimuthal anisotropy that may indicate outward flow from a plume show that a model with average anisotropy of 0.71 ± 0.17% centered SE of Lake Victoria fits the data significantly better than a uniform, single direction of anisotropy. Thus our results agree with the suggestion that an upper mantle plume, centered beneath the Tanzanian cratonic lithosphere, provides the buoyancy required for uplift of the East African Plateau.

Journal of Geophysical Research: Solid Earth, 2000

The goal of this study is to determine whether shear wave splitting observed in subduction zone b... more The goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions, the Tonga subduction zone in particular, can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate. We calculated two-dimensional mantle flow models using known Tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures. Shear wave splitting was predicted for the anisotropy due to lattice preferred orientation (LPO) of olivine and orthopyroxene in the flow model finite strain fields. The predicted shear wave splitting provides a good match to the fast directions (parallel to the azimuth of subducting plate motion) and splitting times (0.5-1.5 s) observed in Tonga, both for models where LPO anisotropy develops everywhere above 410 km and for models where LPO anisotropy is confined to regions of relatively high stress. If LPO anisotropy does develop over the entire upper 410 km of the mantle, the strength of anistropy induced by a given amount of shear strain must be relatively weak (-4% for shear strains of 1.5, with a maximum value of-6% for very large strains). The splitting observations are comparably fit by a wide range of different viscosity models. Anisotropy due to melt-filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting.

Journal of Geophysical Research, 2003

In recent years, a wide range of geophysical results have offered evidence that Earth's lowermost... more In recent years, a wide range of geophysical results have offered evidence that Earth's lowermost mantle is characterized by strong lateral variations in material properties. Among the structures of particular interest are intermittent ultralow-velocity zones (ULVZs), located directly above the core-mantle boundary (CMB), which were originally inferred from the distortion of teleseismic SPdKS phases. ULVZs have been modeled as layers with sharp boundaries and seismic velocity reductions !10% and interpreted as regions of partial melt. In this study, we further constrain local ULVZ structure beneath North America by signal processing and waveform modeling of the SKS coda recorded at broadband seismic arrays. Secondary phases in the SKS coda are effectively isolated by eigenimage processing. Residual (i.e., SKS-less) data sections from various western Pacific events display clear SPdKS arrivals, followed by a secondary phase whose timing and slowness are consistent with CMB origins. One-dimensional modeling of these phases by reflectivity and generalized ray synthetics favors an asymmetric model, with ULVZ present at only one of the CMB intercepts. The preferred ULVZ is characterized by reductions in P and S velocities of 18% and 50%, respectively, and a diffuse upper boundary. These characteristics are consistent with local production and gravitational sinking of dense (e.g., iron rich) partial melt above the CMB. We postulate that a gradational ULVZ beneath North America may mark a lateral transition domain between regions of mantle upwelling, where more uniform ULVZs exist, and regions of downwelling, where ULVZs are either nonexistent or imperceptibly thin.

Journal of Geophysical Research, 2002

Crust and mantle discontinuities across the eastern margin of the North American craton were imag... more Crust and mantle discontinuities across the eastern margin of the North American craton were imaged using P to S converted phase receiver functions recorded by the Missouri to Massachusetts Broadband Seismometer Experiment. Crustal structure constrained by modeling Moho conversions and reverberations shows a variation of Moho depth from a minimum of 30 km near the Atlantic coast to depths of 44-49 km beneath the western Appalachian Province and 38-45 km beneath the Proterozoic terranes in the west. The variation in crustal thickness is substantially greater than that required for local isostasy, unless lower crustal densities are >3110 kg/m 3. In the upper mantle, Ps phases corresponding to a discontinuity at depths of 270-280 km were clearly observed beneath the eastern half of the array. Beneath the western third of the array, the receiver function stacks indicate more complex scattering, but weak Ps phases may be generated at depths of roughly 320 km. The transition between these two regions occurs across the eastern edge of the North American lithospheric keel imaged by tomography. The observed phases may be interpreted as conversions from the base of a low-velocity asthenosphere.

