Receiver Function Inversion Research Articles

Sedimentary Thickness Variations in the Indo-Gangetic Foredeep from Inversion of Receiver Functions

Abstract We utilize receiver functions from ten broadband seismic stations deployed along a north–south profile traversing the Indo‐Gangetic plains in northern India to investigate the sedimentary thickness variations in this hitherto less‐studied foreland basin south of the Himalaya. Parameterization of the velocity structure adopting the neighborhood algorithm approach shows that the data can be satisfactorily modeled for low‐velocity sediments having shear‐wave velocities in the range of 0.72–2.5 km/s with thicknesses varying from 0.5 to 3.7 km beneath the individual stations. The velocity–depth functions obtained in this study are important for earthquake‐hazard assessment of the densely populated urban centers spread over this region, in terms of predicting strong ground motions due to large earthquakes in the Himalaya.

Uncertainty Analysis in the Joint Inversion of Receiver Function and Surface-Wave Dispersion, Parana Basin, Southeast Brazil

The joint inversion of receiver function (RF) and surface-wave dispersion data is popular because it reduces the nonuniqueness of the modeled subsurface- seismic velocities. Whereas various inverse procedures have been used in joint subsur- face imaging, the evaluation of uncertainties in the estimated parameter distribution is usually overlooked or considered qualitatively. We present a quantitative method for determination of uncertainty in velocity models estimated by the joint inversion of RF and surface-wave data by using the prediction of each observation sample to map the objective-function surface and create a statistical distribution of estimated model parameters. The proposed methodology is evaluated in a controlled test using syn- thetic data simulating a realistic shear-wave velocity model. We then apply the method to field data recorded at the POPB seismic station in the Parana Basin, southeast Brazil. The respective range of uncertainty for modeled S-wave velocity distribu- tions in the synthetic and field tests were 0:111-0:412 km=s (2.5%-9.2%), and 0:110-0:341 km=s (2.5%-7.9%).

Earth structure and instrumental seismicity of Madagascar: Implications on the seismotectonics

The aim of this study is to improve the knowledge of the seismotectonics of Madagascar. We first investigate the structure of the Earth beneath Madagascar through the joint inversion of receiver functions and Rayleigh wave group velocities. Then we use the obtained velocity models to relocate local earthquakes in order to analyse the distribution of seismicity. Finally, we use structural models and earthquake coordinates to compute focal mechanisms.Our retrieved Earth structure models confirm a thin lithosphere beneath Madagascar when compared to the nearby East African Rift. The High Plateau in the Central region coincides with the thinnest lithosphere over the slowest asthenosphere. Our results are in good agreement with the gravity anomalies and likely confirm a localised asthenospheric upwelling beneath the central part of Madagascar. The surface expression of the asthenospheric upwelling consists in a horst–graben structure. The moderate seismicity is localised along pre-existing structures reflecting an E-W extension that is mostly accommodated in the lower crust.

Crustal structure of the Khartoum Basin, Sudan

The crustal structure of the northern part of the Khartoum Basin has been investigated using data from 3 permanent seismic stations within 40km of Khartoum and two modeling methods, H–k stacking of receiver functions and a joint inversion of receiver functions and Rayleigh wave group velocities. The Khartoum Basin is one of several Mesozoic rift basins in Sudan associated with the Central African Rift System. Results from the H–k-stacking indicate that crustal thickness beneath the Khartoum Basin ranges between 33 and 37km, with an average of 35km, and that the crustal Vp/Vs ratio ranges from 1.74 to 1.81, with an average of 1.78. From the joint inversion of receiver functions and Rayleigh wave group velocities, we obtained similar results for Moho depth, as well as an average shear wave velocity of 3.7km/s for the crust. These results provide the first seismic estimates of Moho depth for a basin in Sudan. When compared to average crustal thickness for unrifted Proterozoic crust in eastern Africa, our results indicate that at most only a few km of crustal thinning may have occurred beneath the Khartoum Basin. This finding is consistent with estimates of effective elastic plate thickness, which indicate little modification of the Proterozoic lithosphere beneath the basin, and suggests that there may be insufficient topography on the lithosphere–asthenosphere boundary beneath the Sudanese basins to channel plume material westward from Ethiopia.

