Radial Receiver Functions Research Articles

Dʺ Structures Beneath the East China Sea Resolved by P‐Wave Slowness Anomalies

AbstractThe Dʺ layer, defined as 200–400 km in the lowermost mantle, is a thermal and chemical boundary layer between the solid silicate mantle and the liquid outer core. Deciphering the detailed structures of the Dʺ region is essential for unlocking the thermal and chemical states in the deep Earth. Here, we precisely measure the slowness and back‐azimuth of the direct P‐waves by beamforming based on the F‐trace stack at the KZ Array in Kazakhstan, to investigate the detailed Dʺ structures beneath the East China Sea. The P‐wave slowness for rays turning beneath the East China Sea exhibits a significant anomaly as a function of the P‐wave turning depth. Strong correlations between slowness and back‐azimuth anomalies for rays from different directions suggest a tilted Moho, with a tilting direction of ∼103° and a dip angle of ∼15°, beneath the KZ Array, further supported by radial receiver functions. After correcting for the slowness anomalies caused by the tilted Moho and heterogeneities outside the Dʺ layer, we construct a series of Vp Dʺ models to fit the remaining slowness anomalies for rays sampling the East China Sea. We obtain the best Dʺ model with a height of 360 km, a maximum δVp of +1.4%, a Dʺ discontinuity thickness of 120 km, and an 80‐km low‐velocity layer at the base of the mantle by minimizing residuals between the predicted and observed slowness anomalies. Combining the sharpness of the Dʺ discontinuity imaged here with mineralogical analysis suggests a Fe‐enriched region in a cold subduction environment beneath the East China Sea.

Single-Station Back-Azimuth Determination with the Receiver Function Rotation Technique Validated by the Locations of Earthquakes, Impacts, and Explosions

Abstract The success of seismometer installations on the ocean floor, polar regions, remote continental areas, and even other planets’ surfaces has sparked renewed interest in determining the location via the azimuthal direction of a seismic event recorded by a single station, also known as the back azimuth (BAZ). However, classical algorithms for the BAZ estimate, like principal component and polarization analyses based on P-wave particle motions, are prone to ambiguities of 180°. Motivated by the sensor orientation correction techniques used for ocean-bottom seismometers and land stations for known event locations, we explore a receiver function rotation (RFR) method to determine the BAZ for events recorded by a single station. It is a parameter search over a range of horizontal component rotation angles from 0° to 360°. The fundamental feature of the method is that the direct P wave in the radial receiver function (RF) will have the maximum amplitude when the rotation from the ZNE system (vertical, north–south, and east–west) to ZRT (vertical, radial, and tangential) is aligned with the BAZ of the incoming P wave. Hence, the largest amplitude at zero time of the ensemble of RFs computed for different horizontal component rotations shows the optimal BAZ, which is consequently free of the 180° ambiguities. The technique’s performance is validated using the well-documented location of the 2017 Democratic People’s Republic of Korea nuclear explosion and over 1200 cataloged earthquakes on the two permanent stations in Australia. We further benchmark the RFR algorithm by the locations of two ground-truth Martian impact events documented by the orbital camera and recorded by InSight’s seismometer. Our method helps enhance the reliability of BAZ estimation as a complementary scheme to other methods. It can be used in remote areas on Earth and on the future missions to the Moon and other planets.

Constraining the Lithospheric Discontinuity Structure Beneath Hawaiʻi Using Teleseismic Receiver Functions

