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Now showing items 1 - 16 of 26

  • Seismic scatterers in the lower mantle near subduction zones

    Kaneshima, Satoshi  

    We investigate the global distribution of S-to-P scatterers in the shallow to mid-lower mantle beneath subduction zones, where deep seismicity extends down to the bottom of the upper mantle. By array processing broadband and short period waveform data obtained at seismic networks, we seek anomalous later phases in the P coda within about 15-150 s after direct P waves. The later phases usually arrive along off-great circle paths and significantly later than S-to-P conversion from the '660 km' discontinuity, often show positive slowness anomalies relative to direct P, and do not show a conversion depth that is consistent among nearby events. They are thus adequately regarded as scattered waves, rather than conversion at a global horizontal discontinuity. The S-to-P scattered waves often show amplitudes comparable to 'S660P' waves, which indicates that a spatial change in elastic properties by several percent occurs at the scatterers as abruptly as the post-spinel transformation and should arise from compositional heterogeneity. We locate prominent S-to-P scatterers beneath Pacific subduction zones and beneath southern Spain. Nearly half of 137 S-to-P scatterers located in this study and previous studies by the authors are shallower than 1000 km, and the number of scatterers decreases with depth. Scatterers deeper than 1800 km are rare and mostly weak. We examine relations between the locations of the scatterers and recently subducted slabs inferred from seismic tomography. The scatterers of mid-mantle depths, deeper than about 1000 km, are located distant from tomographic slabs. On the other hand, the majority of shallower scatterers are located beneath the slabs rather than near their fastest portions, which would indicate that chemically heterogeneous materials are not extensively entrained within thickened and folded slabs when the slabs impinge on the lower mantle. We also find scatterers near the locations where basaltic rocks of recently subducted oceanic crust are expected to exist, which suggests that oceanic crust is not delaminating when slabs impinge on the lower mantle.
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  • Stratification of earth’s outermost core inferred from SmKS array data

    Kaneshima, Satoshi   Matsuzawa, Takanori  

    SmKS arrivals recorded by large-scale broadband seismometer arrays are analyzed to investigate the depth profile of P wave speed (V p ) in the outermost core. The V p structure of the upper 700 km of the outer core has been determined using SmKS waves of Fiji-Tonga events recorded at stations in Europe. According to a recent outer core model (KHOMC), the V p value is 0.45 % slower at the core mantle boundary (CMB) than produced by the Preliminary Reference Earth Model (PREM), and the slow anomaly gradually diminishes to insignificant values at ∼300 km below the CMB. In this study, after verifying these KHOMC features, we show that the differential travel times measured for SmKS waves that are recorded by other large-scale arrays sampling laterally different regions are well matched by KHOMC. We also show that KHOMC precisely fits the observed relative slowness values between S2KS, S3KS, and S4KS (SmKS waves with m= 2, 3, and 4). Based on these observations, we conclude that SmKS predominantly reflect the outer core structure. Then we evaluate biases of secondary importance which may be caused by mantle heterogeneity. The KHOMC V p profile can be characterized by a significant difference in the radial V p gradient between the shallower 300 km and the deeper part of the upper 700 km of the core. The shallower part has a V p gradient of −0.0018 s −1, which is steeper by 0.0001 s −1 when compared to the deeper core presented by PREM. The steeper V p gradient anomaly of the uppermost core corresponds to a radial variation in the pressure derivative of the bulk modulus, K ′=d K/d P. The K ′ value is 3.7, which is larger by about 0.2 than that of the deeper core. The radial variation in K ′ is too large to have a purely thermal origin, according to recent ab initio calculations on liquid iron alloys, and thus requires a thick and compositionally stratified layering at the outermost outer core.
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  • Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth

    Ohtaki, Toshiki   Kaneshima, Satoshi   Ichikawa, Hiroki   Tsuchiya, Taku  

    The Earth's outer core is believed to be laterally homogeneous because of its low viscosity. However, a hemispherical difference in the inner core likely exists due to its uneven growth, which may be accompanied by localized light-element releases to the outer core. A few seismological studies proposed heterogeneous lowermost outer core (called F layer) but using methods that are not very sensitive to F layer structures. In a previous study we developed a new method sensitive to the F layer structure and insensitive to others. The method analyzes differences in P wave traveltimes reflected on the inner core boundary and those that turn above the boundary as well as dispersion in waves bottoming or diffracting in the F layer, and was applied to obtain an F layer model of P wave velocity for the northeastern Pacific Ocean. In this paper, we examine the F layer structure beneath Australia using the same method. The observed dispersion requires a lower-velocity gradient beneath Australia than beneath the northeastern Pacific, whereas the observed traveltime differences require higher average velocities beneath Australia. The results obtained for the two regions indicate that the F layer is laterally heterogeneous and that the layer beneath Australia has a higher velocity and velocity gradient in its upper part and a much smaller gradient in its lower part than beneath northeastern Pacific. The maximum velocity difference between the two regions is 0.04km/s, which corresponds to 0.8wt% excess oxygen according to ab initio calculations of elastic properties. These results suggest regional light-element concentration beneath Australia.
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  • Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth

    Ohtaki, Toshiki   Kaneshima, Satoshi   Ichikawa, Hiroki   Tsuchiya, Taku  

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  • Seismic structure near the inner core boundary in the south polar region

    Ohtaki, Toshiki   Kaneshima, Satoshi   Kanjo, Kenji  

    It is crucial to obtain good spatial coverage of seismic data points for better understanding the Earth's core, but the core beneath the polar regions remains largely unexplored. We analyzed differential traveltimes and amplitude ratios of core phases whose raypaths run beneath Antarctica for determining the V-p and Q(p) structure near the inner core boundary in the south polar region. The model we obtained (south polar region model, SPR) is described relative to the preliminary reference Earth model (PREM) as follows: a 0.05 km/s lower V-p value at the top of the inner core, a 1.5 times steeper V-p gradient in the upper 300 km of the inner core, a smaller Q(p) (300) in the upper 300 km of the inner core, and a 0.04 km/s lower V-p at the bottom of the outer core. The V-p values of SPR in the lowermost outer core lie between those of PREM and AK135, being closer to those of AK135. The lowermost outer core V-p inside the tangent cylinder is thus close to the global average. In the upper inner core, SPR has lower V-p than AK135 and PREM. The SPR V-p profile is close to that of previous models for the Western Hemisphere, although most of our data sample the Eastern Hemisphere of the inner core. Our results indicate that the inner core does not have a simple hemispherical variation as usually supposed. Our data support an eyeball-shaped high-V-p anomaly with compressional velocity higher than in 1-D reference Earth models, concentrated to a smaller region beneath eastern Asia.
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  • Transition zone structure under a stationary hot spot: Cape Verde

    Helffrich, George   Faria, Bruno   Fonseca, Joao F. B. D.   Lodge, Alexandra   Kaneshima, Satoshi  

    We report on a two-year seismic deployment in the Cape Verde Islands, one goal of which was to study the upper mantle to determine its structure under a hot spot that is stationary in the hot spot reference frame. We find from analysis of P-to-S receiver functions estimated from broadband seismic recordings that, within uncertainty, the time separation between the 410 and 660 km discontinuities is normal compared to radial earth models. Thus, to exist, even stationary hot spots do not require vertical thermal anomalies from deep melting sources anchored in the lower mantle or at the core-mantle boundary or their anomalies are narrower than similar to 250 km in the upper mantle. (C) 2009 Elsevier B.V. All rights reserved.
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  • Outer-core compositional stratification from observed core wave speed profiles

    Helffrich, George   Kaneshima, Satoshi  

    Light elements must be present in the nearly pure iron core of the Earth to match the remotely observed properties of the outer and inner cores(1,2). Crystallization of the inner core excludes light elements from the solid, concentrating them in liquid near the inner-core boundary that potentially rises and collects at the top of the core(3), and this may have a seismically observable signal. Here we present array-based observations of seismic waves sensitive to this part of the core whose wave speeds require there to be radial compositional variation in the topmost 300 km of the outer core. The velocity profile significantly departs from that of compression of a homogeneous liquid. Total light-element enrichment is up to five weight per cent at the top of the core if modelled in the Fe-O-S system. The stratification suggests the existence of a subadiabatic temperature gradient at the top of the outer core.
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  • Seismic scatterers at the shallowest lower mantle beneath subducted slabs