수신함수 역산 및 H-κ 중합법을 이용한 제주도 하부의 S파 지각 속도

제주도 한라산 주위에 위치한 JJU와 JJB 관측소 하부의 S파 속도구조를 규명하기 위하여, 이 관측소에서 2007년 이후 기록한 <TEX>$M_W$</TEX> 5.5 이상인 원격 지진자료 중 150개 수신함수를 이용하여 역산 및 H-<TEX>${\kappa}$</TEX> 중합법에 적용하였다. 모호면에서 변환된 <TEX>$P_S$</TEX>파는 JJU 관측소의 북서쪽(후방위각 <TEX>$207{\sim}409^{\circ}$</TEX>, 평균 <TEX>$308^{\circ}$</TEX>)과 JJB 관측소의 남동쪽(후방위각 <TEX>$119{\sim}207^{\circ}$</TEX>, 평균 <TEX>$163^{\circ}$</TEX>) 방향으로 뚜렷하지 않게 나타났다. 이것은 아마도 모호면의 점이적인 속도변화나 지각 내의 속도 불균질층 때문일 수 있다. 수신함수 역산으로부터 계산된 S파 속도모델은 지각 내의 저속도층을 뚜렷이 보여주며, 30 ~ 40 km 깊이에서 점이적으로 증가하는 양상을 보인다. JJB 관측소 반경 18 km 이내에서 저속도층(<TEX>$v_s{\leq}3.5km/s$</TEX>)은 14 ~ 26 km에 있고, <TEX>$v_s{\geq}4.3km/s$</TEX>으로 정의한 '모호면'은 34 km 깊이에 있는 것으로 분석되었다. 서쪽으로 약 10 km 떨어진 곳에 위치한 JJU 관측소의 반경 16 km 이내에서는 저속도층과 '모호면'이 14 ~ 24 km와 30 km에 각각 존재하여 JJB 관측소에 비해 다소 얕은 깊이에서 나타난다. JJU와 JJB 관측소에 대한 H-<TEX>${\kappa}$</TEX> 분석결과는 지각 두께가 29 km와 33 km이며, 종파/횡파 속도비(<TEX>$v_p/v_s$</TEX>)가 1.64과 1.75임을 각각 나타내어 화산 정상에 가까운 곳에서 상대적으로 낮은 <TEX>$v_p/v_s$</TEX>가 관찰되었다. Shear-wave velocity (<TEX>$v_s$</TEX>) structures beneath two seismic stations, JJU and JJB on the flanks of the volcano Halla on Jeju island, Korea, were estimated by receiver-function inversion and H-<TEX>${\kappa}$</TEX> stacking applied to 150 teleseismic events (<TEX>$M_W{\geq}5.5$</TEX>) recorded since 2007. <TEX>$P_S$</TEX> waves converted at the Moho discontinuity does not appear clearly for northwesterly back-azimuths (<TEX>$207{\sim}409^{\circ}$</TEX>, average <TEX>$308^{\circ}$</TEX>) at station JJU and southeasterly back-azimuths (<TEX>$119{\sim}207^{\circ}C$</TEX>, average <TEX>$163^{\circ}$</TEX>) at station JJB. This may be due to a gradual velocity increase at Moho or heterogeneity within the crust. The <TEX>$v_s$</TEX> models derived by inversion of receiver functions indicate a distinct low velocity layer (<TEX>$v_s{\leq}3.5km/s$</TEX>; LVL) within the crust and a gradual increase in <TEX>$v_s$</TEX> in the depth interval of 30 to 40 km. Within the radius of 18 km beneath station JJB, the LVL occurs at depths of 14 ~ 26 km and the 'Moho' (<TEX>$v_s{\geq}4.3km/s$</TEX>) is at 34 km depth. Ten kilometers to the west, within the radius of 16 km beneath station JJU, both the LVL and the Moho are significantly shallower, at depths of 14 to 24 km and 30 km, respectively. H-<TEX>${\kappa}$</TEX> analyses for stations JJU and JJB yield estimated crustal thickness of 29 and 33 km and <TEX>$v_p/v_s$</TEX> ratios of 1.64 and 1.75, respectively. The lesser <TEX>$v_p/v_s$</TEX> ratio was derived for rocks nearest to th peak of the volcano.