AbstractThe Hawaiian Islands represent the youngest portion of the long‐lived Hawaiian‐Emperor Seamount chain sourced from hotspot volcanism on the northwesterly moving Pacific plate. Despite being one of the best studied hotspots on Earth, our understanding of the lithospheric discontinuity structure of Hawaiʻi is inhibited by a lack of a long‐term distributed broadband seismic network and lithospheric‐scale imaging of other oceanic hotspots. In this study, we analyze 3,530 teleseismic waveforms from 1,130 events recorded at 47 stations to compute P‐wave radial receiver functions. We then incorporate an elevation correction to adaptive common conversion point stacking to create a 3D discontinuity volume of the lithospheric structure associated with an ocean island hotspot. We find a positive amplitude conversion consistently at ∼12 km below sea level, likely representing the crust‐mantle transition below much of the island. However, this conversion occurs at only ∼6 km below Kīlauea, where local tomography images a zone of high P‐wave velocities (>7 km/s) within the crust and deformation of olivine phenocrysts within a magmatic reservoir is thought to occur. At this location, seismicity also extends vertically to 35 km depth before merging with an interpreted sill complex at ∼40 km. We interpret this anomalously shallow high amplitude discontinuity as an olivine‐dominated magmatic reservoir with an estimated maximum 5% melt over the resolution of our data at crustal depths. This represents the shallow seismic signature of a much more extensive magma system extending to depth. These images provide a better understanding of the magmatic structure associated with oceanic hotspot volcanoes.

Crustal Structure Beneath Mexico City From Joint Inversion of Receiver Functions and Dispersion Curves

AbstractOver the years, it has been clear to seismologists that the crustal structure beneath Mexico City is highly complex. This fact may influence the seismic waves' behavior when traveling through it. Consequently, many studies have been done to understand this crustal structure. This article presents a high‐resolution S‐wave velocity model obtained from joint inversion of radial receiver functions and group‐velocity dispersion curves beneath Mexico City (VMRFDC). This model includes azimuthal variations observed with receiver functions, which may be interpreted as crustal discontinuities. Receiver functions were retrieved from velocity and acceleration records, and dispersion curves were collected from a previous tomographic study. We performed a geostatistical analysis that revealed a horizontal spatial correlation of the crustal structure for up to 7.5 km. This distance was obtained from the S‐wave velocity omnidirectional semivariogram range per depth. Using ordinary kriging, we utilized this spatial correlation to estimate S‐wave velocities per depth and introduced two novel techniques: a resolution test for kriging and a geostatistical‐dependent smoothing technique. Finally, we compare our results with previous works, making our model consistent with geological and geophysical features reported in the literature.

Crustal Anisotropy from the Birefringence of P-to-S Converted Waves: Bias Associated with P-Wave Anisotropy

Many researchers have used the birefringence of P‑to‑S converted waves from the Moho discontinuity to constrain the anisotropy of Earth’s crust. However, this practice ignores the substantial influence that anisotropy has on the initial amplitude of the converted wave, which adds to the splitting acquired during its propagation from Moho to the seismometer. We find that large variations in Ps birefringence estimates with back-azimuth occur theoretically in the presence of P‑wave anisotropy, which normally accompanies S‑wave anisotropy. The variations are largest for crustal anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations, but is commonly found in Earth’s crust. We simulated globally-distributed P‑coda datasets for 36 distinct 4‑layer crustal models with combinations of elliptical shear anisotropy or compressional anisotropy, and also incorporated the higher-order anisotropic Backus parameter C. We tested both horizontal and tilted symmetry-axis geometries and tested the birefringence tradeoff associated with Ps converted phases at the top and bottom of a thin high‑ or low‑velocity basal layer. We computed composite receiver functions (RFs) with harmonic regression over back azimuth, using multipletaper correlation with moveout corrections for the epicentral distances of 471 events, to simulate a realistic data set. We estimate Ps birefringence from the radial and transverse RFs, a strategy that is similar to previous studies. We find that Ps splitting can be a useful indicator of bulk crustal anisotropy only under restricted circumstance, either in media with no compressional anisotropy, or if the symmetry axis is horizontal throughout. In other, more-realistic cases, the inferred fast polarization of Ps birefringence estimated from synthetic RFs tends either to drift with back-azimuth, form weak penalty-function minima, or return splitting times that depend on the thickness of an anisotropic layer, rather than the birefringence accumulated within it.