Journal of Geophysical Research, 2003

We have obtained shear velocity structure beneath the northeastern United States and southeastern... more We have obtained shear velocity structure beneath the northeastern United States and southeastern Canada using Rayleigh wave phases and amplitudes. Thin crust (36-42 km) is observed along the Atlantic coast and in the eastern Appalachian orogen, and thick crust (42-46 km) is imaged in the western Appalachians and in the western New York portion of the Grenville Province. The variation of crustal thickness correlates well with the observed Bouguer gravity anomalies. In the upper mantle, the high-velocity continental keel of cratonic North America is present in the western part of the study area, while a broad low-velocity region is imaged in New England from the Hudson River valley to the White Mountains. This low-velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the Monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel. In addition to lateral variations in velocity, we estimate the azimuthal dependence of phase velocity. Strong and relatively uniform shear wave splitting is observed in the study region, but at periods of 100 s or less, the average azimuthal anisotropy of Rayleigh waves is less than 1% and is not significantly different from zero at any individual period. This small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere. Much of the source of the shear wave splitting must lie deeper than 200 km.

Geophysical Research Letters, 2008

An expression for the cross correlation of noise between seismic stations and the 2D Green's func... more An expression for the cross correlation of noise between seismic stations and the 2D Green's function is derived assuming that noise travels as 2D surface waves. The phase velocity is obtained directly from the noise correlation function with a phase shift of +p/4. Mean phase velocity dispersion curves are calculated for the TUCAN seismic array in Costa Rica and Nicaragua from ambient seismic noise using two independent methods, noise cross correlation and beamforming. The noise cross correlation and beamforming methods are compared and contrasted by evaluating results from the TUCAN array. The results of the two methods as applied to the TUCAN array agree within 1%, giving good confidence in the phase velocities extracted from noise.

Science, 2001

Shear-wave splitting analysis of local events recorded on land and on the ocean floor in the Tong... more Shear-wave splitting analysis of local events recorded on land and on the ocean floor in the Tonga arc and Lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate. These observations suggest that the direction of mantle flow rotates from convergence-parallel in the Fiji plateau to north-south beneath the Lau basin and arc-parallel beneath the Tonga arc. These results correlate with helium isotopes that map mantle flow of the Samoan plume into the Lau basin through an opening tear in the Pacific plate.

International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1996

We employed ambient-noise measurements to assess the potential for seismic site response in sedim... more We employed ambient-noise measurements to assess the potential for seismic site response in sediment-filled valleys that intersect beneath downtown Providence, Rhode Island. At eight valley stations and at two sites on an adjacent bedrock highland, we recorded ground motion from two types of sources: pile drivers at a local construction site and ambient microtremors. At all valley sites where sediment thicknesses exceed 10 m, spectral ratios contain amplitude peaks at frequencies of 1.5 to 3.0 Hz. In contrast, spectral ratios from the two sites on the bedrock highland where sediment cover is less than 4-m thick are relatively flat within this frequency range. A variety of borehole logs identified two fundamental sediment types (soft sediment and a consolidated glacial till) and were used to map layer thicknesses over the entire study region. Refraction data constrained P-wave velocity in the bedrock to be 3680 _ 160 m/sec and indicated two soft-sediment layers with P-wave velocities of 300 _ 50 and 1580 __+ 120 m/sec. Using a one-dimensional reflection matrix technique, we matched the spectral-ratio peak observed at each valley site with the frequency of fundamental resonance predicted for local layer thicknesses and velocities. A positive correlation between the best-fitting soft-sediment velocities and bedrock depth may reflect greater compaction in the deepest sediments or a locally twodimensional sediment resonance at the deepest sediment sites. We conclude that unconsolidated sediment layers under downtown Providence have the potential to amplify earthquake ground motion at frequencies damaging to engineered structures.

