Ps Conversion Research Articles

Classical modeling of positronium cooling in silicon nanochannel plates

Silicon nanochannel plates (NCPs) are employed in a variety of experiments as sources of cold positronium (Ps) via positron to Ps conversion. Despite the widespread use of NCPs, no comprehensive model of the dynamics of the cooling process of Ps within the nanochannels has been provided yet. In this work, we present a classical Monte Carlo model of Ps cooling produced in NCPs and validate it against experimental values present in the literature. Our model allows us to estimate crucial characteristics such as the cooling time in NCPs, the angular distribution of emitted Ps, and the fraction of Ps annihilating into three gamma quanta. The simulated energy spectra of emitted Ps closely match experimental results for NCPs held at room temperature. At cryogenic temperature the classical thermalization model does not accurately reproduce the measured spectra, which we attribute to a transition in the cooling process towards a regime where quantum effects play a significant role.

Crustal thickness estimation in Indonesia using receiver function method

The existence of seismic wave velocity difference in the Earth crust and mantle creates the possibility to use earthquake data for estimating the crustal thickness utilizing the Ps conversion phase in the boundary. The radial component signal was deconvolved from the vertical component in the frequency domain to estimate receiver function for Indonesia region. We implemented the water level deconvolution techniques with a Gaussian filter of 2.5 Hz to eliminate the high frequency noise in the receiver function. The H-k stacking technique was performed to all receiver functions from each event to predict the crustal thickness and the Vp/Vs ratio below the stations. We analyzed ten azimuthally distributed teleseismic earthquakes recorded by 108 stations of BMKG. The result shows that the crustal thickness in Indonesia varies from 20 to 39.9 km. The western part of Sumatera, northern part of Sulawesi Island, and North Maluku region show generally thinner crust with value about 20 to 25 km. The North Sumatera, Central Java, and East Java show a considerably thicker crust of up to 36 km. Furthermore, our result also reveals a difference of crustal thickness about 5 km with the previous studies.

Early Results of Time Domain Receiver Function Data Processing in Mt Merapi and Mt Merbabu

The receiver function method is a method to image the earth subsurface by utilizing Ps conversion waves that are formed due to the velocity boundary. In general, the receiver function method estimates depth of structures such as the mantle-crust boundary by deconvoluting the vertical component from the horizontal component. Typical receiver function data processing is done in the frequency domain where the deconvolution process can be seen as a division between two components. In this study, we tried to reprocess the data using a deconvolution technique in time domain, popularly known as iterative time-domain deconvolution. The principle of iterative time domain deconvolution consists of iterative cross-correlation between the horizontal and vertical component. We use data from the DOMERAPI seismic station network located in the vicinity of Mt Merapi and Mt Merbabu. Mt Merapi is one of the most active volcanoes in the world with frequent eruptions and located at the ring of fire chain volcano in Indonesia. Note that the previous receiver function study in this region showed complex signals at some stations that may be related to sediment at shallow sediment and possible layers of low velocity zone that interfering main signal for a crust-mantle boundary. Our current results show iterative time domain RFs have clearer and smoother signal than the frequency domain that help interpreting the waveform signals. We estimate a range of crust thickness between 26-31 km near Mt Merapi. However, we noticed that iterative time domain calculation requires longer computation time and input signal.

Seismic Anisotropy Within an Active Fluid Flow Structure: Scanner Pockmark, North Sea

Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ∼1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55–65 ms TWT after the seabed) consistent with PS conversion at 4–5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160°N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150–160 N or aligned with a local stress gradient at 50–60 N. This study reports the first observation of very shallow anisotropy associated with active methane venting.

Study of Crustal Structure Beneath Station Banjarnegara Indonesia (BJI) Based on Receiver Function Analysis

We applied receiver functions (RFs) analysis to estimated the crustal structures beneath Station Banjarnegara, Indonesia (BJI). We selected teleseismic earthquakes with epicentral distance 30°-90° and magnitude greater than 6 (M > 6) that recorded from 3 component broadband seismograph at the station. We determined crustal thickness, S-wave velocity profiles and ratio of ν p/νs using nonlinear Neighborhood Algorithm (NA). We migrated RFs phase to depth by AK-135 global velocity models to identify the depth of the geological structure. From the Ps conversion phase and inhomogeneity of the arrival time, we detected the near surface has a strong anisotropy. We identified delay P phase near surface that associated with thick sedimentary layer in this area at about 2.5 km. Crustal thicknesses, ‘Moho depth’, beneath the station was estimated at depth 28-32 km. The high RFs amplitude phase that might be associated with the presence of Indo-Australian Slab was identified at a depth 140-160 km according to previous studies.