Crustal anisotropy of the South China block and tectonic implications

Exploiting the teleseismic waveform data from permanent seismic stations distributed in the South China block, we obtained the crustal anisotropy parameters at 51 stations by fitting the azimuthal variations of arrival times of the PMs converted wave of radial receiver functions. The results show the dominant direction of fast wave polarity in the South China block is NW–SE, which is basically consistent with the absolute plate motion direction. Combining the shear wave splitting and GPS results, we speculated that the tectonic extension of the Paleo-Pacific plate subduction dominates the crust–mantle anisotropy, which reflects the large-scale tectonic stress setting of the South China block. The fast wave directions of NE–SW and E–W calculated by a few stations might represent a local complex tectonic environment, and the crustal anisotropy of NE–SW is well related to the tectonic deformation in Paleozoic. The delay time of slow waves caused by crustal anisotropy ranges from 0.14 s to 0.71 s, with an average of 0.33 s. The delay time of Jiangnan orogenic belt, which located in the southeast of Yangtze block, is particularly large, which reflects the complex and intense intra-continental tectonic movements since Neoproterozoic. The crustal anisotropy along the southeast coast can be contributed to the local lithosphere underplating caused by the upwelling of asthenosphere in Cretaceous.

Evidence of structural segmentation of the Uttarakhand Himalaya and its implications for earthquake hazard

The earthquake hazard associated with the Main Himalayan Thrust (MHT) is a critical issue for India and its neighbouring countries in the north. We used data from a dense seismic network in Uttarakhand, India, to model the lateral variations in the depths of MHT (2–6% drop in Vs at 12–21 km depths), Moho (a sharp increase in Vs (by ~ 0.5–0.7 km/s) at 39–50 km depths) and lithosphere (a marked decrease in Vs(~ 1–3%) at 136–178 km depths), across the Himalayan collisional front. Our joint inversion of radial PRFs and group velocity dispersion data of Rayleigh waves detects three NNE trending transverse lithospheric blocks segmenting the lithosphere in Uttarakhand Himalaya, which spatially correlate well with the northward extension of the Delhi -Haridwar Indian basement ridge, an inferred tectonic boundary and great boundary fault, respectively. Our radial receiver function imaging detects highly deformed and segmented crustal and lithospheric structures associated with three mapped transverse lithospheric blocks, suggesting a reduction in rupture lengths of future earthquakes, thereby, reducing earthquake hazards in Uttarakhand.

Crustal structure in and around the East Anatolian volcanic belt by using receiver functions stacking

This study aims to explore the crustal seismic properties across the East Anatolian volcanic belt. Teleseismic receiver functions obtained through deconvolution in the time domain are used to estimate crustal discontinuities and Moho interface. Further, slant stacked method, employed for the first time in the study region, is applied to identify the arrival times of P to S conversion and reverberations (Ps and PpPms) that give Vp/Vs ratio beneath each station. For slant stacking method, radial receiver functions are calculated using deconvolution based on frequency domain. Inversion results indicate that Moho depth varies from at most ∼40 km near the Bitlis-Zagros thrust zone to 45 km in the northeastward of Quaternary volcanic centers. The southwest of study region is characterized overall by a low average crustal Vp/Vs ratio (less than 1.71), implying a felsic crustal composition. A high average crustal Vp/Vs ratio of 1.82–1.87 is found near volcanic centers, characterized by predominantly a mafic crustal composition. The prominent crustal low-velocity zones especially around the Nemrut and Ararat volcanoes are identified below ∼20 km depth and likely imply the presence of partial melt zones or active magma chambers in the region. As a result, the findings of this study provide useful new constraints to the crustal thickness and composition in the study region.

