A Comparison of Synthetic Seismograms for 2D Structures: Semianalytical versus Numerical

Teleseismic wave fields occasionally exhibit rapid changes in travel times and waveforms over distances less than several great-circle degrees when observed at broadband arrays. These rapid changes in wave field suggest the existence of significant structural transitions occurring over scales of several hundred kilometers or less in the mid- and deep mantle. Although approximate analytical methods based on raytracing can be readily adapted to structures having arbitrarily small scale lengths, it is important to validate their accuracy against the predictions of numerical methods. Here we compare synthetics from an approximate ray-based method WKBJ modified (WKM) against the pseudospectral method for a 2D model of the S-velocity anomaly associated with the South African plume. This model consists of a uniform 3% decrease in S velocity over a broad (>10°) region of the mid- and deep mantle beneath South Africa, contiguous at its bottom with a thin (100- to 200-km-thick) zone of low velocity extending 30° westward toward South America along the core-mantle boundary. Transitions between anomalous and radially symmetric structures of the test model are sharp, occurring over l0 km or less. SV and SH wave fields synthesized by the WKM and pseudospectral methods in this model generally agree with each other well. Slight mismatches in the two methods can be understood as the result of either differences in model parameterization or the effects of asymptotic approximations in the ray-based WKM method

Attenuation tomography of the upper inner core

The solidification of the Earth's inner core shapes its texture and rheology, affecting the attenuation and scattering of seismic body waves transmitted through it. Applying attenuation tomography in a Bayesian framework to 398 high-quality PKIKP waveforms, we invert for the apparent Qp for the uppermost 400 km below the inner core boundary at latitudes 45°S to 45°N. We use damping and smoothing for regularization of the inversion, and it seems that the smoothing regularization combined with the discrepancy principle works better for this particular problem of attenuation tomography. The results are consistent with a regional variation in inner core attenuation more complex than hemispherical, suggesting coupling between inner core solidification and the thermal structure of the lowermost mantleThe IRIS DS is fundedthrough the National ScienceFoundation and specifically theGEO Directorate through theInstrumentation and Facilities Programof the National Science Foundationunder cooperative AgreementEAR-000437

Irregular Transition Layer Beneath the Earth's Inner Core Boundary From Observations of Antipodal PKIKP and PKIIKP Waves

Standard Earth models assume a simple uniform inner core boundary (ICB) separating the liquid iron outer core from the solid iron inner core. Metallurgical and geodynamic experiments, however, predict lateral variations along this boundary originating from thermochemical and geodynamic instabilities during solidification. We search for evidence of this lateral heterogeneity by exploiting the sensitivity of antipodal PKIIKP waveforms to the shear wave velocity structure of the uppermost inner core beneath their reflection points on the underside of the ICB. Measuring PKIIKP/PKIKP energy ratios from 33 rare antipodal seismograms in the 178o to 180o distance range, we find this ratio varying between 0.1 and 1.1. Synthetic seismograms demonstrate that a laterally homogeneous liquid-solid ICB cannot account for this variability. Observations instead support a spatially variable ICB transition consisting of either (1) gradients in seismic velocities and density in which they smoothly increase from those at the outer core to those in the bulk of the inner core over a maximum depth of 10 km or (2) a layered transition with localized double discontinuities in velocities and densities separated by 4–10 km. A layered transition can generate a coda following PKIKP if shear velocity is small (<2 km/s) in the transition. Our results imply that the ICB is not uniform and might appear patchy with lateral rigidity variations. Nonuniform small-scale structural features that we infer to be present at the ICB are consistent with nonlinear solidification mechanisms driven by small-scale outer core convection in the lowermost outer core.This work was supported by German Research Foundation grant TH1530/5-1. V. F. C. was supported by NSF grant EAR 17-54498. The data collected for the XB network were supported by NSF grant EAR-080902

Anisotropy of heterogeneity scale lengths in the lower mantle from PKIKP precursors, Geophys