Geochemistry, Geophysics, Geosystems, 2014

Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laborato... more Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry meltfree rocks, but require 1022103extrapolationsingrainsizetomantleconditions;also,theeffectsofwaterandmeltarenotwellunderstood.Atthesametime,bodywaveattenuationmeasuredfromdensebroadbandarraysprovidesreliableestimatesofshearwaveattenuation(Q21S),affordinganopportunityforcalibration.Wereanalyzeseismicdatasetsthatsamplearcandback−arcmantleinCentralAmerica,theMarianas,andtheLauBasin,confirmingveryhighattenuation(QS10 2 210 3 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation (Q 21 S), affording an opportunity for calibration. We reanalyze seismic data sets that sample arc and back-arc mantle in Central America, the Marianas, and the Lau Basin, confirming very high attenuation (Q S 1022103extrapolationsingrainsizetomantleconditions;also,theeffectsofwaterandmeltarenotwellunderstood.Atthesametime,bodywaveattenuationmeasuredfromdensebroadbandarraysprovidesreliableestimatesofshearwaveattenuation(Q21S),affordinganopportunityforcalibration.Wereanalyzeseismicdatasetsthatsamplearcandback−arcmantleinCentralAmerica,theMarianas,andtheLauBasin,confirmingveryhighattenuation(QS 25-80) at 1 Hz and depths of 50-100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300-1450 C. The Q S measurements correlate inversely with the petrologically inferred temperatures, as expected. However, dry attenuation models predict Q S too high by a factor of 1.5-5. Modifying models to include effects of H 2 O and rheologydependent grain size shows that the effects of water-enhanced dissipation and water-enhanced grain growth nearly cancel, so H 2 O effects are modest. Therefore, high H 2 O in the arc source region cannot explain the low Q S , nor in the back arc where lavas show modest water content. Most likely, the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body wave Q S can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs.

Geophysical Monograph Series, 2005

We inverted Rayleigh wave data recorded in the Rocky Mountain Front Broadband Seismic Experiment ... more We inverted Rayleigh wave data recorded in the Rocky Mountain Front Broadband Seismic Experiment for shear-wave velocity structure and azimuthal anisotropy. Distinctive structures are imaged beneath the southern Rocky Mountains, the western Great Plains, and the eastern Colorado Plateau. Beneath the southern Rockies, shear velocities are anomalously low from the Moho to depths of 150 km or more, suggesting replacement or delamination of the mantle lithosphere. The lowest velocities are beneath the extension of the Rio Grande rift into southern Colorado and are probably associated with partial melt. Beneath the Colorado Plateau, a thin, high-velocity lid is underlain by a low velocity layer to a depth of at least 160 km. Under the high plains, the velocities are above average down to ~150 km depth, but not as fast as beneath the cratonic core of the continent. A crustal, low-velocity anomaly is observed beneath the high elevations of central Colorado. Elsewhere, inferred crustal thickness correlates with elevation, with the thickest crust beneath the San Juan Mountains in southwestern Colorado. These crustal anomalies suggest that much of the isostatic compensation for the high topography takes place within the crust. We observe a simple pattern of azimuthal anisotropy in the Rocky Mountain region with fast directions rotated slightly counterclockwise from the absolute plate motion of the North America plate and strength increasing with period. The observed anisotropy can be explained by deep asthenospheric flow dominated by current plate motion and shallower and perhaps laterally variable anisotropy in the upper lithosphere.

