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Now showing items 1 - 16 of 53579

  • On corner frequencies, attenuation, and low-frequency earthquakes

    Bostock, M. G.   Thomas, A. M.   Rubin, A. M.   Christensen, N. I.  

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  • Crustal anisotropy in a subduction zone forearc: Northern Cascadia

    Matharu, G.   Bostock, M. G.   Christensen, N. I.   Tromp, Jeroen  

    S-wave splitting analyses using low-frequency earthquake templates at three-component stations across southern Vancouver Island and northern Washington indicate the presence of a heterogeneous distribution of crustal anisotropy in the North American plate. For southern Vancouver Island, we investigate contributions to anisotropy from the Leech River Complex, a terrane composed of strongly foliated phyllites and schists with steeply dipping foliations striking east-west. Fast directions across mainland southern Vancouver Island are subparallel to the dominant Leech River Complex foliation direction. East-to-west increases in delay times and small-scale azimuthal variations in fast directions indicate heterogeneous anisotropy. We test azimuthally anisotropic Leech River Complex models constrained by previous geological and seismic reflection studies, through forward modeling using 3-D spectral element method simulations. The preferred model of a north/northeast shallowly dipping wedge of Leech River Complex material with varying orientation of anisotropy terminating at midcrustal levels explains the splitting observations at a majority of southern Vancouver Island stations. For stations where anisotropic Leech River Complex models do not recreate observations, fast directions are subparallel to local estimates of maximum compressive horizontal stress, suggesting that fluid-filled cracks could be a source of anisotropy. We assert that the Leech River Complex is the primary source of crustal anisotropy beneath southern Vancouver Island, not cracks as suggested by prior studies. Fast directions at stations on northern Washington exhibit variations with azimuth and incidence angle suggesting complex anisotropy interpreted as due to a combination of cracks and preferred mineral orientation of metamorphosed slates of the Olympic core rocks. These slates may also underlay stations on southern Vancouver Island and represent another source of anisotropy.
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  • The Moho in subduction zones

    Bostock, M. G.  

    The Moho in subduction zones exists in two distinct forms, one associated with the subducting oceanic plate and second with the overriding plate. The seismic expression of both forms is linked to the nature of a landward dipping, low-velocity zone (LW) that has been detected in a majority of subduction zones about the globe and that approximately coincides with Wadati-Benioff seismicity. We review seismic studies that constrain the properties of the LW in Cascadia where it has been extensively studied for over a quarter century. A model in which the LW is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal Layer 2, is consistent with available geological and geophysical data, and reconciles previously conflicting interpretations. In this model, the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic Layer 3 below. The resulting seismic velocity contrast (approaching 50% for S-waves) significantly overwhelms that of a weaker, underlying oceanic Moho. At greater depths, oceanic crust undergoes eclogitization in a top-down sense leading to gradual disappearance of the LVZ. The large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled, forearc mantle wedge. Pervasive serpentinization and free fluids reduce velocities within the wedge, thereby diminishing, erasing or even inverting the seismic contrast associated with the Moho of the overriding plate. This model is tested against observations of LVZs and forearc mantle structure worldwide. (C) 2012 Elsevier B.V. All rights reserved.
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  • Low frequency earthquakes below southern Vancouver Island

    Bostock, M. G.   Royer, A. A.   Hearn, E. H.   Peacock, S. M.  

    The nature and distribution of low frequency earthquakes (LFEs) in subduction zones provide insight into plate boundary deformation downdip of the locked seismogenic zone. We employ network autocorrelation detection to identify LFE families beneath southern Vancouver Island and environs. An initial suite of 5775 LFEs detected in 2004 and 2005 at a select set of 7 stations is grouped into 140 families using waveform cluster analysis. These families are used as templates within an iterative network cross correlation scheme to detect LFEs across different tremor episodes, incorporate new stations, and improve LFE template signal-to-noise ratio. As in southwest Japan, representative LFE locations define a relatively tight, dipping surface several km above the locus of intraslab seismicity, within a prominent, dipping low-velocity zone (LVZ). LFE polarizations for near-vertical source-receiver geometries possess a remarkably uniform dipolar signature indicative of point-source, double-couple excitation. Focal mechanisms determined from P-wave first motions are characterized by a combination of strike-slip and thrust faulting. We suggest that LFEs and regular intraslab seismicity occur in distinct structural and stress regimes. The LVZ, inferred to represent weak, overpressured, porous and mylonitized metabasalts of oceanic crustal Layer 2, separates LFEs manifesting deformation within a plate boundary shear zone from intraslab earthquakes generated by tensional stresses and dehydration embrittlement within a more competent lower oceanic crustal Layer 3 and underlying mantle.
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  • Low frequency earthquakes below southern Vancouver Island

    Bostock, M. G.   Royer, A. A.   Hearn, E. H.   Peacock, S. M.  

