660-km Discontinuity Research Articles

Receiver function image of the mantle transition zone beneath western China: Fragmented subduction and counterflow upwelling

A uniform image of the mantle transition zone (MTZ) beneath western China and neighboring regions is produced through Variable Bin Radius Stacking of receiver functions. We utilized a large data set of 218,050 receiver functions from 1,991 broadband seismic stations. Our results, after 3-D velocity corrections, show significant lateral variations in topography of the 410- and 660-km discontinuity and thickness of the MTZ. The observed lateral variations of the MTZ correlate with seismic-velocity anomalies identified in independent tomographic studies, which are interpreted as cold and hot thermal anomalies from lithospheric downwellings and mantle upwellings, respectively. In the southern Tibetan Plateau, the MTZ topography reveals four segmented zones of up to ∼20 km thicker-than-average MTZ from west to east interfingered with regions of thin-to-normal MTZ. These segmented MTZ thickenings likely originate from a series of cold lithospheric fingers associated with the fragmented subduction of the Indian lithosphere, while the thin-to-normal MTZ may result from an absence of the subducting Indian slab. These observations provide novel evidence for the proposed fragmentation of the Indian subduction. Moreover, we observe regions of thickened MTZ under the Tien Shan orogen and the Qaidam block, which likely result from the foundering of cold delaminated/broken-off lithospheric blocks triggered by the underthrusting of the Junggar and Tarim blocks. Regions of thinned MTZ beneath the Tien Shan region are additionally observed, which could be attributed to counterflow upwellings.

Upper-mantle seismic anisotropy in the southwestern North Island, New Zealand: Implications for regional upper-mantle and slab deformation

We employed shear-wave splitting analysis on both teleseismic SKS and S waves, and S waves from deep (150–250 km) local earthquakes collected from a dense array with 43 temporary broadband seismic stations and nine long-term seismic stations centered at Mount Taranaki to characterize the upper-mantle dynamics in the southwestern North Island of New Zealand, in areas previously unexamined for shear-wave splitting. We observed predominantly trench-parallel fast polarizations and strikingly large delay times over 3 s from teleseismic analysis. In contrast, local S analysis yielded a sharp transition of fast-polarization from trench-parallel in the northeast to trench-normal in the southwest. Trench-parallel fast-polarization from teleseismic analysis may be attributed to sub-slab trench-parallel flow or to trench-parallel fractures in the subducting slab. More importantly, we attribute large delay times to deep upper-mantle (200–400 km depth) deformation, possibly associated with the dynamic interaction between the downgoing slab and the 410-km discontinuity or with the lithosphere delamination near the Taranaki-Ruapehu line. In contrast, the trench-parallel anisotropy from the local S waves in the northeast could be caused by fluid-bearing cracks in the crust of the Taupō Volcanic Zone and/or by trench-parallel fractures in the subducting slab resulting from outer rise bending. The abrupt change to trench-normal may be related to stress variations in the downgoing slab at different depths.

Evidence of low velocity layers at the top and bottom of the Mantle Transition Zone (MTZ) below the Uttarakhand Himalaya, India

The Mantle Transition Zone (MTZ) beneath the Uttarakhand Himalaya has been modelled using Common Conversion Point (CCP) stacking and depth-migration of radial P-receiver functions. In the Uttarakhand Himalaya region, the depths of the 410-km discontinuity (d410) and the 660-km discontinuity (d660) are estimated to be approximately 406 ± 8 km and 659 ± 10 km, respectively. Additionally, the thickness of the mantle transition zone (MTZ) is modelled to be 255 ± 7 km. The average arrival times for d410 and d660 conversions are (44.47 ± 1.33) s and (71.08 ± 1.29) s, respectively, indicating an undisturbed slightly deeper d410 and a deformed noticeably deeper d660 in the area. The model identifies the characteristics of the d410 and d660 mantle discontinuities beneath the Lesser Himalayan region, revealing a thickening of the MTZ towards northeast, which could be due to gradual cooling or thickening of the Indian lithosphere towards its northern limit. We simulate a low-velocity layer (perhaps partially molten) above the d410 discontinuity at depths of 350 to 385 km, indicating the presence of a hydrated MTZ beneath the area. We also interpret a negative phase at d660 as a low-velocity layer between 590 and 640 km depths, which could be attributed to the accumulation of old subducted oceanic materials or increased water content at the bottom of the MTZ. Our results suggest the presence of residues from paleo-subducted lithospheric slabs in and below the mantle transition zone underlying the Uttarakhand Himalayas.

