Vadim Levin

Receiver Functions from Multiple-Taper Spectral Correlation Estimates

Teleseismic P waves are followed by a series of scattered waves, particularly P -to- S converted phases, that form a coda. The sequence of scattered waves on the horizontal components can be represented by the receiver function (RF) for the station and may vary with the approach angle and azimuth of the incoming P wave. We have developed a frequency-domain RF inversion algorithm using multiple-taper correlation (MTC) estimates, instead of spectral division, using the pre-event noise spectrum for frequency-dependent damping. The multitaper spectrum estimates are leakage resistant, so low-amplitude portions of the P -wave spectrum can contribute usefully to the RF estimate. The coherence between vertical and horizontal components can be used to obtain a frequency-dependent uncertainty for the RF. We compare the MTC method with two popular methods for RF estimation, time-domain deconvolution (TDD), and spectral division (SPD), both with damping to avoid numerical instabilities. Deconvolution operators are often biased toward the frequencies where signal is strongest. Spectral-division schemes with constant water-level damping can suffer from the same problem in the presence of strong signal-generated noise. Estimates of uncertainty are scarce for TDD and SPD, which impedes developing a weighted average of RF estimates from multiple events. Multiple-taper correlation RFs are more resistant to signal-generated noise in test cases, though a “coherent” scattering effect, like a strong near-surface organ-pipe resonance in soft sediments, will overprint the Ps conversions from deeper interfaces. The MTC RF analysis confirms the broad features of an earlier RF study for the Urals foredeep by Levin and Park (1997a) using station ARU of the Global Seismographic Network (GSN), but adds considerable detail, resolving P -to- S converted energy up to f = 4.0 Hz.

Mantle flow at a slab edge: Seismic anisotropy in the Kamchatka region

The junction of the Aleutian Island and the Kamchatka peninsula defines a sharp turn in the boundary of the Pacific and North American plates, terminating the subduction zones of the northwest Pacific. The regional pattern of shear-wave birefringence near the junction indicates that trench-parallel strain follows the seismogenic Benioff zone, but rotates to trench-normal beyond the slab edge. Asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge, and may influence the shallowing dip of the Benioff zone at the Aleutian junction.

Shear wave splitting in the Appalachians and the Urals: A case for multilayered anisotropy

Observations of shear wave splitting in the northeastern U.S. Appalachians and in the foredeep of the Urals vary significantly with the back azimuth and incidence angle of the incoming phase. These variations suggest two or more layers within the upper mantle with different anisotropic properties. Synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach. Relying on a subset of back-azimuth and incidence angle may strongly bias the model inferred, especially if the observations are averaged. On the other hand, the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the Earth. Using a new error estimation technique for splitting, we find that individual measurements from broadband data have errors of the order of δ = 3°-7°for the fast direction and 0.1 - 0.2 s for the delay of split shear waves. The azimuthal variation of splitting parameters is broadly consistent throughout the Appalachian terranes in the northeast United States, especially for two long-running stations in the northeast United States, HRV (Harvard, Massachusetts) and PAL (Palisades, New York). Observations can be separated into two distinct populations, with mean fast-axis azimuths of N60°E±4°and N119°E±2°, Delay values within each population range from near zero to ∼1 s. Azimuthal splitting variation for ARU (Arti, Russia) in the foredeep of Uralian mountains is characterized by sharp transitions between different groups of observations. Using synthetic seismograms in simple structures, we develop one-dimensional anisotropic models under stations HRV and ARU. The model for HRV includes two layers of anisotropic material under an isotropic crust, with fast-axis azimuths N53°E and N115°E for the bottom and the top layers, respectively. The model for the upper mantle under ARU includes a layer with a fast-axis at N50°E atop a layer with fast axis azimuth N90°E. Our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under ARU, reported by Levin and Park [1997a]. Our results suggest that both Urals and Appalachians possess a relict anisotropy in the tectosphere, associated with past continental collision and accretion, underlain by anisotropy with orientation similar to the local absolute plate motion, suggesting an asthenospheric component to the signal.

Seismic evidence for catastrophic slab loss beneath Kamchatka

In the northwest Pacific Ocean, a sharp corner in the boundary between the Pacific plate and the North American plate joins a subduction zone running along the southern half of the Kamchatka peninsula with a region of transcurrent motion along the western Aleutian arc. Here we present images of the seismic structure beneath the Aleutian–Kamchatka junction and the surrounding region, indicating that: the subducting Pacific lithosphere terminates at the Aleutian–Kamchatka junction; no relict slab underlies the extinct northern Kamchatka volcanic arc; and the upper mantle beneath northern Kamchatka has unusually slow shear wavespeeds. From the tectonic and volcanic evolution of Kamchatka over the past 10 Myr (refs 3, 4–5) we infer that at least two episodes of catastrophic slab loss have occurred. About 5 to 10 Myr ago, catastrophic slab loss shut down island-arc volcanic activity north of the Aleutian–Kamchatka junction. A later episode of slab loss, since about 2 Myr ago, seems to be related to the activity of the worlds most productive island-arc volcano, Klyuchevskoy. Removal of lithospheric mantle is commonly discussed in the context of a continental collision, but our findings imply that episodes of slab detachment and loss are also important agents in the evolution of oceanic convergent margins.