    The nature and distribution of low frequency earthquakes (LFEs) in subduction zones provide insight into plate boundary deformation downdip of the locked seismogenic zone. We employ network autocorrelation detection to identify LFE families beneath southern Vancouver Island and environs. An initial suite of 5775 LFEs detected in 2004 and 2005 at a select set of 7 stations is grouped into 140 families using waveform cluster analysis. These families are used as templates within an iterative network cross correlation scheme to detect LFEs across different tremor episodes, incorporate new stations, and improve LFE template signal-to-noise ratio. As in southwest Japan, representative LFE locations define a relatively tight, dipping surface several km above the locus of intraslab seismicity, within a prominent, dipping low-velocity zone (LVZ). LFE polarizations for near-vertical source-receiver geometries possess a remarkably uniform dipolar signature indicative of point-source, double-couple excitation. Focal mechanisms determined from P-wave first motions are characterized by a combination of strike-slip and thrust faulting. We suggest that LFEs and regular intraslab seismicity occur in distinct structural and stress regimes. The LVZ, inferred to represent weak, overpressured, porous and mylonitized metabasalts of oceanic crustal Layer 2, separates LFEs manifesting deformation within a plate boundary shear zone from intraslab earthquakes generated by tensional stresses and dehydration embrittlement within a more competent lower oceanic crustal Layer 3 and underlying mantle.
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  • Wave-Field Decomposition of Ocean Bottom Seismograms

    Bostock, M. G.   Trehu, A. M.  

    Seismograms recorded on the seafloor are affected by reverberations in the overlying ocean layer. We examine the feasibility of removing these reverberations through a 1D wave-field decomposition based on a reflection-transmission formulation of the problem. Two decomposition schemes are presented. The first scheme involves a simple manipulation of the fundamental matrix relating stress and displacement of plane harmonic waves to upgoing and downgoing wavevector coefficients and requires measurement of both ocean bottom displacement and pressure. The second approach involves only displacement recordings and knowledge of the water column depth. This latter quantity will generally be known a priori from deployment logs, or, alternatively, itmay be estimated using vertical displacement and pressure seismograms in a preprocessing step. Both approaches require prior information on seabed properties. In the case of P-wave incidence, the decomposition depends primarily on the seabed S velocity beta(0). This quantity can be determined by examining trial decompositions over a range of beta(0)'s and selecting the value that minimizes the energy of the upgoing S-wave component at the arrival time of the incident wave. We apply this approach to synthetic seismograms for a simple Earth structure and to recordings of two large events from the Central Oregon Locked Zone Array (COLZA) on the continental margin of Oregon. The real data indicate that the wave-field decompositions are largely successful at lower frequencies (<0.1 Hz) but that 3D scattering, likely originating near the sediment-basement contact, is manifested at higher frequencies.
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  • Blind deconvolution of seismograms regularized via minimum support

    Royer, A. A.   Bostock, M. G.   Haber, E.  

    The separation of earthquake source signature and propagation effects (the Earth's 'Green's function') that encode a seismogram is a challenging problem in seismology. The task of separating these two effects is called blind deconvolution. By considering seismograms of multiple earthquakes from similar locations recorded at a given station and that therefore share the same Green's function, we may write a linear relation in the time domain u(i)(t) * s(j)(t) - u(j)(t) * s(i)(t) =3D 0, where u(i)(t) is the seismogram for the ith source and s(j)(t) is the jth unknown source. The symbol * represents the convolution operator. From two or more seismograms, we obtain a homogeneous linear system where the unknowns are the sources. This system is subject to a scaling constraint to deliver a non-trivial solution. Since source durations are not known a priori and must be determined, we augment our system by introducing the source durations as unknowns and we solve the combined system (sources and source durations) using separation of variables. Our solution is derived using direct linear inversion to recover the sources and Newton's method to recover source durations. This method is tested using two sets of synthetic seismograms created by convolution of (i) random Gaussian source-time functions and (ii) band-limited sources with a simplified Green's function and signal to noise levels up to 10% with encouraging results.
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  • A low-velocity zone atop the transition zone in northwestern Canada

    Schaeffer, A. J.   Bostock, M. G.  