Global variability of the composition and temperature at the 410-km discontinuity from receiver function analysis of dense arrays

Seismic boundaries caused by phase transitions between olivine polymorphs in Earth's mantle provide thermal and compositional markers that inform mantle dynamics. Seismic studies of the mantle transition zone often use either global averaging with sparse arrays or regional sampling from a single dense array. The intermediate approach of this study utilizes many densely spaced seismic arrays distributed around the globe. We systematically compute teleseismic P-to-S receiver functions for each seismic array and invert for the 1-D seismic velocity structure of the mantle transition zone beneath each array to facilitate a comparison between densely sampled regions. We stack 3,600 receiver functions on average at 67 arrays in total. The stack is used in a probabilistic inversion to estimate the mantle transition zone interface depths and velocities beneath each array. We focus on the 410-km discontinuity (410) because it is a prominent seismic interface that is clearly linked to a single mineral phase transition between olivine and wadsleyite. The depths and velocity contrasts of the 410 are mapped to temperatures and compositions using mineral physics constraints. The depth of the 410 ranges from ∼405–440 km, which is consistent with a ∼360 K temperature range in a dry mantle and a ∼260 K temperature range in a wet mantle (2 wt. % water). The Vs contrast across the 410 ranges from ∼2.5–8 %, which is consistent with ∼20–70 vol. % olivine composition in a dry mantle and ∼25–80 vol. % in a wet mantle. The bulk composition of the upper mantle near the 410-km discontinuity is typically considered to be well-mixed because there is no thermodynamic impediment to convection at the olivine to wadsleyite phase transition. However, the wide range of inferred olivine content from our study suggests that there are large lateral variations in the bulk composition of the upper mantle near the 410-km discontinuity.

No globally detectable seismic interfaces within Earth's mantle transition zone: Evidence for basalt enrichment

Earth's mantle transition zone (MTZ) is characterized by multiple mineral phase changes. The MTZ is bounded seismically by the 410- and 660-km elastic discontinuities, which are interpreted as phase changes from olivine to wadsleyite and from ringwoodite to bridgmanite, respectively. Within the transition zone, the wadsleyite-to-ringwoodite phase change has been assigned to a seismic feature at 520-km depth. Here we show that SS precursors, the most common seismic tool for imaging MTZ structure at global scale, are susceptible to signal-processing artifacts within the transition zone. Owing to the narrow frequency range in which the SS wave is typically examined, S waves scattered by the bounding MTZ discontinuities generate Gibbs-effect overshoots that map into features within the transition zone, including at 520-km depth. These results suggest that inferences of global interfaces at 520-km depth and, in some studies, at 560-km depth, are problematic. If such interfaces are not detectable, Earth's mantle transition zone may depart significantly from the pyrolite composition, enriched in a subducted basalt component that contributes a high proportion of garnet. A basalt fraction (f = ∼0.4) in the MTZ, consistent with some estimates, can render the 520-km discontinuity indistinguishable from signal-processing artifacts.