Developments for pulsed antihydrogen production towards direct gravitational measurement on antimatter

A main scientific goal of the experiment is the direct measurement of the Earth’s local gravitational acceleration g on antihydrogen. The Weak Equivalence Principle is a foundation of General Relativity. It has been extensively tested with ordinary matter but very little is known about the gravitational interaction between matter and antimatter. Antihydrogen is produced in via resonant charge-exchange reaction between cold Rydberg-excited positronium and cooled down antiprotons. The achievements for the development of a pulsed cold antihydrogen source are presented. Large number of antiprotons, necessary for a significant production rate of antihydrogen, are captured, accumulated, compressed and cooled over an extended period of time. Positronium (Ps) is formed through e+-Ps conversion in a silica porous target at 10 K temperature in a reflection geometry inside the main apparatus. The so-formed Ps cloud is then laser-excited to Rydberg levels, for the first time in a 1 T magnetic field. Consequently, a detailed characterization of the Ps source for antihydrogen production in magnetic field needed to be performed. Several detection techniques are extensively used to monitor antiproton and positron manipulations in the formation process of antihydrogen inside the main apparatus. Positronium detection techniques underwent extensive improvements in sensitivity during the last antiproton run. At the same time, major efforts to improve integrate and commission the detectors sensitive to antihydrogen production took place.

Modeling the seismic response of individual hydraulic fracturing stages observed in a time-lapse distributed acoustic sensing vertical seismic profiling survey

In 2017, distributed acoustic sensing (DAS) technology was deployed in a horizontal well to conduct a vertical seismic profiling survey before and after each of 78 hydraulic fracturing stages. From two vibroseis source locations at the surface, time shifts of P- and S-waves were observed but decayed over days. Some stages also showed waves scattered off the stimulated rock volume. We have used 2D finite difference elastic wavefield modeling to understand these observations and connect them to underlying properties of the stimulated rock. We have developed an effective medium model of vertical fractures that close exponentially with time as fluid leaks off into the formation can match the distribution of P- and S-wave time shifts along the well. This has enabled estimates of the height, normal and tangential fracture compliance values, and decay time of the stimulated rock volume. Additionally, the kinematics of scattered waves observed in the data have been found to be consistent with PS conversion across the stimulated rock volume from an individual stage. With higher quality DAS data, stage-by-stage inversion for height, fracture compliance, and decay time attributes may be possible for characterizing variations in the effectiveness of hydraulic fracturing.

Few-Layer Boron Nitride with Engineered Nitrogen Vacancies for Promoting Conversion of Polysulfide as a Cathode Matrix for Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries have become one of the most promising candidates as next-generation batteries, owing to their high specific capacity, low cost, and environmental benignity. Although many strategies have been proposed to restrain the shuttle of lithium polysulfides (LiPSs) through physical trapping and chemical binding, the sluggish kinetics of PS conversion still degrade the capacity, rate, and cycling performance of Li-S batteries. Herein, a novel kind of few-layer BN with engineered nitrogen vacancies (v-BN) has been developed as a cathode matrix for Li-S batteries. The positive vacancies in the BN nanosheets not only promote the immobilization and conversion of LiPSs, but also accelerate the lithium ion diffusion in cathode electrodes. Compared with pristine BN, the v-BN cathodes exhibit higher initial capacities from 775 mA h g-1 to 1262 mA h g-1 at 0.1 C and a high average coulombic efficiency of over 98 % during 150 cycles. Upon increasing the current density to 1 C, the cell still preserves a capacity of 406 mA h g-1 after 500 cycles, exhibiting a capacity decay of only 0.084 % per cycle. The new vacancy-engineered material provides a promising method for achieving excellent performance in Li-S batteries.