Eos, Transactions American Geophysical Union, 2004

This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for unifo... more This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for uniform worldwide coverage of the Earth.A total of 136 GSN stations are now sited from the South Pole to Siberia, and from the Amazon Basin to the sea floor of the northeast Pacific Ocean-in cooperation with over 100 host organizations and seismic networks in 59 countries worldwide (Figure 1). Established in 1986 by the Incorporated Research Institutions for Seismology (IRIS) to replace the obsolete, analog Worldwide Standardized Seismograph Network (WWSSN), the GSN continues a tradition in global seismology that dates back more than a century to the network of Milne seismographs that initially spanned the globe.The GSN is a permanent network of state-of-the-art seismological and geophysical sensors connected by available telecommunications to serve as a multi-use scientific facility and societal resource for scientific research, environmental monitoring, and education for our national and international community. All GSN data are freely and openly available via the Internet both in real-time and from archival storage at the IRIS Data Management System (www. iris.edu). GSN instrumentation is capable of measuring and recording with high fidelity all of Earth's vibrations, from high-frequency, strong ground motions near an earthquake, to the slowest free oscillations of the Earth (Figure 2). GSN seismometers have recorded both the greatest earthquakes on scale (for example, the 1994 Mw-8.2 Bolivia earthquake at 660 km depth; Wallace [1995]),as well as the nano-earthquakes (M < 0) near the sea floor at the Hawaii-2 Observatory [Butler, 2003]. GSN sensors are accurately calibrated, and timing is based on GPS clocks. The primary focus in creating the GSN has been seismology. However, the power, telemetry, site, and logistical infrastructure at GSN stations

Geophysical Journal International, 2014

We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by a... more We have investigated the seismic anisotropy beneath the Central Andean southern Puna plateau by applying shear wave splitting analysis and shear wave splitting tomography to local S waves and teleseismic SKS, SKKS and PKS phases. Overall, a very complex pattern of fast directions throughout the southern Puna plateau region and a circular pattern of fast directions around the region of the giant Cerro Galan ignimbrite complex are observed. In general, teleseismic lag times are much greater than those for local events which are interpreted to reflect a significant amount of sub and inner slab anisotropy. The complex pattern observed from shear wave splitting analysis alone is the result of a complex 3-D anisotropic structure under the southern Puna plateau. Our application of shear wave splitting tomography provides a 3-D model of anisotropy in the southern Puna plateau that shows different patterns depending on the driving mechanism of upper-mantle flow and seismic anisotropy. The trench parallel a-axes in the continental lithosphere above the slab east of 68W may be related to deformation of the overriding continental lithosphere since it is under compressive stresses which are orthogonal to the trench. The more complex pattern below the Cerro Galan ignimbrite complex and above the slab is interpreted to reflect delamination of continental lithosphere and upwelling of hot asthenosphere. The a-axes beneath the Cerro Galan, Cerro Blanco and Carachi Pampa volcanic centres at 100 km depth show some weak evidence for vertically orientated fast directions, which could be due to vertical asthenospheric flow around a delaminated block. Additionally, our splitting tomographic model shows that there is a significant amount of seismic anisotropy beneath the slab. The subslab mantle west of 68W shows roughly trench parallel horizontal a-axes that are probably driven by slab roll back and the relatively small coupling between the Nazca slab and the underlying mantle. In contrast, the subslab region (i.e. depths greater than 200 km) east of 68W shows a circular pattern of a-axes centred on a region with small strength of anisotropy (Cerro Galan and its eastern edge) which suggest the dominant mechanism is a combination of slab roll back and flow driven by an overlying abnormally heated slab or possibly a slab gap. There seems to be some evidence for vertical flow below the slab at depths of 200-400 km driven by the abnormally heated slab or slab gap. This cannot be resolved by the tomographic inversion due to the lack of ray crossings in the subslab mantle.

Science, 2011

Seismic imaging reveals the degree to which extensional forces can pull apart the lithosphere.

Pure and Applied Geophysics, 1998

We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath ... more We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520-660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410-520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow.