Constraining seismic anisotropy on the mantle transition zone boundaries beneath the subducting Nazca slab

Some seismic evidence suggests that the mantle transition zone (MTZ) may become highly hydrous and anisotropic, particularly in the vicinity of subduction zones. The two-dimensional path-integrated anisotropy from the upper mantle to the MTZ has been well established beneath the northwestern region of South America. However, explicit details of azimuthal anisotropy on the MTZ boundaries remains ambiguous. Therefore, we attempted to constrain the azimuthal anisotropy on the MTZ boundaries by implementing the P-to-S anisotropic receiver function analysis. We detected significant seismic evidence of azimuthal anisotropy on the 410-km discontinuity, but weak anisotropy on the 660-km discontinuity. The synthetic waveform modeling indicated the fast symmetry axis of anisotropy trends 50° from the north and plunges 40° downwards from horizontal with an anisotropy strength of 4.0% near 410 km depth. The direction of anisotropy suggests the mantle material moves downwards and towards the subducting Nazca slab near the depth of 410 km. The increased anisotropy strength around the 410 km suggests the hydrous wadsleyite may attribute to anisotropy in the upper MTZ. The lack of detectable seismic anisotropy near the depth of 660 km could be caused by the insufficient amount of aligned anisotropic minerals, even though the mantle material continues moving downwards.

The structure of the mantle transition zone and its implications for mantle upwelling beneath the central Tibetan plateau from triplicated P-waveforms modeling

The Tibetan plateau is considered as an ideal place for studying the continental collisions, tectonic movements and regional geodynamics. Although multiple episodes of igneous activities in Tibetan plateau have been suggested derived from mantle upwelling, their origins and corresponding geodynamic models remain vigorously debate. This study presents a high-resolution upper mantle velocity model of P-wave and its lateral variation beneath the central Tibetan plateau based on triplicated waveform modeling. The results show that, compared with IASP91 model, the 660-km discontinuity is deepened approximately 17–28 km from the Qiangtang terrane to the Beishan block. The lower mantle transition zone (MTZ) is characterized by high P-wave velocity anomalies (HVA) in range of 0.3%–0.9%. We infer that they are mainly related to the hydrous materials with low temperature in the lower MTZ. Besides, the depression of the 410-km discontinuity is about 6–15 km. A remarkable low velocity layer (LVL) is detected atop the 410-km discontinuity, exhibiting the thickness of 31–40 km and low P-wave velocity anomaly of −2.5% to −1.6%. This is against completely temperature-dominated origin of the LVL, and suggests that the material composition has also contributes. Additionally, we propose a new geodynamic model to demonstrate the asthenospheric upwelling beneath the Qiangtang and Songpan-Ganzi terranes, which suggests that the passively ascending water-enriched materials from the lower MTZ and their dehydration are responsible for the observed LVL atop the 410-km discontinuity, as well as the origin of the Cenozoic magmatic intrusion and volcanic eruption of the northern Tibet volcano in Pleistocene.

No basalt accumulation and segregation atop the 660-km discontinuity beneath the Sea of Okhotsk

No basalt accumulation and segregation atop the 660-km discontinuity beneath the Sea of Okhotsk

High-pressure single-crystal elasticity of corundum: Implication for multiple seismic structure of 660-km discontinuity

The complex multi-discontinuity structure at 660‐800 km depth is likely attributable to lateral mantle composition heterogeneities, which are closely related to the mid-ocean ridge basalts (MORB). To decipher the impact of varying composition on the seismic properties of MORB, detailed knowledge of the elasticity of candidate minerals is thus important. Here we employed Brillouin scattering coupled with diamond anvil cells to determine the single-crystal elasticity of corundum up to 14 GPa and 300 K. Using third-order finite strain equation of state, we calculate the pressure derivatives of adiabatic bulk modulus and shear modulus of corundum, which yields KS0’ = 3.8(1), G0’ = 1.8(1) with KS0 = 256(1) GPa and G0 = 163(1) GPa and ρ0 = 3.987(1) g/cm3. Combined with previous high-temperature data, the velocity and anisotropy of corundum have been calculated at 300 K or along normal geotherm. Our results are applied to model the density and velocity profiles of normal and alkali-depleted MORB. Our modeling demonstrates that varying the alkali content and temperature of MORB can significantly impact the discontinuity depth and velocity jump at 660–800 km depth. For normal MORB, reducing the temperature by 300 K from normal mantle geotherm results in a shift of the velocity jump from 670–710 to 650–710 km depth but hardly affects the magnitude of the velocity jump (5.8–6.8(3)% for VP and 10.0–10.8(5)% for VS). By contrast, in alkali-depleted MORB, the discontinuity will occur at a greater depth from 705–730 to 720–745 km depending on temperature with a VP jump of 3.8–4.6(2)% and VS jump of 6.3–7.4(4)%.