SUMMARY The anisotropy of heterogeneity scale lengths in the lower mantle is investigated by modelling its effect on the high-frequency precursors of PKIKP scattered by the heterogeneities. Although models having either an isotropic or an anisotropic distribution of scale lengths can fit the observed coda shapes of short-period precursors, the frequency content of broad-band PKIKP precursors favours a dominantly isotropic distribution of scale lengths. Precursor coda shapes are consistent with 1 per cent fluctuations in P velocity in the wavenumber band 0.05-0.5 km−1 extending to 1000 km above the core-mantle boundary, and with a D◊ region open to circulation throughout the lower mantle. The level of excitation of PKIKP precursors observed in the frequency band 0.02-2 Hz requires a power spectrum of heterogeneity that is nearly white or slowly increasing with wavenumber. Anisotropy of scale lengths may exist in a D◊ layer having larger horizontal than vertical scale lengths and produce little or no detectable effects on PKIKP precursors for P-velocity perturbations as high as 3 per cent when averaged over a vertical scale of several kilometres, and much higher when averaged over scales of hundreds of metres or less. Key words: core-mantle boundary, mantle heterogeneity, scattering. INTRODUC TION Precursors to PKIKP A high-frequency precursor to PKIKP is often observed in the 130°-140°range. It is best observed on short-period instruments from sources enriched in high frequency, such as deep-focus earthquakes, in which P rays traverse the upper-mantle low-Q zone just once, or explosions, which have a higher corner frequency than earthquakes of equivalent moment. When recordings in broad-band and short-period (or long-period and short-period) pass bands are compared, the arrival of the high-frequency scattered precursor can be easily distinguished from the diffraction from the PKP-B caustic by its quite different frequency content and arrival time showed that the high-frequency precursors are best explained in the Banda Sea). Note that the arrival time of the short-period in arrival time and slowness distribution by the scattering precursor to PKIKP is later than the onset of the long-period of P waves by topography on the core-mantle boundary and diffraction from the PKP-B caustic. by heterogeneities in the lower 200 km of the mantle. The location and character of these heterogeneities are recognized 1.2 Observations of heterogeneity in the lowermost mantle to be important in geodynamic models of slab cycling, plume formation and chemical heterogeneity of the lower and Many other observations, in addition to PKIKP precursors (Doornbos 1976(Doornbos , 1988 373 © 1999 RAS 374 V . F. Cormier increased heterogeneity in the lower mantle, particularly the includes scattering into the B caustic as well as inner-core branches, and the diffracted extension of the B caustic to lowermost 250 km (Bullen&apos;s D◊). The diversity and varying sensitivity of different data have complicated the search for a shorter distances. Details of the extension of Paper I, including model construction, are given in the Appendix. simple description of lower-mantle structure. Important constraints from body waves over a broad band of frequencies include wide-angle reflections from discontinuities within D◊ 2 COMPUTATIONAL EXPERIMENTS Heterogeneous models of the lower mantle were constructed using the methods described by Frankel &amp; Clayton (1986 unaffected by the weak perturbations of the background model, which are assumed to average to zero over the Fresnel zones of the direct phases. Note in The recent study of PKIKP precursors by Hedlin et al. (1997) found that small-scale (8 km) heterogeneity persists up to 1000 km above the core-mantle boundary at a relatively uniform perturbation of 1-2 per cent in P velocity. This finding has important geodynamic significance, consistent with mantle circulation extending from the core-mantle boundary to midmantle depths. Hedlin et al.&apos;s study, in common with many previous studies of PKIKP precursors, assumed the validity of ray theory for incident and scattered wavefields. A secondary goal in this paper is to check the accuracy of Hedlin et al.&apos;s result with a calculation that incorporates frequency-dependent diffraction effects from the caustic surface in the outer core

