Moho Discontinuity Research Articles

Seismic mapping of the central and southern segments of the Tanlu fault zone using P-wave receiver functions

The central and southern segments of the Tanlu fault zone, located in the collision boundary between the Yangtze Plate and the North China Craton, underwent complex tectonic deformation associated with the Pacific Plate subduction. The crustal thickness and Vp/Vs ratio are important parameters for comprehending tectonic evolution and geodynamic processes. By integrating a newly dense seismic array, our results based on P-wave receiver function analyses reveal the crust thickness varies significantly across the Tanlu fault zone. The Yangtze Plate is characterized by a thinner crust, contrasting a thicker crust imaged in the North China Craton, indicating the Tanlu fault zone is a prominent structural block boundary. The Vp/Vs ratio within the Tanlu fault zone is visibly higher than its surroundings, probably correlating with abundant fluids activity along the fault zone, as a result of the subduction of the Pacific Plate. Additionally, we observe a gradually deepening Moho discontinuity beneath the Sulu orogenic belt with a low-angle dip, presenting direct seismic evidence for supporting the ancient Yangtze Plate underthrusting beneath the Sulu orogenic belt.

Insights into the crustal and the magmatic feeding structure at the Payunia Volcanic Province highlighted by geophysical methods, in the retroarc of the Southern Central Andes

The Payunia Volcanic Province is a Quaternary volcanic plateau emplaced in the retroarc area in the northern Neuquén Mesozoic Basin, associated with a hydrocarbon system. At deeper levels, this basin is linked to different intrusive systems that developed in the retroarc region at different times, during the Jurassic, Cretaceous, Eocene, Miocene, and even the Quaternary which influenced the hydrocarbon system maturity. We analyzed the Moho structure through this retroarc region, as well as the crustal structure affected by different stages of regional extension. From continuous seismic noise data, we calculated the autocorrelograms to obtain the reflection response below each seismological station. This allowed imaging the surface of primary crustal reflectors and in a few stations the top of an asthenospheric anomaly (SWAP) found by magnetotelluric survey and in concordance with satellite magnetic data. The crustal reflectors were identified in all stations at a mean two-way travel time of about ∼8.5 s and ∼12.5 s using frequency bands of about 1.0–2.4 Hz. Therefore, this is the first geophysical research that estimates the depth of the magmatic system, hosted at the top of the lower crust and the Moho discontinuity. The deepest reflector, only recognized in 4 stations, was observed with a two-way travel time of 17.2 s to 19.6 s. We used a mean one-dimensional Vp model to obtain the corresponding reflector depths which constrain the two-dimensional forward gravity model that fits with the observed regional anomaly for the region. We finally established a relationship between the shallowest sublithospheric electrical conductivity anomalies determined in previous researches and the strong deep reflections observed in some of the seismological stations. This information may help to constrain geochemical and petrological models and re-evaluate the hydrocarbon system maturity of the northern Neuquén basin.

Crustal thickness variations beneath Egypt through gravity inversion and forward modeling: linking surface thermal anomalies and Moho topography

This study aims to quantify the topography of the Moho boundary, the lower crust and uppermost mantle contact of Egypt, in order to estimate the crustal thickness variation and its link to the distribution of thermal anomalies under Egypt. This is accomplished by modeling satellite gravity, supported by the passive seismic constraints throughout Egypt. However, when estimating the thickness of the crust in Egypt using just seismic data, substantial uncertainty and deviation are produced due to the sparsely dispersed stations. Integrating seismic and gravity data minimizes uncertainty and improves estimate accuracy. The investigation is broken down into four stages, the first involving utilizing the Sentinel-3B satellite to create land surface temperature maps. The subsequent steps consist of gravity and seismic data adjustments, inversion and forward modeling. We used seismically restricted nonlinear inversion to look at Goco06s satellite gravity data to model the Moho’s topographic surface. The data gathered from deep seismic refraction and receiver functions adjusted the analyzed data. The inversion process relies on the adapted Bott's approach and Tikhonov regularization, using the assumption of the sphericity of the Earth planet. Reference values for depth of Moho and density contrast were set at 35 km and 500 kg/m3, respectively. The average statistical difference for Moho depth between gravity-based model and seismic data is − 0.10 km. Through forward gravity modeling, five gravity profiles were chosen and interpreted in 2.5D models. The results indicated that the Moho depth in the south varies from 35 to 39 km and decreases in the north and the Mediterranean. In upper Egypt, the highest Moho depth is 39 km. The depth varies beneath the Sinai Peninsula as it is about 35 km in its south, reaches 30 km in the northern portion, and ranges along the Red Sea’s Rift Margin from 29 to 32 km. Moreover, the final model shows the relation between Moho coincides with the surface temperature anomalies approved by satellite images and hot springs. The model reveals a correlation between Moho discontinuity and surface temperature anomalies, revealing the highest geothermal potential in a rectangular area in central Egypt, between latitudes 25°N and 30°N, based on satellite imagery and hot springs distribution.

