Inversion For Velocity Structure Research Articles

Joint inversion method of multipoint ambient noise horizontal-to-vertical spectral ratio for 3-D velocity structure of local site and its application

SUMMARY The diffusion field theory has been widely used to interpret ambient noise wave fields. Based on this theory, 1-D subsurface velocity structure inversion method is developed. However, few studies have referred to the noise horizontal to vertical (NHV) spectral ratio inversion of 3-D subsurface velocity structures, and almost no effective 3-D NHV inversion tools have been developed. To develop a useful tool for obtaining 3-D soil layer velocity structures, we combined the NHV forward calculation formula derived from diffusion field theory with the guided Monte Carlo algorithm and then extended the single-point NHV inversion to multipoint joint inversion through a joint objective function. Subsequently, a new 3-D soil layer velocity structure inversion method was proposed. Subsequently, a synthetic 2-D case was used to verify the proposed method. Finally, the proposed method was applied to the Xiangtang Array in Tangshan, China, to identify the 3-D velocity structures of the site based on noise observations. The results show that the proposed multipoint joint 3-D inversion method is effective for identifying 3-D underground velocity structures.

Velocity structure above seismological bedrock estimated from earthquake recordings: an application of diffuse wave-field concept to strong motions in Iran

We present velocity structure inversion for three stations of Iranian Strong Motion Network (ISMN), and one KiK-net station that is used as a benchmark, for the application of diffuse wave-field concept in tectonic and geological setting of the west of Iran. This study compares the results of two existing computer codes for the velocity structure inversion at these sites. The computer codes use different search space parameterization, and error-minimization algorithms. Firstly, the available information on subsurface structure and surface geology from the strong motion stations is introduced. Then, ground motions of MW larger than 4, with PGA of all components less than 50 cm/s2 are used to calculate horizontal-to-vertical spectral ratios of earthquakes (eHVSR) at each station. The observed eHVSR curves are inverted for the velocity structure of the stations, and the results are compared with each other and the information of previous studies. Finally, the eHVSR curves of the mainshock records of the recent MW 7.3 earthquake at two ISMN sites are investigated. The velocity inversion is repeated by considering the shear modulus degradation of several shallow layers. Empirical nonlinear site amplification functions are calculated based on the modified VS structure and the VS structure of the linear ground response for two ISMN stations.Graphical

Three-dimensional body wave velocity structure and seismogenic structure for the 2022 MS 6.8 Luding earthquake sequence in Sichuan, China

An MS 6.8 earthquake struck the Luding county in the southeastern section of the Xianshuihe fault on 5 September 2022, causing heavy casualties and serious property losses. Using the data of 61 seismic stations surrounding the focal area, we utilize the machine learning-based LOC-FLOW for phase picking and association, and obtain a total of 13,496 aftershocks. We also collate 2,331 preshocks near the mainshock. We then conduct event relocation and three-dimensional P- and S-wave velocity structure inversion by double-difference tomography. The catalog we obtained has higher accuracy and completeness than the manual catalog. The relocation results show that the mainshock struck the Moxi section of the Xianshuihe fault at 9.2 km depth. There is a southeast-striking seismic strip of 35 km long at 5–13 km depth, and two relatively shallow seismic clusters exist to the west and northwest of the mainshock. Furthermore, there is an obvious seismic gap to the northwest of the mainshock. Based on the spatial pattern of the aftershock sequence, we speculate that the Moxi section of the Xianshuihe fault is the seismogenic fault. The earthquakes are mainly located in the regions of low velocity and low Poisson’s ratio, which is consistent with the granite that is geologically distributed along the southeast section of the Xianshuihe fault. There are two high-velocity zones located to the northwest and southeast of the mainshock, which are speculated to have controlled the rupture scale of the MS 6.8 Luding earthquake.

Crustal S-Wave Velocity Structure Beneath the Northwestern Bohemian Massif, Central Europe, Revealed by the Inversion of Multimodal Ambient Noise Dispersion Curves

The northwestern Bohemian Massif and adjacent areas are a tectonically active region associated with complex geodynamic activities, that manifest as Quaternary volcanism, earthquake swarms in the upper and middle crust, degassing of CO2, and crustal fluid migration. The intricate tectonic evolution and activities of this region reflect the complexity of the crustal structure therein. However, the crustal models derived from previous studies in this area offer different, even contradictory information regarding the existence of a mid-crustal low-velocity zone (LVZ). In this study, we apply the frequency-Bessel transform (F-J) method to extract the fundamental-mode and up to five higher-mode Rayleigh wave dispersion curves from ambient seismic noise data recorded in the study area and perform multimodal ambient noise dispersion curves inversion. The addition of higher-mode dispersion curves enhances the vertical resolution of the velocity structure inversion results. Our models support the view that the general S-wave velocity level of the crust is high within the study area. We detect two S-wave LVZs beneath the study area that are distributed mainly in the middle crust rather than the lower crust, and these LVZs are separated by a high-velocity zone. Considering the results of previous studies in the area, we infer that these S-wave LVZs may be the consequence of crustal fluids, plastic deformation and even partial melting of the felsic middle crust at relatively high crustal temperatures. Furthermore, these S-wave LVZs could be responsible for the origin and foci depth distribution of earthquake swarms. S-wave low-velocity anomalies are also observed in the uppermost mantle beneath the study area. These S-wave models based on the joint inversion of multimodal dispersion curves can provide new references for understanding the tectonic activity and geodynamic evolution of the northwestern Bohemian Massif and adjacent areas.