Landfill leachate treatment by sequential combination of activated persulfate and Fenton oxidation

This work assesses the feasibility of sequential persulfate and Fenton oxidation for the decolorization and mineralization of landfill leachate (5600 mg L−1 TOC; pH0: 8.6) in a continuous batch-recirculation system. Firstly, it was analyzed the role of the operational conditions upon the persulfate activation evaluating the effects of electrolysis, ilmenite (FeTiO3) as a source of Fe(II) and UV-LED (at 365 nm). The studied variables include current density (j) (50–200 mA cm−2), persulfate dose (46.8–234 mM) and mineral concentration (500–1500 mg L−1). The increase in j enhanced the hypochlorite generation and PS conversion to SO4− and, consequently, decolorization efficiency increasing the penetration of light through the solution and the photoreduction of Fe(III) to Fe(II) in the FeTiO3 surface. The combined electrolysis/FeTiO3/UV-LED showed synergetic effect compared to the individual processes, achieving mineralization around 53% under the optimum operating conditions (1 g L−1 of FeTiO3, using 234 mM of PS at 200 mA cm−2 under UV-LED radiation). The subsequent Fenton oxidation once the pH decreased up to around 3, led to overall mineralization above 90% after 480 min, confirming the suitability of this combined treatment to deal with recalcitrant and highly colored effluents.

Structure of the Crust and Mantle Down to 700 km Depth beneath the Longmenshan From P Receiver Functions

AbstractSince the Ms 8.0 Wenchuan earthquake occurred in 2008 in the Longmenshan at the eastern margin of Tibet, this region has attracted much attention. In order to investigate the deep structure of the region, an analysis of P receiver functions was carried out, using data from an array of 80 broadband seismic stations operated in 2012 and 2013, and straddling the Longmenshan fault zone around the epicenter of the Wenchuan earthquake. Positive Ps conversions from 5 to 10 km depth indicate the base of the Mesozoic sediments in the Sichuan basin and the base of the Songpan‐Ganzi flysch deposits in Tibet. Later positive Ps conversions represent the Moho, which occurs at 40–50 km depth below the Sichuan basin and deepens to the NW to 55–70 km depth under Tibet. An important finding of this study is that whereas in the NE, the Moho deepening from SE to NW is smooth, in the middle and SW of the region the Moho deepening is abrupt with, in some places, a possible overlap. Negative Ps conversions at stations in Tibet represent the top of low shear velocity zones in the middle and lower crusts below Tibet. Beneath the Sichuan basin, a negative phase about 1 s earlier than the Moho Ps conversion is to a large extent, almost certainly due to multiples from discontinuities in the upper crust. Finally, positive Ps conversions can be observed from the 410 and 660‐km discontinuities at the top and bottom of the mantle transition zone, respectively.

Crustal properties of the northern Scandinavian mountains and Fennoscandian shield from analysis of teleseismic receiver functions

Supplementary data are available at GJ I online. https://academic.oup.com/gji/article-lookup/doi/10.1093/gji/ggy140#supplementary-data Figure S1. (a) Teleseismic events (Mw > 5.8) recorded by SCANLIPS2 between July 2007 and September 2009 for P-receiver function study. (b) Teleseismic events (Mw > 5.8) recorded by SCANLIPS3D between July 2013 and September 2014 for P-receiver function study. In both cases the blue star corresponds to the centre of the seismic array. Figure S2. (a) Synthetic P-receiver functions for a Moho depth of 43 km and Vp/Vs of 1.73 from three crustal models. In red, a model with two layers (upper and lower crust) for the crust and step discontinues. In green, a model with three layers (upper, lower crust and transitional Moho with highVp) and step discontinuities. In blue, a gradual model between upper crust and Moho. (b) Sensibility of amplitude of direct P arrival and Ps conversion for different slowness and for two crustal model (on the left a step Moho model and on the right a gradual Moho model). Table S1. Informations about seismic stations and number of events used in this study for each instruments (70XX: SCANLIPS2 -13XX: SCANLIPS3D). Table S2. Compilation of P-RFs analysis and previous works (Ottemoller & Midzi ¨ 2003; Olsson et al. 2008; Silvennoinen et al. 2014).