The 2021 La Palma (Canary Islands) Eruption Ending Forecast Through Magma Pressure Drop

AbstractForecast of eruptive activity is a core challenge in volcanology. Here, we present an actual forecast for the end of the 2021 La Palma eruption. Using continuous GNSS data, we identified a co‐eruptive quasi‐exponential deflation trend. Assuming mass conservation, magma upflow from an overpressurized reservoir drives the eruptive process. The forecast was carried out during the eruption, however there was uncertainty in the key percentage of drop in driving pressure necessary to stop this eruption. In hindcast, we explore how forecast uncertainty reduces with increase in ingested near‐real time data. We conclude that precise forecasts could have been possible, but only after twice a characteristic exponential decay time‐scale, providing error estimates of 45% of the actual duration. We verify the mass conservation assumption using eruptive material volumes and propose that the eruption dynamics was controlled by a main reservoir at a depth close to Moho discontinuity beneath Cumbre Vieja volcano.

Waveform Fitting of Receiver Functions for Enhanced Retrieval of Crustal Structure in the Presence of Sediments

AbstractThe receiver function technique is widely used to image crustal structure using P‐to‐S converted phases at the Moho discontinuity. However, the presence of sedimentary layer generates additional P‐to‐S conversions and reverberations, which can overprint the Moho phases and pose problems in imaging crustal structure with standard receiver function techniques. We introduce a robust two‐step method that uses H‐κ stacking to determine average thickness and Vp/Vs of the sedimentary layer, followed by waveform‐fitting of the observed receiver function to constrain the average crustal thickness and sub‐sediment Vp/Vs. We tested the method using both synthetic data and real‐data from stations located on sedimentary layers in the Netherlands and USA. We show that the new method outperforms other common approaches in retrieving accurate Moho depth and sub‐sediment Vp/Vs estimates, even in cases where the Moho phases are completely overprinted by large‐amplitude phases related to sedimentary layers.

The crust-mantle velocity structure beneath the Wudalianchi-Erkeshan-Keluo volcanic belt by joint inversion of receiver functions and ambient noise

The Wudalianchi-Erkeshan-Keluo (WEK) volcanic belt is a significant component of intraplate volcanism in Northeast China and is composed of the Wudalianchi, Erkeshan, and Keluo volcanic clusters. Using joint inversion of receiver functions and ambient noise, we construct a high-resolution 3-D S-wave velocity model of the WEK volcanic belt and its adjacent region, taking advantage of a deployed dense seismic array around this volcanic belt. There is a prominent low-velocity anomaly at 8–15 km depth beneath the Wudalianchi volcanic cluster, suggesting the presence of a crustal magma chamber. Low-velocity anomalies are also observed at 30–35 km depth beneath the Erkeshan volcanic cluster and 30–40 km depth beneath the Keluo volcanic cluster, resulting in discontinuous velocity structures at the Moho discontinuity. We further find a distinct low-velocity anomaly in the uppermost mantle beneath the WEK volcanic belt. Combined with previous geophysical and geochemistry studies, we propose a magma system scenario for the WEK volcanic belt. The upwelling molten material from the asthenosphere accumulated in the uppermost mantle and the magma chamber was formed, which provided the same uppermost mantle magma sources for the WEK volcanic belt.