    Kaneshima, Satoshi  

    Data from a western United States short-period seismic network are analyzed in order to investigate anomalous later phases within a time window from about 10 s to 120 s after direct P waves for deep and intermediate-depth earthquakes in circum-Pacific subduction zones. The anomalous phases are best interpreted as S-to-P scattered waves (wavelengths of similar to 10 km) from heterogeneities in the shallow lower mantle (depths <= 950 km). Several S-to-P scatterers where elastic properties of the rocks must substantially change within several kilometers are detected in the shallowest 300 km of the lower mantle beneath four circum-Pacific regions: Kuril, Bonin, Fiji, and Peru. Around most of the observed scatterers the seismic tomography models have delineated high seismic velocity anomalies which are associated with recently subducted Pacific slab or Nazca slab. The most likely origin of the scatterers would be basalt which used to form the oceanic crust. Interestingly enough the majority of the scatterers are located near the bottom boundaries of the slabs. Given rather moderate degrees of folding and/or buckling of the Pacific and Nazca slabs in the shallowest lower mantle at the study areas, the existence of the oceanic crust associated with the most recent slab subduction at the scattering sites is unlikely. Alternative and more likely explanations include: the presence of ancient basaltic rocks, localized dehydration of hydrous minerals, or a sharp boundary between different rock fabrics such as isotropic and anisotropic lower mantle rocks associated with mantle flow dragged by the slab subduction. (C) 2009 Elsevier B.V. All rights reserved.
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  • Lower mantle scattering profiles and fabric below Pacific subduction zones

    Kaneshima, Satoshi   Helffrich, George  

    The Earth's mantle is compositionally evolving principally through the process of mid-ocean ridge basalt production (95% by lithospheric area, but also by way of arc magmatism (> 5%) and continental growth (< 1 %) ( Reymer, A., Schubert, G., 1984, Phanerozoic addition rate to the continental-crust and crustal growth, Tectonics, 3, 63-77; Rowley, D.B., 2002. Rate of plate creation and destruction: 180 Ma to present, Geol. Soc. Am. Bull., 114,927-933). Convection eventually must mix these products back into the mantle, but the details of the mixing process are unknown. We report here new observational data documenting the location of differentiated material in the present-day lower mantle and scattering power profiles with depth. Small-scale (similar to 10 km) objects which efficiently scatter short period S waves are present in the mantle lying below the Pacific Ocean rim at depths between 1100 and 1800 km. We also observe a reduction in scattering power with depth indicating that the objects represent bodies richer in differentiated basalt than the surrounding mantle. The scattering objects appear to be deformed to various degrees and usually form clusters separated from each other by a distance of 100 to 200 km. The clusters probably represent folds of the order of 100 km, probably corresponding to early stage of mixing process. Because the scattering power does not increase with depth, neither does a permanent barrier to flow exist in the mid-mantle, nor does the intrinsic density of basalt prevent re-entrainment into mantle flow from the bottom of the mantle. (C) 2009 Elsevier B.V. All rights reserved.
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  • Small scale heterogeneity in the mid-lower mantle beneath the circum-Pacific area