    Seismic studies over the past decade have identified an S wave low-velocity zone (LVZ) above the transition zone at various locations around the globe. This layer is hypothesized to be a lens of dense, hydrous, silicate melt ponding atop the 410 km discontinuity, beneath the silicate melt-density crossover predicted to exist within the upper mantle. We have assembled a P and S receiver function data set to quantify the physical properties and geographical extent of the layer in northwestern Canada. Geographic profiles formed from 1-D migration of receiver functions computed for the Canadian Northwest Experiment (CANOE) and Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity (POLARIS) Slave arrays reveal an LVZ beneath many stations at a nominal depth of similar to 340 km. To constrain layer thickness and Poisson's ratio, we performed a grid search over a suite of 1-D velocity profiles to model the relative delay times of direct conversions and reverberations from the top of the LVZ and 410 km discontinuity, as recorded at the Yellowknife array. In addition, we performed linearized inversion of transmission coefficient amplitudes to estimate S velocity contrasts at the bounding interfaces. The LVZ is characterized by a thickness of similar to 36 km with an S velocity contrast of -7.8% and Poisson's ratio of 0.42. Taken at face value, the two latter results require an increase in P velocity into the LVZ. The Poisson's ratio lies well above the IASP91 average of similar to 0.29-0.3 for this depth range and favors the presence of high melt or fluid fractions.
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  • Body-wave tomography of western Canada

    Mercier, J. -P.   Bostock, M. G.   Cassidy, J. F.   Dueker, K.   Gaherty, J. B.   Garnero, E. J.   Revenaugh, J.   Zandt, G.  

    In this study, we have produced P- and S-wave velocity models for western Canada using 23.420 delay times measured on vertical component seismograms, and 15,805 delay times measured on transverse component seismograms. respectively, from a range of permanent and temporary networks. Resolution is best in southwestern British Columbia, and along the CANOE (northwestern Alberta, southern Yukon and Northwest Territories) and BATHOLITHS (northwestern BC) arrays where the station density is the highest, and fair elsewhere. We focus our attention on two distinct features 1) the transition from Phanerozoic to Cratonic mantle in northwestern Canada, and 2) the complex tectonic environment at the northern terminus of the Cascadia subduction zone where the plate boundary changes from convergent to transform. We find that the main transition from Phanerozoic to Cratonic mantle in northwestern Canada occurs at the Cordilleran deformation front and represents a sharp jump in seismic velocity from -2% to +2% over a distance of similar to 50 km. In northern Cascadia, we have imaged and characterized the signature of the subducting Juan de Fuca plate and observed evidence of subduction beyond the northern edge of the slab. We also demonstrate that the Anahim hotspot track is underlain by a -2% low-velocity zone possibly extending to 400 km beneath Nazko cone that appears to be the source of volcanism in this area. Consequently, we associate the source of magmatism in this area to a mantle-scale rather than lithospheric-scale process. (C) 2009 Elsevier B.V. All rights reserved.
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  • The teleseismic signature of fossil subduction: Northwestern Canada

    Mercier, J. -P.   Bostock, M. G.   Audet, P.   Gaherty, J. B.   Garnero, E. J.   Revenaugh, J.  

    Between June 2003 and September 2005, 20 broadband, three-component seismometers were deployed along the MacKenzie-Liard Highway in Canada's Northwest Territories as part of the joint Lithoprobe-IRIS Canada Northwest Experiment (CANOE). These stations traverse a paleo-Proterozoic suture and subduction zone that has been previously documented to mantle depths using seismic reflection profiling. Teleseismic receiver functions computed from similar to 250 earthquakes clearly reveal the response of the ancient subduction zone. On the radial component, the suture is evident as a direct conversion from the Moho, the depth of which increases from similar to 30 km to similar to 50 km over a horizontal distance of similar to 70 km before its signature disappears. The structure is still better defined on the transverse component where the Moho appears as the upper boundary of a 10 km thick layer of anisotropy that can be traced from 30 km to at least 90 km depth. The seismic response of this layer is characterized by a frequency dependence that can be modeled by upper and lower boundaries that are discontinuous in material properties and their gradients, respectively. Anisotropy can be characterized by a +/- 5% variation in shear velocity and hexagonal symmetry with a fast axis that plunges at an oblique angle to the subduction plane. The identification of this structure provides an unambiguous connection between fossil subduction and fine-scale, anisotropic mantle layering. Previous documentation of similar layering below the adjacent Slave province and from a range of Precambrian terranes across the globe provides strong support for the thesis that early cratonic blocks were stabilized through processes of shallow subduction.
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  • Transmission to reflection transformation of teleseismic wavefields

    Kumar, M. Ravi   Bostock, M. G.  