Physics of the Earth and Planetary Interiors, 2010

We model 56 segments of subduction zones using kinematically defined slabs based on updated geome... more We model 56 segments of subduction zones using kinematically defined slabs based on updated geometries from Syracuse and Abers (2006) to obtain a comprehensive suite of thermal models for the global subduction system. These two-dimensional thermal models provide insight to the dehydration and melting processes that occur in subduction zones. Despite the wide range of slab geometries, ages, convergence velocities and upper plates the predicted thermal structures share many common features. All models feature partial coupling between the slab and the overriding plate directly downdip of the thrust zone, invoked to replicate the cold nose observed in measurements of heat flow and seismic attenuation. We test four separate assumptions about the causes of the partial coupling: (1) the downdip end of the partial coupling is at a constant depth, (2) it is at constant distance trenchward from the arc, (3) it is defined by a critical surface slab temperature, or (4) it is adjusted such that the hottest part of the mantle wedge beneath the arc is at a constant temperature for all subduction zones. In all of these models, slabs reach temperatures where the top of the oceanic crust and sediments dehydrate before they reach subarc depths, and the overlying mantle wedge is too hot for hydrous minerals to be stable at subarc depths. By contrast, the interior of the oceanic crust and underlying mantle within the downgoing plate remains cold enough for hydrous phases to be stable beyond the arc in all but the hottest subduction zones, allowing water to be carried beyond the arc in the slab.

The Journal of the Acoustical Society of America, 2006

We investigate the resolving power and applicability of a recently developed technique for multic... more We investigate the resolving power and applicability of a recently developed technique for multichannel inversion of scattered teleseismic body waves recorded at dense seismic arrays. The problem is posed for forward-and back-scattered wavefields generated at discontinuities in a 2D isotropic medium, with the backprojection operator cast as a generalized Radon transform (GRT). The approach allows for the treatment of incident plane waves from arbitrary backazimuths, and recovers estimates of material property perturbations about a smoothly varying reference model. An investigation of the main factors affecting resolution indicates that: (1) comprehensive source/station coverage is necessary to optimize geometrical resolution and recover accurate material property perturbations; (2) the range in dip resolution diminishes with increasing depth and is inversely proportional to array width (e.g., reaches [−45°,45°] at depths equivalent to~1/2 array width); (3) distortion of the image due to spatial aliasing is only significant at depths ≤2 × [station spacing]; and (4) unaccounted for departures from model assumptions (i.e., isotropy and 2D geometry) result in defocusing and mismapping of structure. Two applications to field data are presented. The first considers data from the Abitibi 1996 broadband array, in which stations were deployed at~20 km intervals. Imaging results show that this level of spatial sampling, which is characteristic of modern broadband arrays, is sufficient to adequately resolve structure below mid-crustal depths. For these data, we introduce a new preprocessing algorithm that uses eigenimage decomposition of seismic sections to suppress wavefield contamination by PcP and PP phases. The second application involves short period data from the Los Angeles Region Seismic Experiment and shows that images obtained from high frequency records are subject to significant contamination by scattered surface waves.

Journal of Geophysical Research, 2010

SKS and SKKS splitting observations are used to constrain the pattern of mantle flow in the Centr... more SKS and SKKS splitting observations are used to constrain the pattern of mantle flow in the Central American subduction zone beneath Costa Rica and Nicaragua. After removing the effects of shallow wedge anisotropy on SK(K)S waveforms, a best-fitting model of anisotropy beneath the Cocos Plate and in the deeper mantle wedge is determined. Fast polarization directions and model symmetry axis orientations in both regions (as well as the shallow wedge) are dominated by roughly arc-parallel azimuths and, therefore, are not consistent with sublithospheric mantle flow entrained by the subducting Cocos Plate or simple two-dimensional corner flow in the wedge. In conjunction with geochemical data and localS splitting tomography, the SK(K)S splitting observations and anisotropy models are consistent with flow to the WNW within the mantle wedge on the Caribbean side of the Cocos Plate, possibly drawn through a slab window beneath Panama and southern Costa Rica. Anisotropy in the Pacific mantle beneath the Cocos Plate is also best explained by flow with a component that is roughly parallel to the strike of the slab, although the absolute direction of this flow is not uniquely constrained.