    Kaneshima, Satoshi   Helffrich, George  

    Data from a western United States short-period seismic network are investigated in order to detect anomalous later P-wave arrivals within a time window from 50 s to 170 s after direct P waves for deep and intermediate-depth earthquakes at circum-Pacific subduction zones. We interpret the anomalous arrivals as S-to-P scattered waves from heterogeneities in the mid-lower mantle. The searched regions (depths from similar to 800 km to 2200 km) cover approximately 2% of the entire lower mantle. The heterogeneous scattering objects are located by array processing of the seismic waves using semblance, with careful investigations of arrival time, slowness and azimuth of the later phases relative to direct P waves. We usually observe two or three later phases which are separated in arrival time by 5-20 s and have similar arrival azimuth and slowness. This observation suggests that a few scatterers often form a cluster with a typical spacing of 100-200 km vertically. More than ten clusters or groups of S-to-P scattering objects are detected in the mid-lower mantle beneath the circum-Pacific regions; northern Kuril, Izu-Bonin, Mariana, northern Fiji, and west coast of South America. The S-to-P scattering objects are found in a depth range from 1100 to 1800 km, and the most prominent objects of each region tend to be located around the middle of the depth range. The amplitudes of the scattered waves range from 1% to 10% of the direct P waves, with exceptionally large values as large as 40%. The associated magnitudes of the changes in elasticity is not tightly constrained but probably are nearly 1% and may occasionally exceed 4% of shear velocity. Considering their high efficiency at converting short-period waves (wavelengths of 5-10 km), the changes in elastic properties at the heterogeneous objects appear to occur within several kilometers. Seismic tomography studies have not detected prominent seismic wave speed anomalies except for those beneath South America which appear to be located in the former Farallon plate and those at north of Fiji which appear to be located around the Pacific superplume. The mid-lower mantle heterogeneity should represent sharp chemical variations in major element composition of the lower mantle rocks. The most plausible tectonic explanation of the variations is remnant of subducted and folded former oceanic crust which preceded the current subduction of the Pacific plate. The extreme estimates of low shear velocity ( >4%) may indicate the prominent drops in the shear modulus due to ferro-elastic phase transition in stishovite (SiO(2)). Our observations imply that heterogeneity of kilometer-scale persists for billions of years despite of stirring due to convection and therefore is ubiquitous in the lower mantle. It is qualitatively consistent with some recent convection models. (C) 2010 Elsevier B.V. All rights reserved.
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  • Long-term variation of the shallow tremor sources at Aso Volcano from 1999 to 2003

    Takagi, Noriaki   Kaneshima, Satoshi   Ohkura, Takahiro   Yamamoto, Mare   Kawakatsu, Hitoshi  

    In order to investigate the continuous volcanic tremor of Aso Volcano in Japan, we performed a series of temporary short-period seismic array observations near the Nakadake first crater (the active crater) during five years from 1999 to 2003. We deployed in all of the temporary observations a seismic array at the same location about 700 m west of the active crater, in order to investigate long-term changes in the tremor activity. In 1999 and 2003, another array was simultaneously deployed at a different location 700 m north of the crater to help locate the tremor sources. We developed a frequency domain semblance method and applied it to the waveform data of the frequency range where the continuous tremor is dominant (3-6 Hz). We measured arrival azimuths and slownesses of the continuous tremor signals as functions of frequency, which are then used to locate the epicenters of the tremor signals corresponding to the principal peaks of the power spectra. For the observations in 1999 and 2002, the continuous tremor amplitudes are relatively small, and the slowness of the tremor signal observed at the west array takes a local minimum (0.5 to 0.6 s/km) near the frequency (similar to 4.7 Hz for 1999 and similar to 4.8 Hz for 2002) which corresponds to the highest spectral peak. This implies that body waves dominate the tremor signals at the west array around the frequency. The tremor epicenters corresponding to 4.7 Hz for the observation in 1999 are located at the west rim of the currently active crater. While the surface crater activity of Aso remains low and the tremor activity is not clearly linked with the surface activity until early 2003, a close link between the tremor and crater activity appears in the middle of 2003, when a small phreatic eruption occurred a month before the array observation (July 10, 2003). Tremor signals of the observation in 2003 show a large spectral peak (4.2 Hz) where the slowness measured for the west array is very large (1.1 s/km), clearly suggesting that surface waves are dominant. The epicenter is again located at the western rim of the active crater. We interpret these observations as follows: in 1999 and 2002 when the surface activities of Aso were low, the continuous tremor excitation was deep and inactive. In the middle of 2003 when Aso Volcano became active with a series of phreatic eruptions a shallower tremor source was activated, possibly masking the deeper sources. This shallowing of the dominant tremor source could be due to the increase in the volcanic gas flow rate triggered by the phreatic eruptions. (C) 2009 Elsevier B.V. All rights reserved.
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  • Geophysical and mineralogical constraints on the post-spinel transformation for the Tonga slab