Time-interleaved pulse-shrinking time-to-digital converter with reduced conversion time

A silicon-efficient time-interleaved pulse-shrinking time-to-digital converter (TDC) is proposed. The proposed TDC consists of a 16-stage coarse pulse-shrinking TDC with constant per-stage shrinkage 4.8 ns and a pair of 16-stage fine pulse-shrinking TDCs operated in a time-interleaved manner with constant per-stage shrinkage 296 ps. A simple residual time extraction method is proposed to extract the residual pulse of the coarse TDC simultaneously with digitization carried out by the TDC so that the digitization can be carried out by both the coarse and fine TDCs simultaneously to minimize conversion time. The characteristics of the proposed TDC including silicon consumption, power consumption, conversion time, jitter, and mismatch-induced timing errors are investigated. The proposed TDC was implemented in an IBM 130 nm 1.2 V CMOS technology. Simulation results show that the TDC offers 0.296---76.8 ns dynamic range, 850 ps conversion time, 0.285 LSB differential nonlinearity, and 0.78 LSB integral nonlinearity while consuming 7 mW.

Theoretical and experimental analysis on effect of porous silicon surface treatment in multicrystalline silicon solar cells

As widely known in silicon-based solar cells, the intrinsic parameters are decisive factors for internal quantum efficiency (IQE) and solar cells performances. Particularly, the front surface recombination velocity is one of the most important electrical parameters used to quantify the optoelectronic quality of the solar cells. In this study, the impact of the front surface recombination velocities (Sf) and anti-reflection coating are examined. Theoretical analysis shows that a decrease in Sf enhances the IQE for photons with shorter wavelengths (400–700nm). Cells with front porous silicon treatment (PS) was made and studied under ambient condition to improve performance of solar cells. In fact, internal quantum efficiency of mc-Si solar cells with PS was enhanced in shorter wavelengths, while the reflectivity was reduced to about 7% after PS treatment and conversion efficiency was improved by about 5% compared with conventional mc-Si solar cells. Furthermore, the laser-beam-induced current (LBIC) mapping shows that the PS treatment improves their quantum response.

Crustal structure and deformation under the Longmenshan and its surroundings revealed by receiver function data

The convergence of India and Eurasia and the obstruction from the rigid Sichuan Basin cause the Longmenshan (LMS) to have the steepest topographic gradient at the eastern margin of the Tibetan Plateau. However, the mechanisms of surface uplift are still controversial. In this paper, we estimate the crustal structure and deformation under the LMS and its surroundings by analyzing a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration. We apply a comprehensive splitting measurement technique on Ps conversion phase at the Moho (Moho Ps splitting) to calculate crustal anisotropy from azimuthal variations of receiver functions. Our results show that most of the seismic stations beneath the LMS area exhibit significant seismic anisotropy with the splitting time of 0.22–0.94s and a fast polarization direction of NW–SE, while less or even no crustal anisotropy has been observed under the Sichuan Basin. Comparing the fast polarization directions of Moho Ps splitting with the indicators of lithospheric deformation (such as shear wave splitting, absolute plate motion, and global positioning system) imply a consistent tendency of deformation between the lower crust and upper mantle, but decoupling deformation in the crust beneath the LMS area. We further compare Moho Ps splitting time to that estimated from previous SKS splitting, indicating that crustal anisotropy is an important source of the SKS splitting time in this study area. In addition, a thick crust (>50km) with high Vp/Vs values (1.74–1.86) is also observed using the H-κ stacking method. These seismic observations are consistent with the scenario that the LMS area has been built by the lower crustal flow. Combined with the seismic reflection/refraction profile and geology studies, we further suggest that the lower crustal flow may extrude upward into the upper crust along the steeply dipping strike faults under the LMS area, resulting in the surface uplift of the LMS.