Journal of Geophysical Research, 2007

S-toP (Sp) scattered energy independently confirms the existence of a seismic velocity discontinu... more S-toP (Sp) scattered energy independently confirms the existence of a seismic velocity discontinuity at the lithosphere-asthenosphere boundary that was previously imaged using P-to-S (Ps) scattered energy in eastern North America. Exploration of the different sensitivities of Ps and Sp scattered energy suggests that the phases contain independent yet complementary high-resolution information regarding velocity contrasts. Combined inversions of Ps and Sp energy have the potential to tightly constrain associated velocity gradients. In eastern North America, inversions of Sp and Ps data require a strong, 5-10% velocity contrast that is also sharp, occurring over less than 11 km at 87-105 km depth. Thermal gradients alone are insufficient to create such a sharp boundary, and therefore another mechanism is required. A boundary in composition, hydration, or a change in anisotropic signature could easily produce a sufficiently localized velocity gradient. Taken separately, the magnitudes of the effects of these mechanisms are too small to match our observed velocity gradients. However, our observations may be explained by a boundary in hydration coupled with a boundary in depletion and/or anisotropy. Alternatively, a small amount of melt in the asthenosphere could explain the velocity gradient. The tight constraints on velocity gradients achieved by combined modeling of Ps and Sp energy offer promise for defining the character of the lithosphere-asthenosphere boundary globally.

Journal of Geophysical Research: Solid Earth, 2003

The Archean Tanzanian craton, nestled between the eastern and western branches of the East Africa... more The Archean Tanzanian craton, nestled between the eastern and western branches of the East African Rift, presents a unique opportunity to study the interaction of active rifting with stable cratonic lithosphere. The high density of Rayleigh wave paths recorded in a regional seismic array yields unusually precise determinations of phase velocity within the Tanzanian craton. Shear velocities in the cratonic lithosphere are higher than a global average to a depth of 150 ± 20 km. Beginning at 140 km, shear velocity decreases sharply, reaching a minimum of 4.20 ± 0.05 km/s at depths of 200-250 km. The base of the lithosphere, identified by the depth to the center of the maximum negative velocity gradient, is similar to that found beneath other Archean lithospheres. Where Cenozoic rifting crosscuts the southern corner of the craton, velocities up to 130 km depth are reduced, indicating recent disruption of the lithosphere. The anomalously low velocities beneath the Tanzanian craton indicate high temperatures and the presence of melt, consistent with the spreading of a mantle plume head beneath the craton. Tests for the possibility of a radial pattern of azimuthal anisotropy that may indicate outward flow from a plume show that a model with average anisotropy of 0.71 ± 0.17% centered SE of Lake Victoria fits the data significantly better than a uniform, single direction of anisotropy. Thus our results agree with the suggestion that an upper mantle plume, centered beneath the Tanzanian cratonic lithosphere, provides the buoyancy required for uplift of the East African Plateau.

Journal of Geophysical Research: Solid Earth, 2000

The goal of this study is to determine whether shear wave splitting observed in subduction zone b... more The goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions, the Tonga subduction zone in particular, can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate. We calculated two-dimensional mantle flow models using known Tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures. Shear wave splitting was predicted for the anisotropy due to lattice preferred orientation (LPO) of olivine and orthopyroxene in the flow model finite strain fields. The predicted shear wave splitting provides a good match to the fast directions (parallel to the azimuth of subducting plate motion) and splitting times (0.5-1.5 s) observed in Tonga, both for models where LPO anisotropy develops everywhere above 410 km and for models where LPO anisotropy is confined to regions of relatively high stress. If LPO anisotropy does develop over the entire upper 410 km of the mantle, the strength of anistropy induced by a given amount of shear strain must be relatively weak (-4% for shear strains of 1.5, with a maximum value of-6% for very large strains). The splitting observations are comparably fit by a wide range of different viscosity models. Anisotropy due to melt-filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting.