    Kaneshima, Satoshi   Kubo, Tomoaki   Yoshioka, Shoichi  

    The depth of the post-spinel transformation is investigated for the Tonga slab, by using data from short period seismic networks at western United States and Japan for about 100 deep and intermediate-depth earthquakes within a small (similar to 200 km by 200 km) area near 20 degrees S. Later phases in a time window not approximate to 3 to 20 s after direct P waves are analyzed to search for S-to-P converted waves at the 660 km discontinuity, which represents the post-spinel transformation. We find that immediately beneath the foci of the deepest earthquakes the discontinuity is depressed down to the depths of 685 +/- 5 km on average, and that it dips towards WNW by 10 +/- 3 km within 70 km laterally. We constrain the thermal structure near the S to P conversion points based on a plausible assumption that the deepest earthquakes occur around the coldest core of the Tonga slab. The distribution of the hypocenters relocated in this study as well as previously published tomographic images of the same region suggest that the Tonga slab bends upward when approaching the 660 km discontinuity and transiently stagnates around the discontinuity. With these observations as the constraints, we numerically model the thermal structure of the Tonga slab, and estimate the temperature around the conversion points as 1200 +/- 100 degrees C. which is 300 +/- 100 K colder than the surrounding mantle. As the average depression of the discontinuity (down to 685 +/- 5 km) corresponds to an pressure excess over the global average (660 km) by 1.0 +/- 0.2 GPa, the assumption of equilibrium post-spinel transformation results in an estimate of the Clapeyron slope (C-1) of -3.3(-2.7)(+1.3), MPa/K. We also obtain an independent estimate of the Clapeyron slope (C-2) of -2.0 +/- 1.0 MPa/K, based on the observation of the dip of the discontinuity and the computed temperature variation (by about 200 K). The discrepancy between C-1 and C-2 is marginally significant and can be diminished by considering that the slab materials at the conversion points are currently descending across the phase boundary fast enough and thus the depth of the post-spinel transformation is controlled by nucleation kinetics as well as by the temperature. (C) 2012 Elsevier B.V. All rights reserved.
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  • Vp structure of the outermost core derived from analysing large-scale array data of SmKS waves

    Kaneshima, Satoshi   Helffrich, George  

    The Earth'score is composed primarily of iron with several per cent by weight of lighter elements, which are thought to be progressively enriched in the outer liquid core as the core cools and the inner core crystallizes. Detailed profiles of P-wave speed (V-p) of the outermost core might be affected by this process. In this study, we constrain the radial V-p structure in the outermost 600 km of the core by analysing SmKS multiples with m up to 6, which are observed at large-scale broad-band seismometer arrays. Array processing of SmKS waves combined with investigation of their CMB piercing points clearly demonstrate that the array averaged SmKS differential traveltime anomalies primarily reflect the V-p structure in the outermost core. A tau-p inversion is applied to the SmKS (m = 2, 3, 4 and 5) differential times measured by the array processing. The inversion reveals that the V-p is 0.45 per cent slower at the CMB than Preliminary Reference Earth Model (PREM) and the lower V-p anomaly gradually diminishes to zero at 300 km below the CMB. The deviation of the V-p gradient from PREM of the outermost 300 km of the core is nearly an order of magnitude larger than that of the deeper part of the core. The evaluation of Birch's parameter 1 - g(-1)d Phi/dr (Phi = V-p(2)) for the obtained V-p profile as well as the change in the V-p gradient with depth show that adiabatic self-compression of a homogeneous material cannot explain the observations. Waveforms corresponding to the arrivals of S6KS waves are consistent with the V-p profile of the topmost 50 km of the outer core, and agree with the results from the tau-p inversion. This excludes the presence of a high V-p-low density layer thicker than 10 km immediately beneath the CMB.
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  • Seismological and experimental constraints on metastable phase transformations and rheology of the Mariana slab