A glassy lowermost outer core

SUMMARY New theories for the viscosity of metallic melts at core pressures and temperatures, together with observations of translational modes of oscillation of the Earth&apos;s solid inner core, have proposed that the dynamic viscosity of Earth&apos;s liquid outer core may approach 10 11 Pa-sec near the inner core boundary. If the viscosity of the lowermost outer core (F region) were in this range, it may be in a glassy state, characterized by a frequency dependent shear modulus and increased viscoselastic attenuation. Testing this hypothesis, the amplitudes of high frequency PKiKP waves are found to be consistent with an upper bound to shear velocity in the lowermost outer core of 0.5 km/sec at 1Hz. Fitting a Maxwell rheology for the frequency dependent shear modulus to seismic constraints at both low and high frequency results in a model of the F region as a 400 km thick chemical boundary layer. This layer likely has both a higher density and higher viscosity that bulk of the outer core, with a peak viscosity on the order of 10 9 Pa-sec or higher near the inner core boundary. If lateral variations in the F region are confirmed to correlate with lateral variations observed in the structure of the upper most inner core, they may used to map differences in the solidification process of the inner core and flow in the lowermost outer core

On the inner-outer core density contrast from PKiKP/PcP amplitude ratios and uncertainties caused by seismic noise

The inner core boundary (ICB) of the earth is characterized by a discontinuous change in elastic properties between the liquid outer and solid inner core. In the ray theory approximation, a measure of the density contrast at the ICB is given by the amplitude ratio of P waves reflected from the core-mantle boundary (CMB; PcP waves) and the ICB (PKiKP waves), since that ratio conveniently appears in an explicit form in the transmission/reflection coefficient equations. The results for inner-outer core density contrast derived from direct amplitude picks of these waves in the time domain have varied significantly among different authors.The transmission/reflection coefficients on the liquid-solid and solid-liquid boundaries derived from ground displacements enable a direct comparison between the amplitude measurements on displacement seismograms in the time domain and theoretical values. A new approach is proposed and applied to integrate effects of microseismic and signal-generated noise with the amplitude measurements, thus providing a direct maximal uncertainty measure. To suppress the effects of varying radiation pattern and distinctively different ray paths at longer epicentral distances, this new method was applied to high-quality arrivals of PcP and PKiKP waves from a nuclear explosion observed at epicentral distances 10°-20° from recording stations. The resulting uncertainties are high precluding precise estimates of the ICB density contrast, but provide a robust estimate of an upper bound from body waves of about 1100 kg m-3. Median values of two amplitude ratios observed around 17° epicentral distance indicate a small density contrast of 200-300 kg m-3 and suggest the existence of zones of suppressed density contrast between the inner and the outer core, a density contrast stronger than 5000 kg m-3 at the CMB, or a combination of both

Obesrvation of near-podal P'P' precursors: Evidence for back scattering from the 150-220 km zone in the Earth's upper mantle

P′P′ (PKPPKP) are P waves that travel from a hypocenter through the Earth's core, reflect from the free surface and travel back through the core to a recording station on the surface. Here we report the observations of hitherto unobserved near-podal

SKS and SPdKS sensitivity to two-dimensional ultralow-velocity zones

Seismic wave propagation through two-dimensional core-mantle boundary (CMB) ultralow-velocity zones (ULVZs) is modeled using a global pseudospectral algorithm. Seismograms are synthesized for several types of ULVZ models to investigate the effect of these structures on SKS and SPdKS phases. One-dimensional models and two-dimensional models with different, quasi-1-D velocity structures on the source and receiver sides of the CMB provide a baseline for comparison with other 2-D models. Models with finite length ULVZs are used to test the sensitivity of the SPdKS travel time and waveform to different portions of the P diffracted wave path. This test shows that SPdKS waves are only sensitive to ULVZs with lengths >100 km. Our results give three tools for identifying and characterizing 2-D ULVZ structures. First, dual SPdKS pulses indicate exposure to at least two separate CMB velocity structures, either different source and receiver-side CMB velocities or different adjacent velocity regions for which Pdiff inception occurs outside of and propagates into a ULVZ. Second, high-amplitude SKS precursors indicate a very “strong” (i.e., thick and/or large velocity perturbations) ULVZ. Hence, the absence of SKS precursors in most previous ULVZ studies indicates that very strong, sharp ULVZs are not common. Third, mean SPdKS delays relative to PREM constrain the minimum ULVZ strength and length combinations required to produce a given travel time delay.National Science Foundation (U.S.) (EAR 02-29586)National Science Foundation (U.S.) (EAR 07-38492)Massachusetts Institute of Technology. Kerr-McGee Development ChairWoods Hole Oceanographic Institution. Deep Ocean Exploration Institut