Topography of the 660-km discontinuity within the Izu-Bonin subduction zone and evidence of slab penetration near the Bonin Super Deep Earthquake (∼680 km)

The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the ‘660-km discontinuity.’ In this study, the short-period (1–2 Hz) S-to-P conversion phase S660P was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United States and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ∼670 km was found beneath the 300–400-km event zone in the northern Bonin region near 33°N. Meanwhile, the ‘660-km discontinuity’ has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of (690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a (300 ± 100) °C cold anomaly estimated using a post-spinel transformation Clapeyron slope of (−2.0 ± 1.0) MPa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of (695 ± 5) km below the deepest event and with a focal depth of ∼550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin Super Deep Earthquake, which occurred at ∼680 km on May 30, 2015, the discontinuity is depressed to ∼700 km, suggesting a near-vertical penetrating slab and an S-to-P conversion in the coldest slab core, where a large low-temperature anomaly should exist.

Ancient slabs beneath Arctic and surroundings: Izanagi, Farallon, and in-betweens

A detailed 3-D tomographic model of the whole mantle beneath the northern hemisphere (north of ~ 30°N latitude) is obtained by inverting a large amount of P-wave arrival time data (P, pP, and PP) to investigate transition of subducted slabs beneath Eurasia–Arctic–North America. We apply an updated global tomographic method that can investigate the whole mantle 3-D structure beneath a target area with high resolution comparable to that of regional tomography. The final tomographic model is obtained by performing independent calculations for 12 different target areas and stitching together the results. Our model clearly shows the subducted Izanagi and Farallon slabs penetrating into the lower mantle beneath Eurasia and North America, respectively. In the region from Canada to Greenland, a stagnant slab lying below the 660-km discontinuity is revealed. Because this slab has a texture that seems to be due to subducted oceanic ridges, the slab might be composed of the Farallon and Kula slabs that had subducted during ~60–50 Ma. During that period, a complex rift system represented by division between Canada and Greenland was developed. The oceanic ridge subduction and hot upwelling in the big mantle wedge above the stagnant slab caused a tensional stress field, which might have induced these complex tectonic events.

Seismic image of the mantle transition zone beneath northeastern China: evidence for stagnant Pacific subducting slab, lithospheric delamination and mantle upwelling

SUMMARY We provide a comprehensive image of the mantle transition zone (MTZ) beneath northeastern China by performing Variable Bin Radius Stacking of receiver functions. A massive seismic data set consisting of over 133 000 receiver functions recorded by 1208 broad-band stations is processed. Our results reveal fine-scale topography on the 410- and 660-km discontinuities defining the upper and lower bounds of the MTZ, lateral variations in the MTZ thickness and slab interfaces within the MTZ. In particular, unambiguous images of the slab interfaces provide direct evidence for the presence of the stagnant Pacific subducting slab below the eastern portion of the study area. A widespread deepening of the 410-km discontinuity is consistent with a hot and wet low-velocity upper mantle resulting from dehydration of the stagnant slab. Prominent depressions are evident in the depth to the 660-km discontinuity, with a thickened MTZ associated with the cold stagnating slab. Localized uplifts of the 660-km discontinuity are possibly caused by partial melt under the slab. These features attest to the influence of the Pacific plate on the MTZ. Additionally, a pronounced upwarp on the 660-km interface with a thin MTZ agrees with a previously hypothesized mantle upwelling through a slab window, possibly triggered by the sinking of the stagnant slab. Moreover, the western part of the study region is characterized by alternating ups and downs of the 410-km interface, while the topography of the 660-km discontinuity is relatively flat. We propose the western region is dominated by foundering of delaminated lithospheric blocks that induced upward mantle return flows upon entrance into the MTZ.