A 3‐D model of the crust and uppermost mantle beneath the Central and Western US by joint inversion of receiver functions and surface wave dispersion

AbstractRayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the USArray Transportable Array west of 100°W longitude for data recorded in the years 2005 through 2010 to produce a 3‐D model of shear wave speeds beneath the central and Western US to a depth of 150 km. Eikonal tomography is applied to ambient noise data to produce about 300,000 Rayleigh wave phase speed curves, and Helmholtz tomography is applied to data following 1550 (Ms &gt; 5.0) earthquakes so that Rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region. Harmonic stripping generates back‐azimuth independent receiver functions with uncertainty estimates for each of the stations. A nonlinear Bayesian Monte Carlo method is used to estimate a distribution of shear wave speed (Vs) models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties. The assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated Moho depths, improving the determination of the shear velocity jump across Moho, and improving the resolution of the depth of anomalies in the uppermost mantle. A great variety of geological and tectonic features are revealed in the 3‐D model that form the basis for future detailed local to regional scale analysis and interpretation.

Joint inversion of surface wave dispersion and receiver functions: a Bayesian Monte-Carlo approach

A non-linear Bayesian Monte-Carlo method is presented to estimate a Vsv model beneath stations by jointly interpreting Rayleigh wave dispersion and receiver functions and associated uncertainties. The method is designed for automated application to large arrays of broad-band seismometers. As a testbed for the method, 185 stations from the USArray Transportable Array are used in the IntermountainWest, a region that is geologically diverse and structurally complex. Ambient noise and earthquake tomography are updated by applying eikonal and Helmholtz tomography, respectively, to construct Rayleighwave dispersion maps from 8 to 80 s across the study region with attendant uncertainty estimates.Amethod referred to as ‘harmonic stripping method’ is described and applied as a basis for quality control and to generate backazimuth independent receiver functions for a horizontally layered, isotropic effective medium with uncertainty estimates for each station. A smooth parametrization between (as well as above and below) discontinuities at the base of the sediments and crust suffices to fit most features of both data types jointly across most of the study region. The effect of introducing receiver functions to surface wave dispersion data is quantified through improvements in the posterior marginal distribution of model variables. Assimilation of receiver functions quantitatively improves the accuracy of estimates of Moho depth, improves the determination of the Vsv contrast across Moho, and improves uppermost mantle structure because of the ability to relax a priori constraints. The method presented here is robust and can be applied systematically to construct a 3-D model of the crust and uppermost mantle across the large networks of seismometers that are developing globally, but also provides a framework for further refinements in the method.

Receiver Function Inversion Using Genetic Algorithms

Abstract The H‐Kappa (H‐ κ ) stacking algorithm is an important tool to stack receiver functions and thus help determine crustal thickness (H) and crustal P ‐wave to S ‐wave velocity ratio ( κ ). To determine these crustal parameters, the H‐ κ stacking algorithm makes use of a direct search method by assuming some reasonable values for weights assigned to three seismic phases found on receiver functions. In this study, genetic algorithms (GAs) have been implemented to investigate and obtain optimal or near optimal values of the weights assigned to the phases. The results indicate that GA is effective in finding weights for determining crustal parameters as it iteratively searches many combinations of weights without being trapped in local optima. Thus, the GA is implemented here to provide optimal weights for a set of receiver functions to be used by the H‐ κ stacking algorithm. In order to test the validity of the proposed method, we used receiver function sets from a previous work. We found that the resulting weights from the new method are very close to the weights used in the earlier study and that the H and κ values obtained through the two different studies are consistent. Since no human intervention is required while the GA is searching to obtain the different optimal parameters, the GA‐H κ technique could be used in automatic determinations of crustal parameters.

APPLICATION OF GAUSS-MARQUARDT-LEVENBERG METHOD IN THE INVERSION OF RECEIVER FUNCTION IN CENTRAL BRAZIL

Receiver Function studies contribute to the understanding of crustal structure and its evolution by characterizing the internal physical properties of the Earth, such as seismic wave velocity and Poisson ratio. The data inversion technique enables to modeling of crustal structure, seeking a geologically consistent model that provides a quantitative analysis of the related process. The linearized inversion of Gauss-Marquardt-Levenberg method was applied to the Receiver Function synthetic and real data, obtained by geophysical surveys in Central Brazil, using the Parameter Estimation (PEST) software. The Receiver Function inversion was performed varying only S wave velocity parameter. Synthetic data inversion converged and data fitting was optimal when the initial velocity models used were close to the reference model. The inversion results of Receiver Function data, obtained from nine seismographic stations, using models of crustal structure and velocity distribution based on studies of seismic refraction and Receiver Function were deemed reasonable. Crustal mean velocities are compatible with velocity estimates obtained by previous geophysical studies. The inversion method used here is promising for crustal structure investigations, while PEST software proved to be very efficient, enabling inversion control parameter configuration and monitoring of the process by statistical results.