Systematic Mapping of Upper Mantle Seismic Discontinuities Beneath Northeastern North America

AbstractAbrupt velocity gradients in the upper mantle, detectable by receiver functions (RF) techniques, have been known to exist down to the depths of ∼110 km beneath northeastern North America. Comparisons with the surface wave velocity models have designated some negative velocity gradients (NVGs) as the lithosphere‐asthenosphere boundary (LAB), delineating a relatively thin lithosphere beneath this region. This work presents a systematic survey of upper mantle layering in seismic properties using P‐S RF analysis at 62 long‐running sites with dense lateral sampling. We examine both radial and transverse component RF for indicators of seismic anisotropy and adopt the notion of seismic attributes, utilized in active‐source seismology, to characterize the spatial distribution of directionally variant and invariant signal components. We confirm a widespread presence of NVGs at depths 60–100 km throughout the region, consistent with previous studies using mode‐converted body waves. We also find numerous converting boundaries that reflect changes in directional variation (anisotropy) of seismic velocity, indicating complexity of rock texture in the upper mantle. Some of these boundaries appear as deep as 185 km, implying that the lithosphere extends much deeper than the widespread NVGs would suggest. In this, our results agree with recent estimates of the lithospheric thickness in thermodynamically consistent models combining seismic, gravity, and heat flow constraints.

Crustal Thinning From Orogen to Back‐Arc Basin: The Structure of the Pannonian Basin Region Revealed by P‐to‐S Converted Seismic Waves

AbstractWe present the results of P‐to‐S receiver function analysis to improve the 3D image of the sedimentary layer, the upper crust, and lower crust in the Pannonian Basin area. The Pannonian Basin hosts deep sedimentary depocentres superimposed on a complex basement structure and it is surrounded by mountain belts. We processed waveforms from 221 three‐component broadband seismological stations. As a result of the dense station coverage, we were able to achieve so far unprecedented spatial resolution in determining the velocity structure of the crust. We applied a three‐fold quality control process; the first two being applied to the observed waveforms and the third to the calculated radial receiver functions. This work is the first comprehensive receiver function study of the entire region. To prepare the inversions, we performed station‐wise H‐Vp/Vs grid search, as well as Common Conversion Point migration. Our main focus was then the S‐wave velocity structure of the area, which we determined by the Neighborhood Algorithm inversion method at each station, where data were sub‐divided into back‐azimuthal bundles based on similar Ps delay times. The 1D, nonlinear inversions provided the depth of the discontinuities, shear‐wave velocities and Vp/Vs ratios of each layer per bundle, and we calculated uncertainty values for each of these parameters. We then developed a 3D interpolation method based on natural neighbor interpolation to obtain the 3D crustal structure from the local inversion results. We present the sedimentary thickness map, the first Conrad depth map and an improved, detailed Moho map, as well as the first upper and lower crustal thickness maps obtained from receiver function analysis. The velocity jump across the Conrad discontinuity is estimated at less than 0.2 km/s over most of the investigated area. We also compare the new Moho map from our approach to simple grid search results and prior knowledge from other techniques. Our Moho depth map presents local variations in the investigated area: the crust‐mantle boundary is at 20–26 km beneath the sedimentary basins, while it is situated deeper below the Apuseni Mountains, Transdanubian and North Hungarian Ranges (28–33 km), and it is the deepest beneath the Eastern Alps and the Southern Carpathians (40–45 km). These values reflect well the Neogene evolution of the region, such as crustal thinning of the Pannonian Basin and orogenic thickening in the neighboring mountain belts.