Rayleigh wave attenuation tomography based on ambient noise interferometry: methods and an application to Northeast China

SUMMARY Although ambient noise interferometry has been extensively utilized for seismic velocity tomography, its application in retrieving attenuation remains limited. This study presents a comprehensive workflow for extracting Rayleigh wave amplitude and attenuation from ambient noise, which consists of three phases: (1) retrieval of empirical Green's functions (EGFs), (2) selection and correction of amplitude measurements and (3) inversion of attenuation, site amplification and noise intensity terms. Throughout these processes, an ‘asynchronous’ temporal flattening method is used to generate high-quality EGFs while preserving relative amplitudes between stations. Additionally, a novel ‘t-symmetry’ criterion is proposed for data selection along with the signal-to-noise ratio. Furthermore, 2-D sensitivity kernels are utilized to estimate the focusing/defocusing effect, which is then corrected in amplitude measurements. These procedures are designed to deliver reliable attenuation measurements while maintaining flexibility and automation. To validate the effectiveness of the proposed noise-based attenuation tomography approach, we apply it to a linear array, NCISP-6, located in NE China. The obtained results correlate reasonably well with known geological structures. Specifically, at short periods, high attenuation anomalies delineate the location of major sedimentary basins and faults; while at longer periods, a notable rapid increase of attenuation is observed beneath the Moho discontinuity. Given that attenuation measurements are more sensitive to porosity, defect concentration, temperature, melt and volatile ratio than seismic velocities, noise-based attenuation tomography provides important additional constraints for exploring the crustal and upper mantle structures.

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.

Geophysical Evidence of the Collisional Suture Zone in the Prydz Bay, East Antarctica

AbstractThe location and origin of Neoproterozoic‐Cambrian sutures provide keys to understand the formation and evolution of the supercontinent Gondwana. The Larsemann Hills is located near a major Neoproterozoic‐Cambrian suture zone in the Prydz Belt, but has not been examined locally by comprehensive geophysical studies. In this study, we analyzed data collected from a one‐dimensional (1D) joint seismic‐MT array deployed during the 36th Chinese National Antarctic Research Expedition. We found that a sharp Moho discontinuity offset of 6–8 km shows up in the stacked image of teleseismic P‐wave receiver function analysis; coinciding with the abrupt Moho offset, a near‐vertical channel with (a) low resistivity extending to the uppermost mantle depths, and (b) high crustal Poisson's ratio in the crust is identified. These findings provide evidence for the determination of the location and collisional nature of the Prydz belt or a portion of it.

Structural controls of the migration of mantle-derived CO2 offshore in the Santos Basin (Southeastern Brazil)

We present a multi-scale conceptual model based on structural controls of the migration of mantle-derived CO2 offshore in the Santos Basin (Southeastern Brazil). We assembled the model from a regional 2D seismic reflection line integrated with potential gravimetric field data and a local 3D seismic reflection volume integrated with well data (lithologies and in situ stress). (i) The geochemical isotope range of δ13CCO2 falls mostly within −7‰ and −5‰ and shows relatively high values for 3He/4He represented by an R/Ra rate of up to 5.60, indicating CO2 mantle generation and degassing. (ii) Seismic interpretation feasibly validated by potential gravimetric responses of the crustal structure (Moho discontinuity) show CO2 migration through deep-seated faults in a region of highly stretched continental crust with oceanward mantle uprising. (iii) Early Cretaceous basement highs generated in an obliquely syn-rift faulting system control CO2 accumulation in thermogenic travertines (hydrothermal carbonate reservoirs of continental lakes), and Aptian evaporites subsequently trap it.