Seismic evidence for global basalt accumulation in the mantle transition zone.

The mantle's compositional structure reflects the thermochemical evolution of Earth. Yet, even the radial average composition of the mantle remains debated. Here, we analyze a global dataset of shear and compressional waves reflecting off the 410- and 660-km discontinuities that is 10 times larger than any previous studies. Our array analysis retrieves globally averaged amplitude-distance trends in SS and PP precursor reflectivity from which we infer relative wavespeed and density contrasts and associated mantle composition. Our results are best matched by a basalt-enriched mantle transition zone, with higher basalt fractions near 660 (~40%) than 410 (~18-31%). These are consistent with mantle-convection/plate-recycling simulations, which predict that basaltic crust accumulates in the mantle transition zone, with basalt fractions peaking near the 660. Basalt segregation in the mantle transition zone also implies that the overall mantle is more silica enriched than the often-assumed pyrolitic mantle reference composition.

Ultra-low-velocity anomaly inside the Pacific Slab near the 410-km discontinuity

The upper boundary of the mantle transition zone, known as the “410-km discontinuity”, is attributed to the phase transformation of the mineral olivine (α) to wadsleyite (β olivine). Here we present observations of triplicated P-waves from dense seismic arrays that constrain the structure of the subducting Pacific slab near the 410-km discontinuity beneath the northern Sea of Japan. Our analysis of P-wave travel times and waveforms at periods as short as 2 s indicates the presence of an ultra-low-velocity layer within the cold slab, with a P-wave velocity that is at least ≈20% lower than in the ambient mantle and an apparent thickness of ≈20 km along the wave path. This ultra-low-velocity layer could contain unstable material (e.g., poirierite) with reduced grain size where diffusionless transformations are favored.

Sound velocities and single-crystal elasticity of hydrous Fo90 olivine to 12 GPa

Nominally anhydrous minerals (NAMs) may contain significant amounts of water and constitute an important reservoir for mantle hydrogen. The colloquial term ‘water’ in NAMs is related to the presence of hydroxyl-bearing (OH−) point defects in their crystal structure, where hydrogen is bonded to lattice oxygen and is charge-balanced by cation vacancies. This hydrous component may therefore have substantial effects on the thermoelastic parameters of NAMs, comparable to other major crystal-chemical substitutions (e.g., Fe, Al). Assessment of water concentrations in natural minerals from mantle xenoliths indicates that olivine commonly stores ∼0–200 ppm of water. However, the lack of samples originating from depths exceeding ∼250 km coupled with the rapid diffusion of hydrogen in olivine at magmatic temperatures makes the determination of the olivine water content in the upper mantle challenging. On the other hand, numerous experimental data show that, at pressures and temperatures corresponding to deep upper mantle conditions, the water storage capacity of olivine increases to 0.2–0.5 wt%. Therefore, determining the elastic properties of olivine samples with more realistic water contents for deep upper mantle conditions may help in interpreting both seismic velocity anomalies in potentially hydrous regions of Earth's mantle as well as the observed seismic velocity and density contrasts across the 410-km discontinuity.Here, we report simultaneous single-crystal X-ray diffraction and Brillouin scattering experiments at room temperature up to 11.96(2) GPa on hydrous [0.20(3) wt% H2O] Fo90 olivine to assess its full elastic tensor, and complement these results with a careful re-analysis of all the available single-crystal elasticity data from the literature for anhydrous Fo90 olivine. While the bulk (K) and shear (G) moduli of hydrous Fo90 olivine are virtually identical to those of the corresponding anhydrous phase, their pressure derivatives K′ and G′ are slightly larger, although consistent within mutual uncertainties. We then defined linear relations between the water concentration in Fo90 olivine, the elastic moduli and their pressure derivatives, which were then used to compute the sound velocities of Fo90 olivine with higher degrees of hydration. Even for water concentrations as high as 0.5 wt%, the sound wave velocities of hydrous and anhydrous olivines were found to be identical within uncertainties at pressures corresponding to the base of the upper mantle. Contrary to previous claims, our data suggest that water in olivine is not seismically detectable, at least for contents consistent with deep upper mantle conditions. In addition to that, our data reveal that the hydration of olivine is unlikely to be a key factor in reconciling seismic velocity and density contrasts across the 410-km discontinuity with a pyrolitic mantle.