Crustal and uppermost mantle S velocity structure under Hi-CLIMB seismic array in central Tibetan Plateau from joint inversion of surface wave dispersion and receiver function data

Trade-off between the depth of an interface and the average velocity above the interface is well known in receiver function inversion. To resolve model ambiguity, surface wave dispersion data, which are sensitive to vertical shear velocity average, are commonly combined with P receiver functions in the inversion. In this paper, we design a joint inversion scheme using the neighborhood searching algorithm to solve shear velocity structure of the crust and uppermost mantle with receiver function and surface wave dispersion data. Two sets of stretched splines are implemented in the inversion to obtain smooth velocity profiles, separated by a specifically defined Moho discontinuity. We apply this method to the dense Hi-CLIMB seismic array in the central Tibetan Plateau (TP). The Indian crust is observed to subduct and underplate the Tibetan crust between the Yarlung-Zangbo suture and the Bangong-Nujiang suture, at the latitude of about 31.5°N. The crust thickens from about 50km below the Indian foreland to over 75km south of Bangong-Nujiang suture and turns shallower at around 65km under the Qiangtang block. We identify two zones with complicated Moho structure beneath the northern Lhasa block and southern Qiangtang block, indicating highly deformed Moho morphology, and beneath the Himalaya block associated with the underthrusting of the Indian Plate, respectively. A low velocity zone is present in the mid-crust in most parts of the profile under the TP with up to more than 10% velocity reduction. The low velocity structure is not continuous, forming discontinuous patches rather than channels, calling for a revision of the channel flow model.

Relocation of the Yushu M S7.1 earthquake and its aftershocks in 2010 from HypoDD

After the Yushu MS7.1 earthquake on April 14, 2010, a large number of aftershocks were recorded by the surrounding permanent network and temporary seismic stations. Due to the distribution of stations, knowledge about velocity structure, the reliability of seismic phases, and so on, the location result from conventional method is usually of low precision, from which it is difficult to recognize the spatial and temporal distribution and the trends of aftershock activity. In this paper, by using teleseismic waveforms recorded by permanent station, the seismic velocity structure beneath the vicinity is obtained from receiver function stacking and inversion methods. And the Yushu earthquake sequences are relocated from seismic phase data by HypoDD. The results show that the Yushu MS7.1 earthquake occurred at 13 km depth; the aftershock sequences were distributed mainly in the NWW along the Garze-Yushu fault, and most aftershocks were concentrated in a 100 km length and 5–20 km depth. Combined with the velocity structure, it can be inferred that the earthquake mainly destroys the high-velocity layer of the upper crust. In the west of the seismic fault near (33.3°N, 96.2°E), the aftershock sequences were distributed like a straight column, suggesting there was a comminuted break from 25km depth to the ground.

The structure of the Earth’s crust and uppermost mantle along the Baikal rift system based on teleseismic data