Crustal structure of northeastern margin of the Tibetan Plateau by receiver function inversion

Using seismic data of about one year recorded by 18 broadband stations of ASCENT project, we obtained 2547 receiver functions in the northeastern Tibetan Plateau. The Moho depths under 14 stations were calculated by applying the H-κ domain search algorithm. The Moho depths under the stations with lower signal-noise ratio (SNR) were estimated by the time delay of the PS conversion. Results show that the Moho depth varies in a range of ∼40–60 km. The Moho near the Haiyuan fault is vague, and its depth is larger than those on its two sides. In the Qinling-Qilian Block, the Moho becomes shallower gradually from west to east. To the east of 105°E, the average depth of the Moho is 45 km, whereas the west is 50 km or even deeper. Combining our results with surface wave research, we suggest a boundary between the Qinling and the Qilian Mountains at around 105°E. S wave velocities beneath 15 stations have been obtained through a linear inversion by using Crust2.0 as an initial model, and the crustal thickness that was derived by H-κ domain search algorithm was also taken into account. The results are very similar to the results of previous active source studies. The resulting figure indicates that low velocity layers developed in the middle and lower crust beneath the transition zone of the Tibet Block and western Qinling, which may be related to regional faults and deep earth dynamics. The velocity of the middle and lower crust increases from the Songpan Block to the northeastern margin of Tibetan Plateau. Based on the velocity of the crust, the distribution of the low velocity zone and the composition of the curst (Poisson’s ratio), we infer that the crust thickening results from the crust shortening along the direction of compression.

Seismic Rheological Model and Reflection Coefficients of the Brittle–Ductile Transition

It is well established that the upper—cooler—part of the crust is brittle, while deeper zones present ductile behaviour. In some cases, this brittle–ductile transition is a single seismic reflector with an associated reflection coefficient. We first develop a stress–strain relation including the effects of crust anisotropy, seismic attenuation and ductility in which deformation takes place by shear plastic flow. Viscoelastic anisotropy is based on the eigenstrain model and the Zener and Burgers mechanical models are used to model the effects of seismic attenuation, velocity dispersion, and steady-state creep flow, respectively. The stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity, which is obtained by the Arrhenius equation and the octahedral stress criterion. The P- and S-wave velocities decrease as depth and temperature increase due to the geothermal gradient, an effect which is more pronounced for shear waves. We then obtain the reflection and transmission coefficients of a single brittle–ductile interface and of a ductile thin layer. The PP scattering coefficient has a Brewster angle (a sign change) in both cases, and there is substantial PS conversion at intermediate angles. The PP coefficient is sensitive to the layer thickness, unlike the SS coefficient. Thick layers have a well-defined Brewster angle and show higher reflection amplitudes. Finally, we compute synthetic seismograms in a homogeneous medium as a function of temperature.

Stripping Analysis of Ps-Converted Wave Polarization Anisotropy

Abstract The polarization anisotropy of the Ps -converted wave tells us about anisotropic properties of the region between a seismic station at which the Ps phase is observed and a velocity discontinuity at which the Ps conversion takes place. However, it is very difficult to know the anisotropic structure as a function of depth from the polarization anisotropy data because the data are obtained as a quantity integrated along the ray path from the station to the velocity discontinuity. In this study, I present a method to estimate seismic anisotropy of the layers using the shear-wave splitting of Ps phases that originate at velocity discontinuity interfaces in a flat-layered structure. After a description of the method, analyses of synthetic waveforms show that it is of benefit in the study of anisotropic structure of shear waves.

Synthesis and applicable condition of vector wave envelopes in layered random elastic media with anisotropic autocorrelation function based on the Markov approximation