Crustal azimuthal anisotropy in the Jiaodong Peninsula: Evidence for the suture between the North China Craton and South China Block

The Jiaodong Peninsula is an ideal region for studying the geodynamics of collision between the North China Craton (NCC) and South China Block (SCB). It is comprised of the Jiaobei massif and the Northern Sulu UHP massif which are separated by the Wulian suture zone (WSZ). Continent-continent collision is necessarily accompanied with crustal deformation. Consequently a detailed crustal deformation pattern would offer observational constraints for understanding the process of the collision. In this study, we measured the crustal azimuthal anisotropy beneath a broadband seismic profile of 20 stations together with 9 permanent stations in the Jiaodong Peninsula. Sinusoidal moveout of Pms phases from radial receiver functions and azimuth-weighted stacking of transverse receiver functions were jointly applied to ensure the reliability of results. In the WSZ, the fast directions are roughly parallel with the NE oriented fault strike, suggesting the Jiaodong Peninsula may preserve fossilized anisotropy in the crust induced by the Triassic collision of the NCC and SCB although have undergone subsequent lithosphere destruction and thinning since the late Mesozoic. We therefore support that the WSZ represents the NCC-SCB suture east of Tanlu fault. In the Jiaobei massif, the crustal azimuthal anisotropy is controlled by both the nearly E-W local stress and NNE-strike detachment faults. The anisotropy beneath the Northern Sulu massif is quite weak with delay times all less than 0.20 s, probably related to the high-density UHP metamorphic rocks which are difficult to generate strong anisotropy inside.

Mantle Transition Zone Receiver Functions for Bermuda: Automation, Quality Control, and Interpretation

AbstractThe origin of the Bermuda rise remains ambiguous, despite, or perhaps because of, the existence of sometimes incongruous seismic wave‐speed and discontinuity models in the sub‐Bermudian mantle. Hence, whether Bermuda is the surface manifestation of a mantle plume remains in question. Using the largest data set of seismic records from Bermuda to date, we estimate radial receiver functions at the Global Seismographic Network station BBSR in multiple frequency bands, using iterative time‐domain deconvolution. Motivated by synthetic experiments using axisymmetric spectral‐element forward waveform modeling, we devise a quality metric for our receiver functions to aid in the automation and reproduction of mantle transition zone discontinuity studies. We interpret the complex signals we observe by considering the mineralogical controls on mantle transition zone discontinuity structure, and conclude that our results are likely to be indicative of a thicker than average mantle transition zone. Our result is incompatible with the canonical model of a whole mantle plume in an olivine dominated mantle; however, considerations of phase transitions in the garnet system would allow us to reconcile our observations with the possible presence of a through‐going hot thermal anomaly beneath Bermuda.

The crustal thickness and composition in the eastern South China Block constrained by receiver functions: Implications for the geological setting and metallogenesis

The eastern South China Block is characterized by multi-stage intra-continental collisions and orogeny from the Paleozoic to early Mesozoic, and strong magmatism and metallogenesis since the late Mesozoic. In order to further study the tectonics, magmatism and metallogenesis, we constructed a new model of crustal thickness and Vp/Vs ratio for the crust beneath the eastern South China Block using H-κ stacking of teleseismic P-wave radial receiver functions from 845 broadband seismic stations. New knowledge of the crustal structure and properties pertinent to the tectonics and metallogenesis is as follows: (1) the average crustal thickness beneath the eastern South China Block is 33 km. It is thinner and varies smoothly in the Cathaysia Block and Jiangnan Orogen, and gradually thickens to the northwest and reaches ~40 km in the Yangtze Block; (2) the average crustal Vp/Vs ratio of the eastern South China Block is ~1.72, lower than the global average value ~1.78. This implies an absence of a mafic lowermost crustal layer; (3) the Jiangnan Orogen is characterized by a conspicuous belt of lower Vp/Vs ratios, different from the Yangtze and Cathaysia Blocks on either side. This is interpreted as caused by synorogenic crustal thickening and the subsequent lower crustal delamination; (4) the crustal thickness and the Vp/Vs ratios in the eastern South China Block are negatively correlated, also indicating tectonic thickening of a felsic upper and middle crust. This is consistent with folding and thrusting during orogenesis and the later delamination of the mafic lower crust; (5) the Anhua-Luocheng fault is considered to be the southwestern segment of the boundary between the Yangtze Block and Jiangnan Orogen; (6) lithospheric extension caused by subduction of the Pacific Plate and the corresponding slab rollback during the late Mesozoic can explain the crustal thickness, the Vp/Vs ratio distribution patterns and the metallogenesis in the eastern South China Block.