Detailed data on the velocity structure of the Earth's crust and uppermost mantle are very impor� tant to study the complex structure of the Baikal rift system. Such data can be obtained not only from deep seismic sounding (DSS) but also on the basis of the teleseismic method of Pwave receiver function (PtoS). The analysis of the wave forms in this method is based on the approach of seismic prospecting using the records of strong distant earthquakes (1, 2). Owing to the sensitivity of converted shear seismic waves to velocity inhomogeneities at different depths, this method gave good results in the study of the velocity structure based on the records of temporary stations in the south of Siberia and in Mongolia (3, 4). In the past decade, after the recording at the sta� tionary Baikal seismic network was reconstructed and became digital, a large amount of data was accumu� lated in the Baikal region and Transbaikalia, which is applicable to study the deep structure in the regions around seismic stations (Fig. 1). Longterm observa� tions at stationary stations can compete with short� term seismic experiments in the amount of records of strong remote earthquakes providing greater reliability of the velocity models obtained on their basis. In this work, we used these data and applied the method of receiver function to determine onedimensional mod� els of Swave propagation velocities up to a depth of 70 km. Twodimensional models ( VStransects) were constructed on the basis of onedimensional models (VS(h)) in the regions of dense locations of stations along the lengthy Baikal Rift system. We separated the receiver functions of longperiod waves formed by converted SVwaves in the observa� tion region from the records of earthquakes with a magnitude greater than 6.0 that arrive from distances of 3000-9000 km. Taking into account the extremely complex geomorphological structure of the region and possible existence of seismic azimuthal anisotropy not only in the mantle but also in the Earth's crust (5), we used the records of remote earthquakes from the northeastern and southwestern directions. Thus, in this work, we analyzed only the earthquakes records that make possible sounding of the velocity structure approximately under the lines of stations, i.e., along the rift zone (Fig. 1). Summing of the receiver func� tions from two opposite directions evens the influence of irregular inhomogeneities amplifying the contribu� tion of the main structural peculiarities in the forma� tion of the wave forms. Using a program of receiver function inversion developed by G.L. Kosarev (2), we restored one� dimensional velocity vertical sections of shear seismic waves, VS(h), from the separated received functions. The detailed structure of velocity models is deter� mined by their parameterization, which we developed on the basis of numerical modeling (3). The experi� ments with numerical models demonstrated that divi� sion of the studied structure within the Earth's crust into elementary layers not thicker than 1 km is opti� mal. In order to determine the depth of the crust- mantle boundary and its character (sharp or gradient) more accurately within the upper 5-10 km of the mantle the thickness of the elementary layers should be close to 1 km. Deeper in the mantle, the depth step can increase to 5 km or more. The smoothed velocity function V0(h) calculated from the mean results of investigations based on the DSS method in the SayanyBaikal mountain region (6) was used as the first approximation in all cases of the inversion procedure. According to the DSS, the mean velocity of longperiod waves in the Earth's crust is equal to 6.4 km/s in the first approximation, while the velocity of the shear seismic waves is 3.7 km/s. The velocity of Pwaves in the mantle under the crust was assumed 8.0 km/s, and the ratio was V P /V S = 1.80. Using a spline based on the method of linear interpo� lation, we constructed twodimensional models of the velocity structure (Fig. 2) from the restored one� dimensional stationary models of V S (h). The models

Lithospheric structure below seismic stations in Cuba from the joint inversion of Rayleigh surface waves dispersion and receiver functions

SUMMARY The joint inversion of Rayleigh wave group velocity dispersion and receiver functions has been used to study the crust and upper mantle structure at eight seismic stations in Cuba. Receiver functions have been computed from teleseismic recordings of earthquakes at epicentral (angular) distances in the range from 30° to 90° and Rayleigh wave group velocity dispersion relations have been taken from earlier surface wave tomographic studies in the Caribbean area. The thickest crust (∼30 km) below Cuban stations is found at Cascorro (CCC) and Maisi (MAS) whereas the thinnest crust (∼18 km) is found at stations Rio Carpintero (RCC) and Guantanamo Bay (GTBY), in the southeastern part of Cuba; this result is in agreement with the southward gradual thinning of the crust revealed by previous studies. In the crystalline crust, the S-wave velocity varies between ∼2.8 and ∼3.9 km s–1 and, at the crust–mantle transition zone, the shear wave velocity varies from ∼4.0 and ∼4.3 km s–1. The lithospheric thickness varies from ∼65 km, in the youngest lithosphere, to ∼150 km in the northeastern part of the Cuban island, below Maisi (MAS) and Moa (MOA) stations. Evidence of a subducted slab possibly belonging to the Caribbean plate is present below the stations Las Mercedes (LMG), RCC and GTBY whereas earlier subducted slabs could explain the results obtained below the Soroa (SOR), Manicaragua (MGV) and Cascorro (CCC) station.

An Effective Technique to Constrain the Non‐Uniqueness of Receiver Function Inversion

AbstractThe relation curve between the apparent velocity of S wave and the filter‐parameter is obtained by low‐pass filtering the radial and vertical receiver function, then it is transformed into the S velocity structure beneath the station with empirical formula; moreover, the S velocity structure will be regarded as initial model for receiver function inversion. The numerical simulation indicated that the inversion processes would approach quickly to the true model once the initial S velocity can supply the effective constraint to inversion, even if the depth of Moho is not accurate. In addition, for the same initial S structure, the inversion results also approach to the true model quickly, even adding 10% of random noises to the “observed” receiver function, and 5% of positive and negative perturbation to the velocity ratio (corresponding to the Poisson's ratio of 0.27 and 0.23) respectively. We processed the teleseismic receiver function recorded by Baoshan station. The result from our method is similar to that of sounding section. These confirmed that the inversion processes of receiver function are not sensitive to the initial P velocity if the initial S wave velocity can provide the effective constraint information.

수신함수와 표면파 분산곡선의 복합역산 및 수신함수 H-κ 중첩법을 이용한 원주 KS31 지진관측소 하부의 S파 지각 속도구조

To estimate the S-wave velocity structure beneath the KS31 broad-band station in Wonju, Korea, we used H- κ stacking and joint inversion of receiver functions and surface-wave dispersion curves derived from 297 teleseismic events (Mw > 5.5) recorded during the period between 2002 and 2009. We thereby determined that the average depth to a nearly flat Moho is 32.4 ± 0.5 km within tens of kilometer radius of the seismic station. For the crust at this location, we estimate an average shear-wave velocity of 3.69 km/s and a ratio of P- to S-wave velocities, VP/VS, of 1.72 ± 0.04, as is typical for continental crust. A negative phase in the receiver functions at 1 s indicates the presence of a shear- wave low velocity layer in a depth interval of 10 to 18 km in the upper crust beneath the KS31 station.

Transdimensional inversion of receiver functions and surface wave dispersion

We present a novel method for joint inversion of receiver functions and surface wave dispersion data, using a transdimensional Bayesian formulation. This class of algorithm treats the number of model parameters (e.g. number of layers) as an unknown in the problem. The dimension of the model space is variable and a Markov chain Monte Carlo (McMC) scheme is used to provide a parsimonious solution that fully quantifies the degree of knowledge one has about seismic structure (i.e constraints on the model, resolution, and trade‐offs). The level of data noise (i.e. the covariance matrix of data errors) effectively controls the information recoverable from the data and here it naturally determines the complexity of the model (i.e. the number of model parameters). However, it is often difficult to quantify the data noise appropriately, particularly in the case of seismic waveform inversion where data errors are correlated. Here we address the issue of noise estimation using an extended Hierarchical Bayesian formulation, which allows both the variance and covariance of data noise to be treated as unknowns in the inversion. In this way it is possible to let the data infer the appropriate level of data fit. In the context of joint inversions, assessment of uncertainty for different data types becomes crucial in the evaluation of the misfit function. We show that the Hierarchical Bayes procedure is a powerful tool in this situation, because it is able to evaluate the level of information brought by different data types in the misfit, thus removing the arbitrary choice of weighting factors. After illustrating the method with synthetic tests, a real data application is shown where teleseismic receiver functions and ambient noise surface wave dispersion measurements from the WOMBAT array (South‐East Australia) are jointly inverted to provide a probabilistic 1D model of shear‐wave velocity beneath a given station.

Receiver function structure beneath four seismic stations in the Sumatra region

We estimated the velocity structure beneath four three-component broad-band seismic stations in the Sumatra region by the joint inversion of teleseismic receiver functions and surface wave group velocities. The stations, part of GEOFON network GE, are located in diverse geologic settings including backarc basins, forearc basins, and the forearc ridge. The stations are distributed roughly trench-parallel, providing samples of conditions along the length of Sumatra. 143 receiver functions were computed by employing an iterative, time-domain deconvolution in order to minimize acausal noise. The teleseismic observations at each station were grouped by back azimuth and ray parameter and then stacked to increase the signal to noise ratios. Surface wave group velocity dispersion was measured for paths across the Sunda block from events to the northeast of the stations. P-wave velocity profiles were derived by inverting the stacks and group velocity data using a linearized time-domain inversion scheme with an interpolated CRUST2.0 global model for input. The results differ significantly from CRUST2.0, and reflect the diversity of geologic environments within which the stations are located. Crustal thicknesses beneath the stations range from 16.0km at the forearc to 30.0km at a backarc basin. These results add to the available velocity and crustal thickness data for the Sumatra region.