A three-dimensional Moho depth model beneath the Yemeni highlands and rifted volcanic margins of the Red Sea and Gulf of Aden, Southwest Arabia

Knowing Moho discontinuity undulation is fundamental to understanding mechanisms of lithosphere-asthenosphere interaction, extensional tectonism and crustal deformation in volcanic passive margins such as the study area, which is located in the southwestern corner of the Arabian Peninsula bounded by the Red Sea and the Gulf of Aden. In this work, a 3D Moho depth model of the study area is constructed for the first time by inverting gravity data from the Earth Gravitational Model (EGM2008) using the Parker-Oldenburg algorithm. This model indicates the shallow zone is situated at depths of 20 km to 24 km beneath coastal plains, whereas the deep zone is located below the plateau at depths of 30 km to 35 km and its deepest part coincides mainly with the Dhamar-Rada’a Quaternary volcanic field. The results also indicate two channels of hot magmatic materials joining both the Sana’a-Amran Quaternary volcanic field and the Late Miocene Jabal An Nar volcanic area with the Dhamar-Rada’a volcanic field. This conclusion is supported by the widespread geothermal activity (of mantle origin) distributed along these channels, isotopic data, and the upper mantle low velocity zones indicated by earlier studies.©2023 China Geology Editorial Office.

Segmented Up‐Bending of the Arabian Continental Plate Revealed by Pn Attenuation Tomography

AbstractThe Zagros orogen in the Iranian Plateau serves as a natural laboratory for studying the tectonic evolution of the transition from oceanic subduction to continental collision. Various observations from both geology and geophysics show that deep oceanic slabs may have detached one after another from the Arabian continental margin, forming a complex plate front beneath the Zagros orogen. However, where slab detachment occurred and how slab detachment affects shallower continental underthrusting remain poorly understood. The mechanism behind the complex post‐collision magmatism remains elusive. While the uppermost mantle is often considered the heart of the mantle lid, its rheological variations can provide insights into the underlying thermal structure. Here, we construct a high‐resolution Pn‐wave attenuation model for the uppermost mantle beneath the Iranian Plateau and surrounding areas using a newly compiled data set and constrain the lithospheric structure by combining the Lg‐wave attenuation within the crust. Weak Pn‐wave attenuation outlines the boundary of the Arabian Plate near the Moho discontinuity, extending further in the direction of underthrusting in the northwestern and southeastern regions. This is likely due to the up‐bending and underplating of the Arabian lithosphere following the cessation of slab pull. The overlying crusts in northwestern and southeastern Zagros thickened under the compressive force from plate underplating. The correlations among the surface Miocene‐Quaternary volcanism, strong Lg attenuation in the crust and strong Pn attenuation in the uppermost mantle suggest that asthenospheric materials escaped from slab windows and further intruded into the upper mantle and crust to feed the post‐collision volcanoes.

Moho Depth Estimation and The Presence of Subducting Slab in West Sumatra, Jambi, and Riau Islands Regions Using Teleseismic Receiver Function Method: A Preliminary Result

The teleseismic receiver function method was used to investigate the crustal structure in West Sumatra, Jambi, and Riau Islands regions. These regions have a high level of seismicity because it is in an active tectonic zone where the oblique subduction between the Indo-Australian and Eurasian plates occurs. This study aims to determine the depth of Moho discontinuity and the presence of subducting slab beneath 6 BMKG seismic stations that form a line perpendicular to the trench and cover 3 different geological zones. PPSI and RASM are in the forearc zone, SKSM is in the volcanic arc zone, and MBBI, JMBI, and DSRI are in the back-arc zone. This study used teleseismic earthquake record data with epicentral distance between 30°-90° from the receiver and magnitude 6 or more. The iterative time-domain deconvolution and receiver function migration techniques were applied to estimate the depth of Moho discontinuity and the presence of subducting slab. The depth of Moho discontinuity in West Sumatra, Jambi, and Riau Islands regions ranges from 21-39 km and generally deeper in the volcanic arc zone possibly due to the isostatic effect. Moho is at a depth of 21-29 km in the forearc zone, 36-39 km in the volcanic arc zone, and 30-36 km in the back-arc zone. Then the subducting slab was observed at a depth of 20 km under PPSI station to 200 km under MBBI station.