On The Efficiency of P‐Wave Coda Autocorrelation in Recovering Crustal Structure: Examples From Dense Arrays in the Eastern United States

AbstractDue to a sharp contrast in elastic properties across the basement rocks of sedimentary basins (SBs), strong reverberations are generated during the passage of seismic waves. Traditional receiver function methods become inadequate for imaging crustal structure due to the existence of these strong reverberations. We investigate the feasibility of an autocorrelation technique to extract vertical component receiver functions from teleseismic earthquake data and the efficiency of the method to image the crustal architecture in presence of a SB. The method involves spectral whitening followed by autocorrelation and stacking in the depth domain. We show promising results when using temporary seismic networks in the eastern United States. Using synthetic and field‐data examples, we demonstrate that vertical autocorrelations are more efficient than classical radial receiver functions for interpretation purposes in an SB context. We also perform a joint analysis of the amplitudes on radial and vertical receiver functions for characterizing the thickness of the Mohorovičić discontinuity (Moho). We find that the Moho in the eastern United States is a transitional layer (up to 5‐km thick) instead of a sharp boundary. Further, we point out that it is challenging to unambiguously pick and interpret reflected phases on autocorrelations because of the effects of reverberations, cross‐mode contaminations, and a narrow frequency band limiting the resolution of velocity gradients. We therefore send a message of caution for future interpretations based on this technique.

Seismological evidence of a dehydration reaction in the subducting oceanic crust beneath western Shikoku in southwest Japan

SUMMARYThe Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the downdip of the source region of M8-class earthquakes that have occurred every 100–150 yr. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the backazimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the updip directions of the dipping slab. In this profile, the parallel negative and positive P-to-S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab.

Crustal shear-wave velocity structure in Western Java, Indonesia from analysis of teleseismic receiver functions

We analysed receiver functions from teleseismic events recorded at 11 broadband seismometers in the western part of Java Island, Indonesia. The stations are mostly located at three main geological environment including Northwest Java Basin, Bogor Zone, and Southern Mountains Arc. A total of about 341 receiver functions were computed using iterative time domain deconvolution. We derived shear-wave velocity structure and crustal Vp/Vs ratio by inverting stacked radial receiver functions using non-linear neighbourhood algorithm. Inversion results show sediment thickness varies between 1 and 2 km thick in Western Java. Our inversion shows that crustal thickness in this region varies between 25 and 32 km. Average crustal Vp/Vs ratio is estimated to be about 1.69–1.78. We hope the study may provide useful information for velocity model and crustal thickness for Indonesia region.

Crustal structure study based on principal component analysis of receiver functions

The receiver function (RF) technique is an effective method for studying crustal structure. For a single station, the average 1-D crustal structure is usually derived by stacking the radial RFs from all back-azimuths, whereas structural variations (such as dipping discontinuities or anisotropy) can be constrained through analysis of waveform dependence on the back-azimuth of both the radial and tangential RFs. However, it is often difficult to directly extract information about structural variations from the waveform of RF, due to the common presence of noise in real data. In this study, we proposed a new method to derive structural variation information for individual stations by applying principal component analysis (PCA) to RFs sorted by back-azimuth. In this method (termed as RF-PCA), a set of principal components (PCs), which are uncorrelated with each other and reflect different characteristics of the RF data, were extracted and utilized separately to reconstruct new RFs. Synthetic tests show that the first PC of the radial RFs contains the average structural information of the crust beneath the corresponding station, and the second PC of the radial RFs and the first PC of the tangential RFs both reflect the variations of the crustal structure. Our synthetic modeling results indicate that the new RF-PCA method is valid for a variety of synthetic models with intra-crustal dipping discontinuities and/or anisotropy. We applied this method to the real data from a broadband temporary seismic station (s233) in the central part of the Sichuan Basin. The results suggest that the RF data can be best explained by the presence of two nearly parallel dipping discontinuities within the crust. Combining with previous logging data, seismic exploration and deep sounding observations, we interpret the shallow dipping discontinuity as the top boundary of the Precambrian crystalline basement of the Sichuan Basin and the deep one corresponding to the Conrad interface between the upper and lower crust, consistent with the geological feature of the study area. In this work, both synthetic tests and real data application results demonstrate the effectiveness of the RF-PCA method for studying crustal structures.