Structure and evolution of the Australian plate and underlying upper mantle from waveform tomography with massive data sets

SUMMARY We present a new S-wave velocity tomographic model of the upper mantle beneath the Australian Plate and its boundaries that we call Aus22. It includes azimuthal anisotropy and was constrained by waveforms from 0.9 million vertical-component seismograms, with the densest data sampling in the hemisphere centred on the Australian continent, using all available data covering this hemisphere. Waveform inversion extracted structural information from surface waves, S- and multiple S-waves and constrained S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle, down to the 660-km discontinuity. The model was validated by resolution tests and, for particular locations in Australia with notable differences from previous models, independent inter-station measurements of surface-wave phase velocities. Aus22 can be used to constrain the structure and evolution of the Australian Plate and its boundaries in fine detail at the regional scale. Thick, high-velocity (and, by inference, cold) cratonic lithosphere occupies nearly all of western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it collides with island arcs, without subducting. Diamondiferous kimberlites and lamproite deposits are underlain by cratonic lithosphere, except for the most recent diamondiferous lamproites in the King Leopold Orogen. The rugged eastern boundary of the cratonic lithosphere resolved by the model provides a lithospheric definition of the Tasman Line. Just east of the Tasman Line, an area of intermediate-thick lithosphere is observed in the southern part of the continent. The eastern part of Australia is underlain by thin, warm lithosphere, evidenced by low seismic velocities. All the sites of Cenozoic intraplate volcanism in eastern Australia are located on thin lithosphere. A low-velocity anomaly is present in the mantle transition zone (410–660 km depths) beneath the Lord Howe and Tasmanid hotspots, indicative of anomalously high temperature and consistent with a deep mantle upwelling feeding these hotspots and, possibly, also the East Australia hotspot. High seismic velocities at 200–410 km depth below New Guinea indicate the presence of slab fragments, probably linked to the subduction of the Australian Plate. High seismic velocities are observed in the transition zone below northeast Australia and indicate the presence of subducted lithospheric fragments trapped in the transition zone, possibly parts of the former northern continental margin of Australia.

The topography of the 660-km discontinuity beneath the Kuril-Kamchatka: Implication for morphology and dynamics of the northwestern Pacific slab

The 660-km discontinuity (660) plays a potentially important role in deep slab dynamics and mantle convection. Increasing numbers of seismic observations suggest controversial morphologies of the Pacific slab beneath the Kuril-Kamchatka, highlighting the poorly understood interaction of the slab and mantle discontinuities. Here we collect near-source S-to-P converted waves from a large dataset with several dense seismic networks and systematically image the topography of the 660 around the Pacific slab beneath the Kuril-Kamchatka. We conduct detailed comparisons of the 660 depths and seismicity along some vertical cross sections. In comparison with the discontinuity depth in the IASP91 model, the 660 exhibits broad depressions up to 32–63 km with apparent downward deflections beneath the Kamchatka Peninsula and northern Kuril (region I), supporting slab penetration into the lower mantle; in contrast, the 660 depressions beneath southern Kuril (region II) are less than 21–28 km with a relatively flat configuration, implying a stagnating slab with possible hot entrained mantle materials and/or partial melts below it. We interpret these regional variations in the 660 topography as reflecting local low-temperature anomalies due to different slab morphologies associated with contrasting subduction modes. We suggest compound effects of pressure-driven mantle flow and trench retreat for inducing the inferred subduction mode change of the Pacific slab from region I to region II. Our results can provide more insight into subduction dynamics in the northwestern Pacific.