    Kubo, Tomoaki   Kaneshima, Satoshi   Torii, Yoku   Yoshioka, Shoichi  

    A wedge with a low seismic wave velocity implying the presence of metastable olivine and the depression of the 660 km seismic discontinuity have been observed inside the Mariana slab. Based on these seismic observations and numerical calculation of thermal structures of the slab, we suggest that the phase transformations from olivine to ringwoodite, and from ringwoodite to perovskite + ferropericlase (the post-spinel transformation) occur at the depth of 630 km and 550 degrees C, and at 690 km and 700 degrees C, respectively, in the central coldest part of the Mariana slab. Combining this information with recent experimental kinetic data, we constrain the details of non-equilibrium phase transformations and rheology of the Mariana slab. The observed depth of the metastable olivine inside the Mariana slab can be explained by growth-controlled olivine-ringwoodite transformation under relatively dry condition such as the water content of about 150 wt. ppm H(2)O. On the other hand, nucleation process controls the depth of the post-spinel transformation in the Mariana slab, and therefore the observed depressions of the 660 km discontinuity should depend on both the overpressure needed for the nucleation and the negative Clapeyron slope for the transformation. When the overpressure for the nucleation is 0.5 GPa, the observed depressions can be explained by the Clapeyron slope of -0.7 MPa/K. Grain-size evolution and viscosity structures in the Mariana slab are estimated based on these metastable phase transformations. The slab weakening due to the grain-size reduction is limited in about 40 km in width below the metastable olivine wedge at the depths from 630 to 690 km, whereas the width of the weakened portion of the slab increases to more than 120 km at deeper than the 690 km depth after the post-spinel transformation. These viscosity structures are consistent with the behaviors of the Mariana slab estimated from seismic tomography: the slab vertically descends to the bottom of the transition zone, and substantially deforms and thickens at the top of the lower mantle. (C) 2009 Elsevier B.V. All rights reserved.
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  • Anisotropic loci in the mantle beneath central Peru

    Kaneshima, Satoshi   Silver, Paul G.  

    Seismic anisotropy of the upper mantle beneath the central part of Peru is examined by analyzing shear waves observed at broad-band stations and a temporary array of short-period stations. Shear-wave splitting is seen on various shear phases, such as direct S waves from local intermediate to deep earthquakes, ScS waves from regional deep earthquakes, and SKS waves from teleseismic earthquakes. It is inferred that the shear-wave anisotropy in the uppermost 100 km of the subcontinental mantle overlying the subducted Nazca plate is 0.5%at most, while the anisotropy in the subslab asthenosphere (depth range of about 150-350 km) is 2%or greater. The fast shear-wave polarization direction in this depth range, as observed at two broad-band stations, is 30°-40° different from the absolute motion of the Nazca plate. This does not fit simple two-dimensional (2-D) models of olivine alignment caused by slab-induced mantle flow, and implies either the existence of a complex flow pattern in the asthenosphere underneath the Nazca plate or the presence of an unknown mechanism for the anisotropy formation. The lower mantle beneath central Peru is found to be effectively isotropic for nearly vertically propagated shear waves.
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  • Seismic structure near the inner core-outer core boundary

    Kaneshima, Satoshi   Hirahara, Kazuro   Ohtaki, Toshiki   Yoshida, Yasuhiro  

    A model of P-wave velocity of the Earth's core 500 km above and below the inner core boundary (ICB) beneath North America is constructed from travel time analysis and broad-band waveform modeling of core phases at distances from 130 degrees to 160 degrees. Differential travel times between PKPBC (Cdiff) and PKPDF (T-BC-T-DF) are about 0.5 sec shorter than those computed from PREM at distances from 146 degrees to 152 degrees and increase with distance to nearly the same as those from PREM at 155 degrees. Differential travel rimes between PKPCD and PKPDF (T-CD-T-DF) at distances from 130 degrees to 143 degrees are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKPBC (Cdiff) relative to PKPDF (A(BC)/A(DF)) at distances 150 degrees to 156 degrees are about 40% larger than those from PREM. These observations require a smaller P-wave velocity gradient (0.0005 sec(-1)) in the lowermost 300 km of the outer core than that from PREM (similar to 0.00065 sec(-1)), a slower P-wave velocity (10.96 km/sec) at the top of the inner core, and a larger velocity gradient (0.0005 sec(-1)) in the uppermost 300 km of the inner core.
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