Calculation of Station-Representative Isotropic Receiver Functions

The estimation of one-dimensional (1-D) isotropic structures is routine work in most receiver function (RF) analyses that generally use a reference radial RF (RRF) for each station. However, the assumption of negligible back-azimuthal dependencies in a set of RRFs for a station may not be valid because of anisotropic layering, dipping structures, or incorrect sensor orientations. This work presents a comprehensive procedure to obtain a station-representative isotropic RRF, which can be applied automatically to prepare RRF data. The method incorporates a harmonic stripping method with a grid-search for sensor orientations. An optimum angle for the sensor orientation is determined by searching for the angle that minimizes the energy in the tangential component RF (TRF). For each searched angle, possible effects by anisotropy and dipping structures are iteratively suppressed by an inversion process to exclude two- and four-robe back-azimuthal patterns. The performance of the method was first confirmed with a test using a set of highly noisy composite RRFs and TRFs. The method was then applied to RF data from the southern Korean Peninsula and southwestern Japan. Obtained isotropic RRFs and measured station orientations were found to be reliable in comparisons with results from neighboring stations and previous studies. As an automatized routine pre-process, the obtained isotropic RRF data are particularly useful for estimating 1-D isotropic structures in migration or inversion studies, which are potentially affected by back-azimuthal dependencies in RF data calculated through conventional averaging methods.

Preliminary Moho depth determination from receiver function analysis using AlpArray stations in Hungary

Receiver function analysis is applied to the western part of the Pannonian Basin, a rather complex region both geologically and geodynamically. Previous receiver function analyses in this region had to deal with much smaller station density and time span than those available to us. In the analysis we used the data of some 48 seismological stations. These include not only the permanent stations from Hungary and permanent stations from neighbouring countries (Slovakia and Slovenia), but also the temporary broadband stations that were installed within the framework of the AlpArray project. Having applied rather strict manual quality control on the calculated radial receiver functions we stacked the receiver functions. Using the H–K grid search method we determined the Moho depth and the Vp/Vs ratio beneath the seismological stations in the western part of the Pannonian Basin. The unprecedented density of the AlpArray network, combined with the permanent stations, allowed us to derive high resolution Moho and Vp/Vs maps for the West Pannonian Basin, together with uncertainty estimates. Our preliminary results agree well with previous studies and complement them with finer details on the Moho topography and crustal thickness.

GLImER: A New Global Database of Teleseismic Receiver Functions for Imaging Earth Structure

Teleseismic receiver functions (RFs) provide a simple means to image structure in the Earth’s subsurface. Despite its ease of use, the method has traditionally required considerable user input to ensure that key processing operations run in a stable manner and yield robust results. As a consequence, large‐scale applications to global datasets have been limited. Here, we present preliminary results from a global database of RF data products dedicated to imaging structure in the crust and upper mantle. Earthquake signals recorded at seismic stations worldwide are downloaded, quality controlled, and processed using a range of deconvolution approaches to generate robust RFs. We describe our automated workflow and apply it to the entire data holding of the Incorporated Research Institutions for Seismology, producing 1,300,424 individual radial and transverse Ps RFs computed at 8258 stations. A series of tools designed to visualize the results will be made publicly available on a website. These include a map‐based interface that allows users to inspect RFs at individual stations, and interactive programs that can generate cross sections through 3D volumes of RFs.