SUMMARY The Markov approximation is a powerful stochastic method for the direct synthesis of wave envelopes in random velocity fluctuated media when the wavelength is shorter than their correlation distance. To apply the Markov approximation to realistic cases, we consider horizontal layered random media characterized by an anisotropic autocorrelation function (ACF), where different layers have different randomnesses and different background velocities having step-like changes. Solving the parabolic master equation for the two frequency mutual coherence function (TFMCF) in random elastic media for the vertical incidence of a plane wavelet from the bottom, we calculate the angular spectrum just before the first velocity boundary. Multiplying transmission or PS conversion coefficients of the boundary by the angular spectrum, we calculate the angular spectrum and TFMCF on the other side of the boundary. Then we solve the master equation for the forward propagating wavelet in the second layer. Taking the same procedure for each layer boundary, we finally obtain the mean square (MS) vector envelopes on the free surface on the top layer. For the practical simulation, we use 2-D random media characterized by a Gaussian ACF. We numerically confirm the validity of the envelope synthesis for a specific case of layered random media with anisotropic ACF by comparing with finite difference (FD) simulations of elastic waves. Considering the Earth structure, the horizontal correlation distance is larger than the vertical one and the velocity fractional fluctuation becomes weak as depth increases, the Markov approximation is good for modelling the primary wavelet and also the converted wavelet for the vertical incident wavelet. We derive an applicable range of the Markov approximation for random media with anisotropic ACF by comparing with FD simulations. The results show that the Markov approximation is accurate when the wavelength is comparable or shorter than the both of vertical and horizontal correlation distances and the MS fractional fluctuation is much smaller than the ratio of squared horizontal correlation distance to the product of the vertical correlation distance and propagation distance.

A major step in the continental Moho and its geodynamic consequences: the Taranaki-Ruapehu line, New Zealand

Receiver function analysis reveals a 7 km step-like change in crustal thickness across the Taranaki–Ruapehu Line (TR-line) of North Island, New Zealand. The TR-line runs east–west between the active andesite volcanoes of Mt Taranaki and Ruapehu and marks the southern-most extent of subduction zone volcanism in New Zealand. North of the TR-line receiver functions show a strong and sharp P-to-S (Ps) conversion at 25 ± 1.5 km depth, which is interpreted as a shallow Moho. At the TR-line the Moho Ps conversion deepens across a step to ∼32 km depth and weakens. Further south the Moho deepens to >35 km. Most of the 7 km step-change in crustal thickness occurs over a lateral distance of ∼8 km, yet there is little surface or topographic manifestation of the line. Given Fresnel zone considerations, the dip of the Moho offset could vary between 90° and 45°. Gravity, seismic attenuation and electrical data all show that the TR-line is not only a step in crustal thickness but also a profound lithospheric boundary as mantle properties, such as attenuation (Qp−1), implied density, and electrical resistivity change abruptly across the line. An east–west oriented cluster of earthquakes with hypocentral depths of 20–40 km is centred on the Moho step. We propose that the Moho step, the earthquakes and the rapid change in mantle properties across the TR boundary are causally related. Processes that could be responsible for the phenomena described here include the rapid removal of mantle lithosphere and lower crust to the north of the TR-line.

Mapping seismic anisotropy using harmonic decomposition of receiver functions: An application to Northern Apennines, Italy

Isotropic and anisotropic seismic structures across the Northern Apennines (Italy) subduction zone are imaged using a new method for the analysis of teleseismic receiver functions (RFs). More than 13,000 P‐wave coda of teleseismic records from the 2003–2007 Retreating‐Trench, Extension, and Accretion Tectonics (RETREAT) experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of Mediterranean evolution. A new methodology for the analysis of receiver functions is developed, which combines both migration and harmonic decomposition of the receiver function data set. The migration technique follows a classical “Common Conversion Point” scheme and helps to focus on a crucial depth range (20–70 km) where the mantle wedge develops. Harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of P‐to‐S converted phases. The separation of the back‐azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures. The application of our method to the RETREAT data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the Northern Apennines chain, where previous studies failed to clearly retrieve the geometry of the subducted interfaces. We present our results in closely spaced profiles across and along the Northern Apennines chain to highlight the convergence of the Tyrrhenian and the Adriatic microplates which differ in their crustal structure where the Adriatic microplate subducts beneath Tuscany and the Tyrrhenian sea. A signature of the dipping Adriatic Moho is clearly observed beneath the Tyrrhenian Moho in a large portion of the forearc region. In the area where the two Mohos overlap, our new analysis reveals the presence of an anisotropic body above the subducted Moho. There is a strong Ps converted phase with anisotropic characteristics from the top of the Adriatic plate to a depth of at least 80 km. Because the Ps conversion occurs much deeper than similar Ps phases in Cascadia and Japan, dehydration of oceanic crust seems unlikely as a causative factor. Rather, the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting.