Teleseismic P-wave Receiver Functions Research Articles

Fault-scale crustal structure across the Dunhua-Mishan fault (Tanlu northern segment) constrained from teleseismic P-wave receiver functions

The Dunhua-Mishan fault, located in the northern segment of the Tanlu fault zone, experienced multiple tectonic processes associated with the effects of the Pacific Plate subduction and the Indo-Asia collision. The high-resolution fault-scale structure is critical for understanding the fault evolution and potential fault damage. However, the well-defined deep structure of the Dunhua-Mishan fault is still unclear due to the lack of the dense seismic array. In this study, we construct a high-resolution P-wave receiver function imaging based on linear dense seismic array across the fault. Our results reveal the strong Moho depth variation across the Dunhua-Mishan fault zone. The slightly higher Vp/Vs ratio values within the fault zone indicate the presence of a small amount of mafic crust composition. Interestingly, the significant double positive Ps converted phases are observed within the fault zone, which may represent double Moho discontinuities. The double Moho structure may be related to multiple significant tectonic activities in the Tanlu northern segment. These newly observed structures provide new seismic constraints on the formation and evolution of the Tanlu fault zone and probably reflect that the lithospheric structure of the Dunhua-Mishan fault has been modified by a series of tectonic processes.

Crustal structure and geodynamics of the eastern Qilian orogenic belt, NE margin of the Qinghai-Tibet plateau, revealed by teleseismic receiver function

The eastern segment of the Qilian orogenic belt, comprising the Linxia block and Longzhong block, is at the Qinghai–Tibet Plateau’s northeastern margin. The area has experienced multiple tectonic events, including closure of the Qilian Ocean, convergence of the North China block and Qilian terrane, and collision of the Indian and Eurasian plates, forming a complex tectonic framework. To investigate the area’s geological evolution and the suture’s current location between the blocks, we used 3-year data recorded by 33 portable ChinArray II broadband stations (2013–2016). Using three teleseismic P-wave receiver function methods, H-κ stacking and common conversion point stacking (CCP), crustal structure, Poisson’s ratio, and Moho morphology were obtained at 33 stations. The results are described as follows: 1) The Maxianshan fault is an important boundary fault that divides the Linxia block and Longzhong block. The Linxia block’s layered crustal structure is obvious, and there is a low-velocity anomaly in the middle and lower crust, which may contain saline fluid and has Japanese-type island arc characteristics. 2) The layered structure of the Longzhong block’s upper crust is significant, while the middle and lower crust’s layered structure is weak with weak low-velocity characteristics and oceanic-island basaltic crust characteristics. The Longzhong block may have originally been formed by Mariana-type island arcs. 3) The Conrad interface and Moho lateral variation in the Ordos block’s southwestern margin are weak, showing stable craton characteristics. 4) Our results show that the Maxianshan fault cuts through the Earth’s crust and is a continuous west-dipping negative seismic phase in the Common Conversion Point section. The fault zone is the suture line between the Linxia block and Longzhong block. 5) The middle and upper crust of the Liupanshan tectonic belt is thrust upwards on the Ordos block’s southwestern margin, providing deep structural evidence of the Cenozoic uplift of the Liupanshan structural belt.

Quality classification and inversion of receiver functions using convolutional neural network

SUMMARYConvolutional neural network (CNN) is presented to implement quick quality classification and inversion for teleseismic P-wave receiver functions (RF). For the first case, a CNN is trained using field measured RFs from NE margin of the Tibetan Plateau to efficiently predict the quality of each input waveform. Signal-to-noise ratio and correlation are introduced to quantitatively determine the quality label of RF, avoiding the subjectivity of manual labelling. The trained network reduces the time needed for data processing and has higher accuracy and efficiency than conventional methods. Its good performance is confirmed by comparing it with manually selected data from NE of the Tibetan Plateau. The second case is an example of joint inverting teleseismic P-wave RF and surface wave dispersions for the estimation of earth S-wave structure and associated uncertainties. We train a UNet based on synthetic global Crust 5.1 models and standard earth models, as well as associated perturbed models to ensure enough generalization capacity. We find that the UNet inversion is robust and has a better performance to reconstruct subsurface ${V}_s$ distributions than the damping least-squares method, but at the expense of slightly higher data misfits. The pre-trained network can predict subsurface ${V}_s$ models and associated uncertainties beneath NE of the Tibetan Plateau, which is consistent with the published models.

Remnants of the amalgamation of the east and west Cathaysia blocks revealed by a short-period dense nodal array

The Cathaysia block located at the southeast South China block (SCB) is considered formed by the amalgamation of the east and west Cathaysia blocks along the Gaoyao–Huilai and Zhenghe–Dapu deep faults (here referred as GHF and ZDF, respectively). Although the extension of the ZDF to the northeast, which represents the amalgamation of the two sub-blocks has been confirmed, the development of the GHF to the southwest remains to be verified. To better constrain the detailed deep structure beneath the southwest Cathaysia, which hold great significance for revealing the evolution of the SCB, a linear seismic array with 331 nodal geophones was deployed across the Sanshui basin (SSB). Combining with the regional 10 permanent stations (PA), we obtained two profiles with teleseismic P-wave receiver function stacking. The most obvious feature in our results is the ascending Moho towards the coastal area, which is consistent with the passive margin continental and extensional tectonic setting. The stacking profile from the dense nodal array (DNA) shows that the Moho is offset beneath the transition zone of the Nanling orogeny and SSB. We deduce that this offset may be casued by the deep extension of the GHF, which represents the remnants of the amalgamation of the Cathaysia block. From the other evidences, we infer that the widespread and early erupted felsic magmas in the SSB may have resulted from lithospheric materials that were squeezed out to the surface. The relative higher Bouguer gravity and heat flow support the consolidation of magmas and the residual warm state in the shallow crustal scale beneath the SSB. The sporadic basaltic magmas in the middle SSB may have a close relation to deep extension of the GHF, which serves as a channel for upwelling hot materials.

Crustal structure beneath the northeastern margin of the Tibetan Plateau constrained by receiver functions and gravity data

The northeastern margin of the Tibetan Plateau is a key area for studies of the crustal thickening and outward expansion of the plateau and its interactions with surrounding blocks. Detailed crustal structure information can provide important constraints for understanding the tectonic evolutionary process in this region. We obtained the crustal thicknesses and average Vp/Vs ratios via joint estimation from teleseismic P-wave receiver functions and the complete Bouguer gravity anomaly. Our results show that the crustal thickness gradually decreases northward and eastward from ∼65 km beneath the Qilian Tectonic Belt to ∼40 km within the Alxa and Ordos blocks. A low Vp/Vs ratio (1.71–1.78) is found in the northeast Tibetan Plateau, which is indicative of its felsic crustal composition. A maximum 9.3 km deviation from the Airy isostatic crustal thickness is revealed beneath the Songpan–Garze Terrane and the West Qinling Orogen. The correlation between the elevations and corresponding crustal thicknesses within these two terranes is weak, suggesting that contributions of the isostatic compensation come not only from the crust but also from the lithospheric mantle. We found that the crustal thicknesses below the northeast Tibetan plateau are anti-correlated with the average crustal Vp/Vs ratios and densities, which is in favor of the previous hypothesis of upper-crust thickening or lower-crust delamination of the coherent thickening crust.

Crustal Velocity Structure of Western Iraq from Inversion of Receiver Functions at Anbar Seismic Station

The inversion of the teleseismic P-wave receiver functions was employed to determine the structure of the crust under Anbar broadband seismic station. Some of the computer programs in seismology were used to analyze the receiver functions of nine teleseismic earthquakes. Results of inversion show that the crustal structure (Moho depth) beneath the ANB1 station is 44 km. The sedimentary cover thickness (basement depth) at the ANB1 station is 12km. In general, the obtained sedimentary cover and crustal thicknesses are consistent with the results of some previous studies in the Mesopotamian plain. As well as the inversion results show that the crustal velocity model beneath the ANB1 station has four distinct discontinuities; the first at 6km with Vs of 2.83km/s, the second one at 12km with Vs of 3.63, the third one at 36km with Vs of 4.21km/s, and the fourth one at 44km with Vs of 4.37km/s. The obtained velocity model may be employed to locate and relocate the local and teleseismic earthquakes and in moment tensor solutions.

Joint inversion for 1-D crustal seismicS- andP-wave velocity structures with interfaces and its application to the Wabash Valley Seismic Zone

SUMMARYInterfaces are important part of Earth’s layering structure. Here, we developed a new model parametrization and iterative linearized inversion method that determines 1-D crustal velocity structure using surface wave dispersion, teleseismic P-wave receiver functions and Ps and PmP traveltimes. Unlike previous joint inversion methods, the new model parametrization includes interface depths and layer Vp/Vs ratios so that smoothness constraint can be conveniently applied to velocities of individual layers without affecting the velocity discontinuity across the interfaces. It also allows adding interface-related observation such as traveltimes of Ps and PmP in the joint inversion to eliminate the trade-off between interface depth and Vp/Vs ratio and therefore to reduce the uncertainties of results. Numerical tests show that the method is computationally efficient and the inversion results are robust and independent of the initial model. Application of the method to a dense linear array across the Wabash Valley Seismic Zone (WVSZ) produced a high-resolution crustal image in this seismically active region. The results show a 51–55-km-thick crust with a mid-crustal interface at 14–17 km. The crustal Vp/Vs ratio varies from 1.69 to 1.90. There are three pillow-like, ∼100 km apart high-velocity bodies sitting at the base of the crust and directly above each of them are a low-velocity anomaly in the middle crust and a high-velocity anomaly in the upper crust. They are interpreted to be produced by mantle magmatic intrusions and remelting during rifting events in the end of the Precambrian. The current diffuse seismicity in the WVSZ might be rooted in this ancient distributed rifting structure.

Structure of the crust and upper mantle beneath the Bransfield Strait (Antarctica) using P receiver functions

The Bransfield Strait is a tectonically active back-arc rift basin located between the South Shetland archipelago (SSI) and the Antarctic Peninsula (AP). We use teleseismic P-wave receiver functions from twelve seismic stations in order to improve the current understanding of the crustal and upper mantle structure of the region. We calculate the thickness and average Vp/Vs ratio of the crust, the thickness of the Mantle Transition Zone (MTZ) and their spatial variability along the study area. Our analysis reveals a homogeneous crustal thickness in the AP, averaging 33.5 ± 1.2 km. The VP/VS ratio increases from east to west, with an average of 1.78 ± 0.04 typical of continental crust. In the SSI results are highly heterogeneous. The minimum crustal thickness is observed in Deception Island (~15 km), near the rift axis, while the crust thickens towards the edges of the basin to ~30 km. High crustal VP/VS ratios (>1.95) in the archipelago correlate with a shallow Moho and may be linked to the presence of melts in the crust, in accordance with the active volcanism in the region. We detected an underplating layer and the Moho of the Phoenix slab beneath the SSI. There is evidence of a low velocity zone (LVZ) in the mantle wedge under the SSI and also under the AP, suggesting there is widespread partial melting of the upper mantle. We infer that the Phoenix slab dips steeply, as we observe a thickened MTZ under the AP.

Shallow Shear-Wave Velocity Structure in Oklahoma Based on the Joint Inversion of Ambient Noise Dispersion and TeleseismicP-Wave Receiver Functions

ABSTRACTAmbient noise cross-correlations, used to obtain fundamental-mode Rayleigh-wave group velocity estimates, and teleseismic P-wave receiver functions are jointly modeled to obtain a 3D shear-wave velocity model for the crust and upper mantle of Oklahoma. Broadband data from 82 stations of EarthScope Transportable Array, the U.S. National Seismic Network, and the Oklahoma Geological Survey are used. The period range for surface-wave ambient noise Green’s functions is from 4.5 to 30.5 s constraining shear-wave velocity to a depth of 50 km. We also compute high-frequency receiver functions at these stations from 214 teleseismic earthquakes to constrain individual 1D velocity models inferred from the surface-wave tomography. Receiver functions reveal Ps conversions from the Moho, intracrustal interfaces, and shallow sedimentary basins. Shallow low-velocity zones in the model correlate with the large sedimentary basins of Oklahoma. The velocity model significantly improves the agreement of synthetic and observed seismograms from the 6 November 2011 Mw 5.7 Prague, Oklahoma earthquake suggesting that it can be used to improve earthquake location and moment tensor inversion of local and regional earthquakes.

Significance of crustal extension and magmatism to gold deposits beneath Jiaodong Peninsula, eastern North China Craton: Seismic evidence from receiver function imaging with a dense array

The Jiaodong Peninsula, located in the eastern North China Craton (NCC), is a key region to study the genetic linkage between large-scale gold mineralization and craton destruction. We construct a fine crustal structural image across the Jiaodong Peninsula, using teleseismic P-wave receiver function data recorded by a dense linear array of 340 short-period seismic stations. The results reveal significant similarities and differences between the eastern and the western Jiaodong Peninsula. The presence of an intracrustal low-velocity zone, and the north-dipping interfaces beneath the Jiaolai Basin imply widespread crustal extension in the whole region. In the northwest Jiaodong metallogenic belt, the identified detachment faults, small vertical velocity gradients in the upper part of the lower crust and the thick crust-mantle transition zone provide evidences for intense crustal extension in this area, probably coeval with the peak of lithospheric thinning and destruction of the eastern NCC in the Late Mesozoic. In the Muru metallogenic belt, distinct differences are found on the opposite sides of the Wulian-Yantai Fault Zone (WYFZ), in the depth and structure of the Moho, intracrustal low-velocity zone, upper crustal Vp/Vs ratios, and Bouguer gravity anomalies, indicating that the WYFZ may be a steep transcrustal fault. We propose that the coupled effects of intense crustal extension, voluminous magmatism and water released from the subducting Paleo-Pacific slab may have been responsible for the concentrated gold deposits in the northwest Jiaodong during the peak of the NCC destruction, while limited gold deposits in the Muru metallogenic belt may have been attributed to relatively weak crustal extension and steep faults.

Distribution of the crustal low velocity zones beneath the central and northeastern Tibetan Plateau: Insights from joint analysis of receiver functions and surface wave dispersion observations

A crustal and uppermost mantle shear-wave velocity model beneath the central and northeastern Tibetan Plateau is obtained from the joint analysis of Rayleigh wave phase dispersions and teleseismic P-wave receiver functions at 188 broadband seismic stations deployed in this region. This newly presented model reveals wide-spread low velocity zones in the middle crust, and these zones are mainly confined to the region south of the Kunlun Fault. The velocities in the low velocity zones are extremely low (<∼3.3 km/s), suggesting the presence of actively melting or previously melted materials in the crust, and further implying the possibly large contributions of the crustal channel flow model to the deformation in the Tibetan Plateau. Additionally, a continuous low velocity anomaly in the uppermost mantle beneath the eastern Kunlun Fault, the Songpan-Ganzi terrane and part of the Qiangtang terrane is observed, which may suggest an induced local mantle upwelling in this region.

Crustal Structure of Yunnan Province of China from Teleseismic Receiver Functions: Implications for Regional Crust Evolution

Yunnan Province is located on the southeastern margin of Tibet and represents an important marker in understanding the tectonic evolution of Tibetan Plateau. In this study, we calculated teleseismic P-wave receiver functions at 49 permanent broadband seismic stations in Yunnan Province and estimated crustal thickness and the bulk crust ratios of P-wave to S-wave velocities using the H-κ method together with more detailed crustal structural profiles from the common conversion point stacking method. There is a significant transition of Moho interface and lower crustal composition along latitude 26°N in northwestern Yunnan. Decrease of crustal thickness with a concomitant increase of Poisson's ratio occurs at station CUX. An interesting phenomenon is that a step-like Moho fashion is observed at several stations, which might correspond to local thermal activities, such as partial melt/lower crust delamination. Our results show changes in crustal properties appear to be associated with varieties in upper mantle structure and compositions. We propose the controlling factor of the dynamic processes below 26°N is the result of eastern forward subduction of the Indian Plate; the northern part is controlled by the redirected material flow from the SE Tibet.

Crustal structure and deformation beneath eastern and northeastern Tibet revealed by P-wave receiver functions

The present-day Tibetan crust records the shallow response of the Cenozoic continental collision between the Indian and Eurasian plates. An analysis of the deep crustal structure beneath eastern and northeastern Tibet is of vital significance for studying the geodynamic processes of crustal thickening and expansion of the Tibetan Plateau. We herein provide detailed images of the crustal structure of eastern and northeastern Tibet and the adjacent Sichuan Basin using teleseismic P-wave receiver function (P-RF) data from a NW–SE-trending linear seismic array. Our P-RF imaging result reveals distinct structural features of the study region, including marked lateral variations in the depth to basement beneath the Songpan–Ganzi block and the Sichuan Basin, a seismically slow mid-lower crust beneath the Songpan–Ganzi block and a low-velocity anomaly just above the Moho around the easternmost Kunlun fault area, and obvious Moho offsets near the boundaries of tectonics blocks. These structural features may reflect various crustal responses within the continental interior to the India–Eurasia collision at the plate margin. The rigid crust of the Sichuan Basin might have wedged into the Tibetan crust in the Longmenshan area, which probably facilitated crustal thickening and enabled channelized mid-lower crustal flow in the Songpan–Ganzi block to the west. Being a pre-existing tectonic boundary, the Kunlun fault could have acted as a focus of heating and hot mantle upwelling associated with the deep processes of the Indian plate underthrusting and subduction, possibly resulting in localized weakening and modification of the lower crust around this fault area. The observed significant differences in the crustal structure of eastern and northeastern Tibet suggest that crustal shortening in this region may have been absorbed by not only vertical thickening in the interiors of the tectonic blocks but also complex local deformation along the boundary zones.

Constraints on the structure of the crust and lithosphere beneath the Azores Islands from teleseismic receiver functions

The Azores Archipelago is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, resting on both sides of the ridge. Various methods including seismic reflection, gravity and passive seismic imaging have previously been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 and 30 km, but until now models of the more fine-scale crustal structure have been lacking. Pending questions include the thickness of the volcanic edifice beneath the islands and whether crustal intrusions or even underplating can be observed beneath any island. In this study, we use data from nine seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. Our results indicate that the base of the volcanic edifice is located approximately 1 to 4 km depth beneath the different islands and that the crust–mantle boundary has an average depth of ∼17 km. There is strong evidence for magmatic underplating beneath the island of Sao Jorge, and indications that the underplating is also present beneath Sao Miguel and possibly Santa Maria. Additionally, the seismological lithosphere–asthenosphere boundary, defined as a seismic velocity drop in the uppermost mantle, seems to deepen with increasing distance from the MAR. It has a depth of ∼45 km beneath the islands close to the MAR, compared to depths >70 km beneath the more distal islands.

Lithospheric structure across the central Tien Shan constrained by gravity anomalies and joint inversions of receiver function and Rayleigh wave dispersion

Shear wave velocity structure across the central Tien Shan orogeny was generated by jointly inverting Rayleigh wave phase and group velocity with teleseismic P-wave receiver functions at 40 broadband seismic stations. The inferred seismic structure was validated by forward modeling of the complete Bouguer anomaly data. The joint inversion result reveals larger crust thicknesses beneath the Kokshaal (∼68–72km) and Kyrgyz ranges (∼62–64km), while other units have crustal thicknesses between 48 and 58km. A fast velocity layer (Vs=3.6–3.9km/s) in the upper crust is found in some seismic stations within the Kazakh Shield. Our models show the presence of high velocity and density layers in the lowermost crust throughout the region, consistent with the presence of mafic/ultramafic lithologies. The large crustal thickness is associated with a thickened mafic layer in the lower crust, indicating that the thickened crust may be partly caused by magmatic underplating. The low velocity and density anomaly in the middle crust, and low upper mantle velocity observed in our model beneath the middle Tien Shan reflect the presence of partial melt in the crust due to the intrusion of hot mantle material. The lack of correlation between Moho depth and topography, together with the gravity results, suggests that the topographic compensation in the central Tien Shan is not confined to the crust. This requires significant support from the mantle to account for the relative high elevation of the middle Tien Shan.

Estimation of the Crustal Bulk Properties Beneath Mainland Portugal from P-Wave Teleseismic Receiver Functions

In this work, we present results from teleseismic P-wave receiver functions (PRFs) obtained in Portugal, Western Iberia. A dense seismic station deployment conducted between 2010 and 2012, in the scope of the WILAS project and covering the entire country, allowed the most spatially extensive probing on the bulk crustal seismic properties of Portugal up to date. The application of the H–κ stacking algorithm to the PRFs enabled us to estimate the crustal thickness (H) and the average crustal ratio of the P- and S-waves velocities V p/V s (κ) for the region. Observations of Moho conversions indicate that this interface is relatively smooth with the crustal thickness ranging between 24 and 34 km, with an average of 30 km. The highest V p/V s values are found on the Mesozoic–Cenozoic crust beneath the western and southern coastal domain of Portugal, whereas the lowest values correspond to Palaeozoic crust underlying the remaining part of the subject area. An average V p/V s is found to be 1.72, ranging 1.63–1.86 across the study area, indicating a predominantly felsic composition. Overall, we systematically observe a decrease of V p/V s with increasing crustal thickness. Taken as a whole, our results indicate a clear distinction between the geological zones of the Variscan Iberian Massif in Portugal, the overall shape of the anomalies conditioned by the shape of the Ibero-Armorican Arc, and associated Late Paleozoic suture zones, and the Meso-Cenozoic basin associated with Atlantic rifting stages. Thickened crust (30–34 km) across the studied region may be inherited from continental collision during the Paleozoic Variscan orogeny. An anomalous crustal thinning to around 28 km is observed beneath the central part of the Central Iberian Zone and the eastern part of South Portuguese Zone.

Lithospheric detachment of India and Tibet inferred from thickening of the mantle transition zone

To spatially and temporally interpret eruptive volcanic activity and plateau uplift, the dynamic model of the Himalayan–Tibetan orogen requires several scenarios in which the deep part of the lithosphere is removed. The removed cold, dense material sank deeply and may rest in the mantle transition zone, which is considered as the graveyard for descending mantle lithosphere. Beneath the Himalayas and southern Tibet, stacking teleseismic P-wave receiver functions reveal thickening of the mantle transition zone (MTZ), which is caused by decreasing temperatures. We interpret the MTZ thickening beneath southern Tibet as being a result of a remnant of detached thickened Tibet mantle lithosphere, whereas the other thickening is most likely caused by a lithospheric slab that detached from the Indian plate and is sinking into the MTZ beneath the Himalayas.

Detailed Moho geometry beneath southeastern China and its implications on thinning of continental crust

We used teleseismic P-wave receiver functions and the H–κ stacking method to obtain crustal thicknesses beneath 121 permanent stations in the Middle-lower Yangtze craton and its adjacent areas, using nearly 700 teleseismic events in 2009 and 2010. We then combined them with results of previous work to map detailed Moho geometry in the region. The results show that in addition to overall thin crust of ∼30km in thickness throughout southeastern China, there are two NS-oriented narrow zones of extensive crustal thinning in the region. The western zone passes through the Xiangzhong Basin, the Jianghan Basin, and the Nanxiang Basin, while the eastern zone follows the Tanlu fault. The two merge in the south, forming a V-shaped thin crust area in southeastern China. We suggest different geodynamic mechanisms of crustal thinning north and south of the 29°N Parallel. Crustal thinning in the northern part was caused by the westward subduction of the Pacific plate and its eastward migration of the trench. Crustal thinning in the southern part is mainly controlled by the interaction between the Philippine and the Eurasia plates. The change of subduction polarity from the northward subduction of the Philippine plate under Eurasia to the eastward high-angle subduction of the Eurasian plate under the Philippine plate in Taiwan is responsible for diverse extensional stress regime in South China. The V-shape crustal thinning zone was formed due to a mantle corner flow in the rear of subducting Eurasia plate, in a form of “hot fingers” growing from the mantle corner flow to cause the Moho uplift in the southern part of the region.

Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong–Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show ∼34–38km thick crust beneath the Shillong Plateau which increases to ∼37–38km beneath the Brahmaputra valley and ∼46–48km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2–8km along with a low velocity zone within ∼8–13km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range ∼10–18km and low velocity zone within ∼18–36km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 300 toward northwest.

The oceanic nature of the African slab subducted under Peloponnesus: thin-layer resolution from multiscale analysis of teleseismic P-to-S converted waves

In the Hellenic subduction zone, the lithospheric slab may comprise continental and oceanic units juxtaposed downdip and along strike. For stations along eastern Peloponnesus, teleseismic P-wave receiver-function (RF) processing in the standard frequency band produces an image of a low-velocity layer (LVL) at the top of the slab apparently twice thicker than for an oceanic crust. To assess if this could come from a lack of resolution of the standard processing, we develop a multiscale approach with the RFs based on the wavelet-response of the medium, akin to the wavelet-transform of the velocity-depth function. The synthetic response in conversion is obtained for a multiscale singularity formed by two opposite velocity-steps at the boundaries of a crust embedded in mantle material. This indicates that only wavelet periods shorter than about 0.8 s will allow to identify clearly a 7 km thin oceanic crust. Going to longer periods leads to underestimate or overestimate the time-thickness of the LVL, due to interference phenomena. The analysis of the response in conversion from full waveform synthetic seismograms in a dipping slab model validates a multiresolution approach to real observations. With earthquakes of broad-enough spectrum towards high frequencies, yielding energy to provide wavelet periods significantly shorter than 1 s, the P-to-S conversions obtained allow us to resolve for the first time a standard oceanic crust at the slab top beneath the eastern coast of Peloponnesus. This documents the subduction of a purely oceanic slab of most reduced buoyancy since 4–5 Myr under the rapidly southwestward extending upper plate continental material.