Crust and upper mantle of Kamchatka from teleseismic receiver functions

Teleseismic receiver functions (RFs) from a yearlong broadband seismological experiment in Kamchatka reveal regional variations in the Moho, anisotropy in the supra-slab mantle wedge, and, along the eastern coast, Ps converted phases from the steeply dipping slab. We analyze both radial- and transverse-component RFs in bin-averaged epicentral and backazimuthal sweeps, in order to detect Ps moveout and polarity variations diagnostic of interface depth, interface dip, and anisotropic fabric within the shallow mantle and crust. At some stations, the radial RF is overprinted by near-surface resonances, but anisotropic structure can be inferred from the transverse RF. Using forward modeling to match the observed RFs, we find Moho depth to range between 30 and 40 km across the peninsula, with a gradational crust–mantle transition beneath some stations along the eastern coast. Anisotropy beneath the Moho is required to fit the transverse RFs at most stations. Anisotropy in the lower crust is required at a minority of stations. Modeling the amplitude and backazimuthal variation of the Ps waveform suggests that an inclined axis of symmetry and 5–10% anisotropy are typical for the crust and the shallow mantle. The apparent symmetry axes of the anisotropic layers are typically trench-normal, but trench-parallel symmetry axes are found for stations APA and ESS, both at the fringes of the central Kamchatka depression. Transverse RFs from east-coast stations KRO, TUM, ZUP and PETare fit well by two anisotropic mantle layers with trench-normal symmetry axes and opposing tilts. Strong anisotropy in the supraslab mantle wedge suggests that the mantle ‘‘lithosphere’’ beneath the Kamchatka volcanic arc is actively deforming, strained either by wedge corner flow at depth or by trenchward suction of crust as the Pacific slab retreats. D 2002 Elsevier Science B.V. All rights reserved.

Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica

Abstract Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.

Crustal anisotropy in the Ural Mountains Foredeep from teleseismic receiver functions

Radial and transverse teleseismic receiver functions (RFs) at GSN station ARU, in central Eurasia, display variation in back-azimuth ψ consistent with a 1-D anisotropic crustal structure. In a broad ψ range, the transverse RFs possess a strong phase at ∼5-sec delay relative to direct P, with a polarity reversal at ψ ∼ 50°. The radial RFs peak at the transyerse-RF polarity reversal for this converted phase. The first motion of the transverse RFs varies with ψ also, reversing polarity at ψ ∼ 345°. The azimuthal variation can be modeled by a 5-layer velocity profile with substantial (15%) seismic anisotropy in both the lowermost crust and a low-velocity surface layer. Assuming hexagonal symmetry, the lowermost crust has a tilted “slow” symmetry axis i.e. an oblate phase velocity surface. The strike of the axis is oblique to the north-south Urals trend, but deviates <20° from the mantle fast-axis inferred from SKS splitting. The magnitude and tilt of the models anisotropy suggests that fine layering and/or aligned cracks augment mineral-orientation anisotropy near the top and bottom of the crust.

Cold crust in a hot spot

Shear waves from locally-recorded Icelandic earthquakes exhibit very little anelastic attenuation, even though they turn in 0–5 Ma lower (15–25 km) crust just above the Moho. Lower crustal shear wave quality factors are about Q=150-225 for paths that cross central Iceland and about Q=300 for paths in southwestern Iceland, values that indicate that the temperature of the lower crust is at least 250–325°C below the solidus of gabbro (that is, about 875–950°C, based on published experimental studies of the effect of temperature on shear wave attenuation in gabbro). These low temperatures are not consistent with existing models of crustal accretion, which predict partial melt at lower crustal depths.

No regional anisotropic domains in the northeastern U.S. Appalachians

A fabric “frozen” within the continental lithosphere should lead to “regional anisotropic domains” with close correlation between tectonics, seismic anisotropy, and shear wave travel time delays. Alternatively, seismic anisotropy could be maintained by active deformation in the sub-lithospheric mantle. This scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting, surface tectonics, and travel time delays. A coherent pattern of shear wave splitting is observed in the northeastern United States, covering large parts of New York and New England, spanning the Appalachian Orogen and areas underlain by the Grenvillian basement. The direction of fast shear wave propagation, as measured from core-refracted SKS, SKKS, and PKS phases, varies systematically with event back azimuth at all sites examined, and is fit well by two-layers of anisotropy with subhorizontal symmetry axes. Relative travel time delays of shear waves that sample the region vary over 100 km length scale, and correlate with surface geology. Tomographic images suggest lateral shear velocity variations ∼3%, including a slow anomaly beneath the Grenvillian basement exposed in the Adirondack Mountains. There is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting. We therefore conclude that in the northeastern US the concept of “regional anisotropic domains” (i.e. distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation) does not apply.

Slab portal beneath the western Aleutians

Tomographic images of the distribution of shear-wave speed beneath the northwestern Pacific delineate the configuration of the subducted oceanic lithosphere beneath the western Aleutian arc. At ;100 km depth, a fast shear-wave speed anomaly is beneath the Aleutian arc everywhere east of 1738E. Between 1648E and 1738E, however, seismic velocities at this depth are slow relative to the surrounding mantle. The lateral termination of the fast shear-wave speed anomaly at depth coincides with a gap in deep seismicity beneath the Aleutians. The absence of these two distinctive traits of subducting slabs leads us to conclude that this segment of the Aleutian arc overlies a very large window in the otherwise continuous lithospheric slab that we term a ‘‘slab portal.’’ This portal is likely to facilitate the production of distinctive volcanic rocks (adakites or high-Mg# andesites) by partially melting the adjacent edges of the slab. The Miocene age of most adakites in the westernmost Aleutians where no slab is present may indicate that the portal formed relatively recently. The chemistry of western Aleutian adakites approximates that of typical continental crust, so their genesis and subsequent lateral transport toward Kamchatka is a plausible mechanism for new continent formation.