Depth Estimation of Moho Discontinuity Layer at 5 BMKG Seismic Stations in East Java Using Receiver Function Method

East Java is one of the areas with a high level of seismicity due to the existence of a subduction zone in the south of Java Island. In addition, there are also active faults and active volcanos in East Java. This study aims to determine the depth of the Moho Discontinuity layer and subduction slab using receiver function method based on the iterative time domain deconvolution. This study was conducted using teleseismic earthquake data with an epicenter distance of 30°–90° from the receiver station and magnitude more than 6 (M≥6). 5 BMKG seismic stations that form a straight line are used, including those in the Bawean Arc (BWJI), Rembang Zone (BAJI), Kendeng Zone (SIJM), Modern Mountain Arc (PPJI), Southern Mountain Zone (GEJI). The depth of the Moho Discontinuity layer at BWJI Station was observed in ranges from 36–38 km, at BAJI Station it ranges from 39–40 km, at SIJM Station it ranges from 39–40 km, at PPJI Station it ranges from 46–48 km, and at GEJI Station it ranges from 33–36 km. In general, the Moho Discontinuity layer in the mountainous region is deeper due to the isostasy effect.

Identification of Moho Discontinuity Depth and Subduction Slab in Bengkulu and South Sumatra Region using Receiver Function Method: A Preliminary Result

Bengkulu and South Sumatra regions are areas with high seismic activity due to faults and subduction zones. This study aims to analyze the depth of the Moho discontinuity layer and subduction slab under 4 BMKG seismic stations that form a perpendicular line of trench in the Bengkulu and South Sumatra areas. The distribution of stations used are Enggano Station (EGSI) in the front arc islands, Manna Station (MNAI) in the front arc basin, Pagar Alam Station (PKSI) in the volcanic-arc zone, and Lahat Station (LHSI) in the proto-volcanic zone. We selected teleseismic earthquake data with a distance of 30°-90° from the stations and magnitudes above 6 (M>6). For the identification of Moho and slab depths, we used analysis of receiver functions with iterative time domain deconvolution and migration of receiver functions with the AK-135 velocity model. The Oceanic Moho layer and the subduction slab under the EGSI Station were identified at depths of 34 km and 23 km respectively, under the MNAI Station the Moho layer and the subduction slab were identified at a depth of 18 km and 87 km respectively, under the PKSI Station the Moho layer and the subduction slab were identified at a depth of 34 km and 129 km respectively, and under the LHSI Station the Moho layer and subduction slab were identified at depths of 35 km and 175 km respectively.

Reference seismic crustal model of the Dinarides

Abstract. Continental collision zones are structurally one of the most heterogeneous areas intermixing various different units within a relatively small space. A good example of this is the Dinarides, a mountain chain situated in the central Mediterranean, where thick carbonates cover older crystalline basement units and remnants of subducted oceanic crust. This is further complicated by the highly variable crustal thickness ranging from 20 to almost 50 km. In terms of spatial extension, this area is relatively small but covers tectonically differentiated domains making, any seismic or geological analysis complex, with significant challenges in areas that lack seismic information on crustal structure. Presently there is no comprehensive 3D crustal model of the Dinarides (and surrounding areas). Using the compilations of previous studies and employing kriging interpolation, we created a vertically and laterally varying crustal model defined on a regular grid for the wider area of the Dinarides, also covering parts of Adriatic Sea and the SW part of the Pannonian Basin. The model is divided by three interfaces, Neogene deposit bottom, carbonate rock complex bottom and Moho discontinuity, with seismic velocities (P and S waves) and density defined at each grid point. To validate the newly derived model, we calculated travel times for an earthquake recorded on several seismic stations in the Dinarides area. The calculated travel times show significant improvement when compared to the simple 1D model used for routine earthquake location in Croatia. The model derived in this work represents the first step towards improving our knowledge of the crustal structure in the complex area of the Dinarides. We hope that the newly assembled model will be useful for all forthcoming studies (e.g., as a starting model for seismic tomography, as a model for earthquake simulations) which require knowledge of the crustal structure.