Evidence for back scattering of near-podal seismic P&apos;P&apos; waves from the 150-220 km zone in Earth&apos;s upper mantle Evidence for back scattering of near-podal seismic P&apos;P&apos; waves from the 150-220 km zone in Earth&apos;s upper mantle

The deepest and most inaccessible parts of Earth&apos;s interior -the core and coremantle boundary regions can be studied from compressional waves that turn in the core and are routinely observed following large earthquakes at epicentral distances between 145° and 180° (also called P&apos;, PKIKP or PKP waves) 10 . Our understanding of Earth&apos;s composition and dynamics has evolved dramatically in the last several decades, especially with the expansion of modern broadband seismic networks and development of seismological methods such as tomographic imaging. Many details about Earth&apos;s deep interior, however, remain elusive or completely unknown. Faster progress toward understanding of Earth&apos;s lowermost mantle and core regions has been hindered by the difficulty of observing and analyzing core-sensitive seismic phases. For example, core-mantle and inner-outer core boundaries can be studied from P waves that reflect from these boundaries (PcP and PKiKP waves, respectively). These phases, however, are not widely observed on seismograms, particularly at very short epicentral distances 12? . Moreover, with the current configuration of stations and the world&apos;s seismicity, the existing collection of PKP waves does not sample equally well all parts of the inner core. For this reason seismic phases with more complex geometry, like PnKP (where n is a number of multiple reflections from the inner side of the core mantle boundary) and P&apos;P&apos; An alternative scattering hypotheses 26? A common characteristic of early observations of P&apos;P&apos; and their precursors was that they were mostly assembled at epicentral distances of about 50°-70°. This characteristic, considered with the geometry of two separate single legs of PKP, can be explained by the fact that a maximum amplitude of PKP waves due to triplication (simultaneous arrivals and interference of 3 branches of PKP waves) is observed between 145° and 155°. Thus, 360°-2Δ={70°,50°} for PKP epicentral distance Δ={145°,155°}. Unlike the majority of these earlier observations of P&apos;P&apos; precursors, we report unprecedented observations at near-podal epicentral distances (less then 10°) of very clear and energetic P&apos;P&apos; precursor arrivals An example of P&apos;P&apos; precursor observations at the short-period ILAR array network in Alaska is illustrated in Scattering of P&apos;P&apos; waves could take place anywhere along the ray path beneath the receiver, core-mantle boundary, inner-core boundary, or antipodal bounce point area. If P&apos;P&apos; waves were forward-scattered, simple travel time calculations reveal that all forward scattering would arrive as energy following P&apos;P&apos; rather than as a precursor. This is a distinguishing property of such short epicentral distance geometry and corresponding seismic wave arrivals and is true regardless of the location or distribution of scatterers in the mantle. For some strong velocity anomalies conncentrated near the receiver, it is possible to create some P&apos;P&apos; precursors but they would arrive with very different slowness or apparent angle of incidence than the main P&apos;P&apos; phase. It is feasible that a specific near-source geometry-related effect, such as a dipping slab, could cause multi-pathing, in which case a denser lithospheric slab would propagate compressional energy faster in the direction of the slab. It is highly unlikely, however, that this is the case in our observations. First, mechanisms with a compressional energy radiation pattern favorable to produce a podal P&apos;P&apos; phase are thrust faulting source mechanisms, where one lobe with maximum compressional energy extends vertically downward from the source region. The seismic energy that is released in the direction of the slab and travels through it is thus much smaller than the main P&apos;P&apos; energy (unless the slab itself is not vertical). Even if this were the case in our observations at ILAR, which were recorded in Alaska above the subducting Pacific plate, it would be difficult to acquire as much as 55 seconds of advance time. We also observed similar P&apos;P&apos; precursors on another, independent set of broadband recordings, for an Afghanistan-Tajikistan border earthquake, recorded at the Kazakhstan regional seismic network. The precursor in the recordings from the Afganistan-Tajikistan border earthquake had advance times similar to that observed at ILAR, also inconsistent with forward scattering from any dipping structure associated with the intermediate depth earthquakes in this region. Further evidence against forward scattering or multipathing near the receiver is given by a slowness analysis from beams formed at the ILAR array, which found strikingly similar slowness and back-azimuth for both the precursor and main P&apos;P&apos; phase that agree well with those predicted for from the location of the earthquake Results of our slowness analyses are also inconsistent with precursor origin from scattering near the core-mantle boundary. It has been suggested that various scattered phases of PKP as well as PK(KKP) or PKK(KP) (where parentheses indicated scattered part of the signal) might account for precursors of PKKKP (double reflection from the inner side of the core mantle boundary) and P&apos;P&apos; waves, and could be an alternative explanation to underside reflections from 410-and 660-km discontinuities at epicentral distances equal and longer than 30°2 Therefore, we suggest that the observed P&apos;P&apos; precursors are back-scattered energy from reflectors in the upper mantle (see illustration in One curiosity is that we do not observe any 410-or 660-km related precursors to P&apos;P&apos;. Because PKP waves have maximum amplitudes near 150º, it is not surprising that most observations of P&apos;P&apos; and their precursors are made near epicentral distances of 60º (see previous comments and a note on relationship between geometry of P&apos;P&apos; and PKP). An interesting factor to consider is the observability of reflections off upper mantle discontinuities having topography The observation of different frequency content between the main P&apos;P&apos; phase and the precursors as well as high amplitudes of the observed precursors persisting at higher frequencies motivated us to investigate this phenomenon more closely. The higher frequency content of the precursors to P&apos;P&apos; could be explained by a combination of the effects of higher attenuation in the uppermost mantle and the frequency dependence of backscattered energy from small-scale heterogeneities. The effects of upper mantle attenuation are relatively simple to model. The modeling of the effects of the backscattered radiation pattern of small-scale heterogeneities is necessarily more speculative. The largest effect on frequency content, however, will undoubtedly be the effect of the exponential attenuation of amplitude with frequency due to intrinsic attenuation rather than the simple first and second power law increase in amplitude with frequency due to scattering by heterogeneities of varying scale length and shape. Hence, we first consider the effects of mantle attenuation on the backscattered attenuation. The difference in the attenuation experienced by P&apos;P&apos; relative to the back-scattered precursors is simply given by the effect of the travel time accumulated by the additional two legs that the main P&apos;P&apos; phase spends in the attenuating uppermost 150-220 km of the mantle. In while in In conclusion, we interpret our best fit to the frequency content and slowness of nearpodal P&apos;P&apos; precursor as backscattering from horizontally connected small-scale heterogeneity concentrated in the uppermost 150-220 km of the mantle. Possible candidate scatterers include compositional blobs of variable size and elastic impedance or lenses of partial melt. Compositional heterogeneities may be eclogitic slab fragments. The impedance contrasts of the heterogeneities may also be associated with a rheologic change from dislocation creep to diffusion creep, which Karato 35 has proposed as a mechanism to account for a transition from an isotropic uppermost mantle to an isotropic lower mantle. Partial melt lenses will be more effective than either compositional or solid-solid phase changes in accounting for the large impedance contrasts needed to account for the amplitude of the observed P&apos;P&apos; precursors at ILAR. Our best observations of P&apos;P&apos; precursors back-scattered from this depth range at ILAR occur beneath oceanic regions, far from mid-ocean ridge. Little or no partial melt, however, has ever been postulated in the upper mantle as deep as 150-220 km, far from mid-ocean ridges. Compared to P&apos;P&apos; precursors observed at ILAR, however, precursors observed from P&apos;P&apos; in the Afganistan-Tajikistan Border region have relatively lower frequency content, perhaps related to an antipodal bounce point near a mid-ocean ridge. Important future observations include an assessment in regional variations in the frequency content of P&apos;P&apos; precursors, especially whether similar back-scattering is observed beneath continental regions. Perhaps the mechanism producing the backscattering from a diffuse depth 150 km zone beneath oceanic regions is identical to the mechanism producing occasional observations of a Lehmann discontinuity near 220 km depth beneath continental regions. The vertical cross-section shows main subdivisions and discontinuities as well as podal P&apos;P&apos;df ray-paths connecting the source with the receiver. A podal P&apos;P&apos;-DF ray-path consists of two antipodal PKIKP ray-paths with bottoming points in the inner core very close to Earth&apos;s center. b, Theoretical travel time curves of P&apos;P&apos; and PKKKP seismic phases from a source at 0 km depth, shown by thick and thin lines, respectively. The P&apos;P&apos;-DF branch corresponds to the waves bottoming in the inner core. The BC branch corresponds to the waves bottoming in the lower, while the AB branch corresponds to the waves bottoming in the middle parts of the outer core. PKKKP waves could be observed in the same epicentral distance range preceding the arrivals of P&apos;P&apos; waves, although with significantly different slowness. Also shown (by dashed line) is a theoretical P&apos;P&apos; travel time curve from a 500 km deep source. Reference model ak135 11 was used. c, Map of Earth with surface projections of P&apos;P&apos; raypaths for the observed podal P&apos;P&apos; precursors. Locations of 9 earthquakes as well as the location of ILAR short period network in Alaska are shown by stars and a triangle, respectively. Also shown (by stars in the southern hemisphere) are reflection points near the antipode. Circles are surface projections of the corresponding bottoming points in the inner core (one on the source, and one on the receiver side). d, Schematic representation of the reflection of P&apos;P&apos; waves in the antipodal mantle (indicated by a rectangle in part a). Thin lines show geometry of back-scattered P&apos;P&apos; responsible for the observed precursor energy. Back scattering originates in a zone between 150 and 220 km in the upper mantle. P&apos;P&apos; waves continue their way through the lithosphere to the surface, reflect from it and travel to the receiver with similar slowness to P&apos;P&apos; precursors. They are attenuated, however, significantly with respect to the precursors, owing to two additional leg paths through the antipodal lithosphere. Figure 2 P&apos;P&apos; observations at podal epicentral distance. Vertical component records at the short-period ILAR array are shown for two bandpass filters: a, 0.2-0.7 Hz and b, 1.0-1.5 Hz. This earthquake was located in the southern Alaska, about 7 degrees southwest of the ILAR seismic network. Both the main P&apos;P&apos; phase and precursors are visible at lower frequencies. Precursors to P&apos;P&apos; are characterized by several distinct arrivals in 55-30 second interval before P&apos;P&apos;. Note a difference in frequency content between the precursor and the main P&apos;P&apos; energy. At higher frequencies, the main P&apos;P&apos; phase is below the noise level and not visible. Figure 3 Amplitude spectra for the observed main P&apos;P&apos; phase (thick black line), P&apos;P&apos; precursor (thick gray line) and noise preceeding the precursors (dashed gray line). The spectrum of the main phase was used to calculate predictions of precursor spectra (thin black lines). Only frequency effect of Q (quality factor) without frequency dependence on scattering was taken into account. a, Q was assumed to be constant in the antipodal lithosphere, with values used shown above the theoretical curves. b, Q has a flat relaxation spectrum for frequencies below 0.1 Hz and increases with as the first power of frequency above a given corner frequency. For Q=200, frequency corners of 0.1 (Qf 1 ) and 0.05 Hz (Qf 2 ) were used. c. Q varies with frequency in the same way as in b, but with frequency dependence proportional to the first power of frequency in order to account for integrated effect of connected small-scale heterogeneity or lenses of partial melt. References