High-resolution mid-mantle imaging with multiple-taper SS-precursor estimates

SUMMARY SS-precursor imaging is used to image sharp interfaces within Earth’s mantle. Current SS-precursor techniques require tightly bandpassed signals (e.g. 0.02–0.05 Hz), limiting both vertical and horizontal resolutions. Higher frequency content would allow for the detection of finer structure in and around the mantle transition zone (MTZ). Here, we present a new SS-precursor deconvolution technique based on multiple-taper correlation (MTC). We show that applying MTC to SS-precursor deconvolution can increase the frequency cut-off up to 0.5 Hz, which potentially sharpens vertical resolution to ∼10 km. Furthermore, the high-pass frequency can be lowered (≪ 0.01 Hz), allowing more long-period energy to be included in the calculation, to better constrain the signal and reduce side lobes. Our method is benchmarked on full-waveform synthetic seismograms computed via AxiSEM3D for the PREM 1-D Earth model. We apply our novel MTC-SS-precursor deconvolution to ∼7000 seismograms recorded at broad-band borehole sensors of the Global Seismographic Network with source–receiver bounce points in the North-Central Pacific Ocean. The MTZ in this region appears to be thin, which agrees with previous results. We do not observe the 520-km discontinuity in our SS-precursor estimates. Additionally, we detect a low-velocity zone above the MTZ to the north of the Hawaiian Islands that has previously been inferred from asymmetry in side lobe amplitudes. Our high-frequency analysis demonstrates this feature to be a sharp interface (≤ 10-km thickness), rather than a thick wave speed gradient.

Nature of mantle discontinuities beneath the Ontong Java Plateau

We determined the depths of the mantle discontinuities with a receiver function method using broadband data recorded on the seafloor and islands of the Ontong Java Plateau (OJP) and its vicinity. The 410-km discontinuity is broadly elevated by 10–20 km beneath the OJP. The elevation is observed in the low-velocity region above a stagnant paleo-Pacific slab that had been subducted until 25 Ma. The elevated 410-km discontinuity and the slow anomalies may be caused by partial melting induced by hydration with water released from the paleo-Pacific slab. The 660-km discontinuity is close to the global average beneath the OJP, suggesting that the stagnant Pacific slab is bottoming in the mantle transition zone. The 410-km discontinuity is depressed by 10–20 km beneath the Caroline volcanic chain, where a low-velocity sheet has been observed by tomographic studies, implying that a sheet-like upwelling occurs at least from the top of the mantle transition zone. The 660-km discontinuity is also depressed by 10–30 km beneath the Caroline volcanic chain, which may be attributed to the majorite-bridgmanite phase change in a high-temperature environment and/or an underestimated velocity correction in the moveout correction of receiver functions. It is thus unclear whether the sheet-like mantle upwelling has its roots in the lower mantle.

Subduction-zone parameters that control slab behavior at the 660-km discontinuity revealed by logistic regression analysis and model selection

The potential mechanisms that drive the behavior of subducted oceanic plates at the 660-km discontinuity are subject to debate. Here we conduct logistic regression analysis and model selection to determine the key subduction-zone parameters in natural subduction zones that discriminate the plate behavior along the discontinuity. We select the key variables based on three information criteria: leave-one-out cross-validation score (LOO), Akaike Information Criterion (AIC), and Bayesian Information Criterion (BIC). Among the 17 subduction-zone parameters analyzed, only the trench velocity, convergence rate, and trench width are selected in the simplest model that minimizes BIC. The thermal parameter and several other variables are also selected to minimize AIC and LOO. Our results suggest that a stagnant slab occurs along the 660-km discontinuity when there is a narrow oceanic plate and a retreating trench in natural subduction zones, which has also been modeled in previous numerical simulations. Neither the stress nor the deformation rate of the upper-plate margin is selected in the three optimal models, which suggests that back-arc spreading in natural subduction zones does not globally characterize plate behavior at the 660-km discontinuity, although back-arc spreading and a stagnant slab coincide in some numerical simulations. The combination of subduction-zone data analysis and numerical simulations will therefore provide deep insights into the dynamics of Earth’s deep interior.