    [ 1] In this paper we review the transformation of P and SV transmission Green's functions into the corresponding reflection quantities for one-dimensional, elastic media at precritical slownesses. To obtain estimates of the transmission Green's functions from observed data, we apply a recently developed approach that exploits the minimum-phase nature of the direct waves and employs autospectra and cross-spectra of raw component seismograms representing multiple sources recorded at a single station. To accomplish transformation to reflection Green's functions, we outline a practical recipe that involves amplitude balancing, energy normalization, cross correlation, and removal of free surface effects. We assess its performance through application to both synthetic seismograms and data from station Hyderabad. In all cases we are able to identify the first-order scattering contributions from the continental Moho. The transmission-to-reflection transformation has potential applications for imaging hitherto elusive shallow mantle discontinuities, since interfering forward and back scattering contributions are effectively separated by component in the resulting reflection Green's functions.
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  • An inverted continental Moho and serpentinization of the forearc mantle

    Bostock, M. G.   Hyndman, R. D.   Rondenay, S.   Peacock, S. M.  

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  • Insight into the assembly and evolution of the Slave craton from teleseismic data analyses

    Rondenay, S.   Snyder, D. B.   Chen, C. W.   Straub, K. M.   Bank, C. -G.   Bostock, M. G.  

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  • Split from slip and schist: Crustal anisotropy beneath northern Cascadia from non-volcanic tremor

    Bostock, M. G.   Christensen, N. I.  

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  • Split from slip and schist: Crustal anisotropy beneath northern Cascadia from non-volcanic tremor

    Bostock, M. G.   Christensen, N. I.  

    Polarization and splitting analyses of minute-long windows of non-volcanic tremor at 3-component stations in the vicinity of southern Vancouver Island provide new evidence for the presence of anisotropy within the crustal portion of the North American plate beneath Cascadia. As reported previously, tremor particle motions are predominantly horizontal and inferred to manifest an upgoing wavefield composed primarily of S-waves. Under this assumption, estimates of incidence angle and back azimuth can be deduced from orientation of the smallest principle direction of motion. When subjected to standard S-wave splitting analysis, most stations yield systematic and reproducible fast polarization directions. Estimates of original polarization direction are more scattered and splitting times often exhibit multimodal distributions. Numerical simulations, wherein band-limited synthetic seismograms comprising contributions from multiple sources are analyzed for splitting, neatly reproduce these characteristics and thereby provide a framework for interpretation. A number of stations in our study sit astride the San Juan fault which, based on reflection studies, dips approximately 60 to 70 degrees to the north. Underlying these stations is the Leech River Complex consisting of strongly foliated greenschist facies phyllites with steeply dipping foliations striking parallel to fast polarization directions of split S-waves. Detailed laboratory velocity measurements at elevated pressures show that as little as 2 to 3 km of vertically dipping Leech River phyllite are required to produce the observed splitting. These phyllites have some of the highest S-wave anisotropies measured to date and are part of a once continuous belt of anisotropic crust extending over 2000 km along the western North American coast from southern Vancouver Island to southwest Alaska.
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  • Upper mantle stratigraphy beneath the southern Slave craton

    Bostock, M. G.   Cassidy, J. F.  

    A large teleseismic data set comprising 724 broadband, three-component P-wave seismograms has been compiled for the southern Slave craton with the objective of characterizing underlying mantle stratigraphy. Coherent P to S wave conversions are identified by simultaneously deconvolving seismograms as functions of epicentral distance and along theoretical moveout curves corresponding to possible mantle phases. Clear PDs conversions are observed from the 410 and 660 km discontinuities at times that are only slightly faster than those predicted from the IASP91 model, and over 1.0 s slower than corresponding times observed at other stations on the Canadian Shield and the south African Kaapvaal craton. The PDs times show very little azimuthal variation, implying an absence of major lateral velocity variations in the lithospheric mantle underlying the Slave craton, and adjacent Wopmay orogen and Taltson magmatic zone. Considered in light of other geophysical and geological evidence, these results suggest that the root underlying the Slave province has been modified along its margins and may remain intact only toward a central core. Another important result involves the observation of a PDs conversion from a negative velocity contrast interface at approximately 360 km depth. It and a similar phase, observed on the Kaapvaal craton, would appear not to be directly related to tectospheric structure, but may originate at the top of a layer containing a dense silicate partial melt just above the mantle transition zone.
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