Joint inversion of teleseismic P waveforms and surface-wave group velocities from ambient seismic noise in the Bohemian Massif

Joint inversion of teleseismic P-waveforms and local group velocities of surface waves retrieved from ambient seismic noise has been performed to model velocity structure of the crust and uppermost mantle of the Bohemian Massif. We analysed P-waveforms of 381 teleseismic earthquakes recorded at 54 broadband seismic stations located on the territory of the Czech Republic and in its close surroundings. Group velocities of Rayleigh and Love surface waves were obtained by cross-correlating long-term recordings of seismic noise. The basis for waveform inversion is the well-known methodology of P-to-S receiver functions constructed from converted phases. Due to instabilities in direct inversion of receiver functions caused by the necessity of applying deconvolution, we propose an alternative formulation to fit observed and calculated radial components of P waveforms. The joint inversion is transformed into a search for the minimum of the cost function defined as a weighted sum of waveform and group velocity misfits. With the use of the robust stochastic optimizer (Differential Evolution Algorithm), neither derivatives nor a starting model are needed. The task was solved for 1D layered isotropic models of the crust and the uppermost mantle. We have performed a sequence of inversions with models containing one, two, three and four layers above a half-space. By using statistical criteria (F-test) we were able to select the simplest velocity models satisfying data and representing local geological structures. Complex crustal models are typical for stations located close to boundaries of major tectonic units. The relatively low average P to S wave-velocity ratio is in agreement with the generally accepted view that the BM crust is predominantly felsic.

S-velocity structure beneath the Bohemian Massif from Monte Carlo inversion of seismic receiver function

The paper contains an analysis of S-velocity distribution in the crust and upper mantle beneath the Bohemian Massif, which is the second biggest Variscan outcrop in Europe. It occupies mainly the west part of Czech Republic and also part of south-west Poland and south-east Germany. We use data from 10 permanent stations set in the region. Some previous papers relate to the same scope but use linear methods to inverse receiver function. Our new approach involves Monte Carlo techniques for inversion procedure, which is more convenient and robust for such a non-linear task. The result of Monte Carlo inversion is compared with the previously achieved one. The obtained Moho depths vary from 29 km in the north-west part of the Bohemian Massif to 38 km in the south and south-east and are consistent with other papers. Some discrepancies occur in the middle and upper crust.

Seismogenesis of the lower crustal intraplate earthquakes occurring in Kachchh, Gujarat, India

Large intraplate continental earthquakes like the 1811–12 New Madrid (M w ⩾ 8.0) and the 2001 Bhuj (Mw7.7) were highly destructive because they occurred in strong crust, but the mechanisms underlying their seismogenesis are not understood. Here we show, using local earthquake velocity tomography, and joint inversion of receiver functions and surface wave group velocity dispersion that the crust and uppermost mantle beneath the 2001 Bhuj earthquake region of western India is far more complex than hitherto known through previous studies. A new image of the crust and underlying mantle lithosphere indicates the presence of a 18-km thick high velocity (Vp: 7.15–8.11 km/s) differentiated crustal and mantle magmatic layer above a hot and thin lithosphere (only 70 km) in the epicentral region of 2001 Bhuj earthquake. This magmatic layer begins at the depth of 24 km and continues down to 42 km depth. Below this region, brittle-ductile transition reaches as deep as the Moho (∼34 km) due to the possible presence of olivine rich mafic magma. Our 1-D velocity structure envisages an initial phase of plume activity (Deccan plume at 65 m.y. ago) resulting in basaltic magma in the eclogitic layers at sub-lithospheric levels, wherein they were subjected to crystallization under ultra-high pressure conditions. Our study also delineates an updoming of Moho (∼4–7 km) as well as asthenosphere (∼6–10 km) below the Kachchh rift zone relative to surrounding areas, suggesting the presence of a confined body of partial melts below the lithosphere-asthenosphere boundary. Restructuring of this warm and thin lithosphere may have been caused due to rifting (at 184 and 88 m.y. ago) and tholeiitic and alkalic volcanism related to the Deccan Traps K/T boundary event (at 65 m.y. ago). Recent study of isotopic ratios proposed that the alkalic basalts found in Kachchh are generated from a CO 2 rich lherzolite partial melts in the asthenosphere that ascended along deep lithospheric rift faults into the lithosphere. It appears that such kind of crust–mantle structure, deepening of brittle-ductile transition and a high input of volatiles containing CO 2 emanating from mantle control the seismogenesis of lower crustal earthquakes in the Kachchh continental rift zone.