Gas geochemical evidence for the India-Asia lithospheric transition boundary near the Karakorum fault in western Tibet

The lithospheric structure of the Tibetan Plateau plays a key role in explaining the underlying plateau formation mechanism(s) and growth history. Although many studies have focused on the Moho discontinuity, lithosphere boundary, and lithospheric structure of central and eastern Tibet, there are few corresponding constraints for western Tibet. In this contribution, we systematically investigated the lithosphere in western Tibet by analyzing the He and C isotopes of seven hydrothermal spring systems along the southeastern segment of the Karakorum fault. We found a high Rc/Ra ratio (1.298) for springs along the Karakorum fault, suggesting that the fault plays a key role as a lithospheric-scale conduit for fluid uplift after mantle degassing. In addition, we identified two different origins of the gases at thermal springs, which may reflect two distinct mantle sources. We infer that the locations of these springs reflect the boundary between the Indian and Asian mantles, which is consistent with the geophysics-based interpretations of a “mantle suture” along the Karakorum fault. Our work confirms the boundary between the Indian and Asian mantle, which provides geochemical support to the model of the subduction of Indian lithosphere.

Lithosphere density structure of southeastern South America sedimentary basins from the analysis of residual gravity anomalies

We conduct a gravity study of the lithosphere beneath three large sedimentary basins in southeastern South America: Paraná, Chaco-Paraná, and Pantanal. We compile a massive gravity database and estimate the free-air and Bouguer gravity anomalies, resulting in a novel complete Bouguer anomaly map for the study area. To discern the influence of crustal loads with known lithologies, including sediments, basalts, and topography variations of the Moho discontinuity, we calculate their gravity effects and subsequently remove them from the complete Bouguer anomaly, leading to the development of our residual Bouguer anomaly map. This map highlights unknown anomalous masses within the lithosphere. To aid in the interpretation of these residual anomalies, we perform a 2D forward modeling. Based on our results, we propose new boundaries for the Paranapanema block and the Luiz Alves craton. Additionally, we propose that the Ponta Grossa swarm dike has a more substantial impact on the crust and lithosphere than previously considered, and delimit the region of influence of this magmatism in the lithosphere. Moreover, tectonic features such as the São Francisco paleocontinent and the Rio de La Plata craton appear to be associated with negative residual Bouguer anomaly regions. Furthermore, we identify and emphasize the significance of the Western Paraná Suture, which acts as a demarcation between the Paraná Basin region and the Pantanal and Chaco-Paraná basins. Remarkably, this suture appears to play a more important role in shaping the density structure of the southwest of South America than the age and tectonic history of the sedimentary basins.

Preliminary Results on Receiver Function Study in Mt Merapi, Central Java, Indonesia

Mount Merapi, a stratovolcano, is the world’s most active volcano, with a relatively short eruption period. Mount Merapi formed in the Java region as a result of regional tectonics dominated by the Sunda Arc, resulting in a large earthquake. Many earth scientists are interested in studying the volcano’s subsurface conditions due to its relatively short eruption period and interesting geological features. The Receiver Function method was used in this study to determine the crust’s depth and assess the presence of a LVZ (low velocity zone) by reprocessing receiver function data. The Receiver function is used to identify the Moho discontinuity area by converting P to S waves. A total of 100 earthquake data from 8 teleseismic stations were successfully downloaded from the IRIS website, that was distributed into sections A-A′ (west side of Mount Merapi) and B-B’ (east side of Mount Merapi). The processing of the receiver function data, as shown by the stacking align results, shows that the closest teleseismic station at west side of Mount Merapi has a very strong negative amplitude response, which is represented as a LVZ or magma reservoir after the arrival of P wave. To estimate the zone for LVZ, a forward modeling receiver function technique was used to find the best correlation between the Synthetic Receiver Function curve and the Receiver Function observation curve. A forward modeling receiver function technique was used to find the best correlation between the Synthetic RFcurve and the RF observation curve to estimate the zone for the LVZ. The correlation between the synthetic RF curve from Ramdhan et al’s (2019) tomographic velocity model and the observed RF curve is poor. To improve the correlation, include the main signal source that affects the receiver function curve in the form of seismic wave velocity particularly Vs, LVZ Zone, thin sedimen layer or shallow reservoir, and depth of discontinuity by Suhardja et al (2019). The estimated depth of the LVZ at 10 - 17 km is thinning towards the south or towards Mount Merapi, according to the results of the synthetic receiver function curve modelling at the closest station to Mount Merapi.

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.