Journal of Geophysical Research, 2003

In recent years, a wide range of geophysical results have offered evidence that Earth's lowermost... more In recent years, a wide range of geophysical results have offered evidence that Earth's lowermost mantle is characterized by strong lateral variations in material properties. Among the structures of particular interest are intermittent ultralow-velocity zones (ULVZs), located directly above the core-mantle boundary (CMB), which were originally inferred from the distortion of teleseismic SPdKS phases. ULVZs have been modeled as layers with sharp boundaries and seismic velocity reductions !10% and interpreted as regions of partial melt. In this study, we further constrain local ULVZ structure beneath North America by signal processing and waveform modeling of the SKS coda recorded at broadband seismic arrays. Secondary phases in the SKS coda are effectively isolated by eigenimage processing. Residual (i.e., SKS-less) data sections from various western Pacific events display clear SPdKS arrivals, followed by a secondary phase whose timing and slowness are consistent with CMB origins. One-dimensional modeling of these phases by reflectivity and generalized ray synthetics favors an asymmetric model, with ULVZ present at only one of the CMB intercepts. The preferred ULVZ is characterized by reductions in P and S velocities of 18% and 50%, respectively, and a diffuse upper boundary. These characteristics are consistent with local production and gravitational sinking of dense (e.g., iron rich) partial melt above the CMB. We postulate that a gradational ULVZ beneath North America may mark a lateral transition domain between regions of mantle upwelling, where more uniform ULVZs exist, and regions of downwelling, where ULVZs are either nonexistent or imperceptibly thin.

Journal of Geophysical Research, 2002

Crust and mantle discontinuities across the eastern margin of the North American craton were imag... more Crust and mantle discontinuities across the eastern margin of the North American craton were imaged using P to S converted phase receiver functions recorded by the Missouri to Massachusetts Broadband Seismometer Experiment. Crustal structure constrained by modeling Moho conversions and reverberations shows a variation of Moho depth from a minimum of 30 km near the Atlantic coast to depths of 44-49 km beneath the western Appalachian Province and 38-45 km beneath the Proterozoic terranes in the west. The variation in crustal thickness is substantially greater than that required for local isostasy, unless lower crustal densities are >3110 kg/m 3. In the upper mantle, Ps phases corresponding to a discontinuity at depths of 270-280 km were clearly observed beneath the eastern half of the array. Beneath the western third of the array, the receiver function stacks indicate more complex scattering, but weak Ps phases may be generated at depths of roughly 320 km. The transition between these two regions occurs across the eastern edge of the North American lithospheric keel imaged by tomography. The observed phases may be interpreted as conversions from the base of a low-velocity asthenosphere.

Journal of Geophysical Research, 2003

We have obtained shear velocity structure beneath the northeastern United States and southeastern... more We have obtained shear velocity structure beneath the northeastern United States and southeastern Canada using Rayleigh wave phases and amplitudes. Thin crust (36-42 km) is observed along the Atlantic coast and in the eastern Appalachian orogen, and thick crust (42-46 km) is imaged in the western Appalachians and in the western New York portion of the Grenville Province. The variation of crustal thickness correlates well with the observed Bouguer gravity anomalies. In the upper mantle, the high-velocity continental keel of cratonic North America is present in the western part of the study area, while a broad low-velocity region is imaged in New England from the Hudson River valley to the White Mountains. This low-velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the Monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel. In addition to lateral variations in velocity, we estimate the azimuthal dependence of phase velocity. Strong and relatively uniform shear wave splitting is observed in the study region, but at periods of 100 s or less, the average azimuthal anisotropy of Rayleigh waves is less than 1% and is not significantly different from zero at any individual period. This small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere. Much of the source of the shear wave splitting must lie deeper than 200 km.