Transdimensional simultaneous inversion of velocity structure and event locations in downhole microseismic monitoring

In downhole microseismic monitoring, the velocity model plays a vital role in accurate mapping of the hydraulic fracturing image. For velocity model uncertainties in the number of layers or interface depths, the conventional velocity calibration method has been shown to effectively locate the perforation shots; however, it introduces nonnegligible location errors for microseismic events, especially for complex geologic formations with inclinations. To improve the event location accuracy, we have exploited the advantages of the reversible jump Markov chain Monte Carlo approach in generating different dimensions of velocity models and adopted a transdimensional Bayesian simultaneous inversion framework for obtaining the effective velocity structure and event locations simultaneously. The transdimensional inversion changes the number of layers during the inversion process and selects the optimal interface depths and velocity values to improve the event location accuracy. The confidence intervals of the simultaneous inversion event locations estimated by Bayesian inference enable us to evaluate the location uncertainties in the horizontal and vertical directions. Two synthetic examples and a field test are presented to illustrate the performance of our methodology, and the event location accuracy is shown to be higher than that obtained using the conventional methods. With less dependence on prior information, our transdimensional simultaneous inversion method can be used to obtain an effective velocity structure for facilitating highly accurate hydraulic fracturing mapping.

Quantitative damage, fracture mechanism and velocity structure tomography of sandstone under uniaxial load based on acoustic emission monitoring technology

Rock is an opaque medium body, and it is impossible to visually see its internal damage, which brings difficulties to the investigation of rock damage. In the light of the problems of rock damage, fracture mechanism and velocity structure evolution under load, the acoustic emission (AE) technology is adopted in this paper to monitor and analyze AE events in sandstone specimens during uniaxial loading. To investigate the damage law and fracture mechanism of sandstone under uniaxial load, a mechanical model that can quantitatively describe rock damage is constructed. Finally, the time-lapse double-difference tomography (tomo-TDD) is used to invert the velocity structure in the loaded rock. The results show that the fracture mode of the sandstone under uniaxial load is mainly shear fracture accompanied by a small amount of tensile fracture. The fracture modes of the three stages divided by the characteristics of AE signals, during loading process of the sandstone, is shear fracture, tensile fracture and shear fracture respectively. The damage mechanics model including the whole process of stress-strain curve in the post peak stress stage and the stage of primary micro-pores’ compaction and closure can quantitatively describe the damage degree and accurately predict the failure of rock under different stress levels, which is consistent with the velocity structure inverted by tomo-TDD. The velocity structure inversion based on tomo-TDD can reflect the damage degree, damage distribution and velocity structure evolution of any section under any stress level. The research results can be used for rock stability analysis and risk prediction, and at the same time, it provides a more intuitive reference for the non-destructive testing of rock masses in engineering.

3-D isotropic and anisotropic shallow crustal structure on Pingtan Island, Fujian, southeastern coast of China

Pingtan Island is located on the southeastern coast of China between the Changle-Zhao'an and Binhai fault zones. We use P- and S-wave traveltime data from marine large-volume airgun sources for 3-D velocity structure inversion, and the spatial resolution is high at depths of 1–3 km. The isotropic inversion results show that the velocity in the island is higher in the midwestern area, which consists of sedimentary plains. The velocity is lower in the southern and eastern areas, which consist of eroded hills. Then we perform azimuthally anisotropic inversion using the obtained isotropic model as the initial model. The fast directions in the island are predominantly NE-SW and NNE-SSW, consistent with the main fault structures along the southeastern coast of China. The magnitude of anisotropy gradually decreases from south to north, consistent with the decreasing intensity of fault activity from south to north. On the southeastern coast of China, the fast direction in the upper crust is nearly orthogonal to that in the upper mantle from previous studies, which is nearly parallel to the absolute plate motion in the lower crust and upper mantle. The faults along the Fujian coastal area and Taiwan formed at different time period. But the fast directions in these two areas are similar. Therefore, the fast direction is mainly dominated by the trends of faults and fractures in the upper crust. And it is dominated by aligned olivine crystals induced by plate motion in the lower crust and upper mantle.

Improvement of the objective function in the velocity structure inversion based on horizontal-to-vertical spectral ratio of earthquake ground motions

SUMMARYGiven the improvements that have been made in the forward calculations of seismic noise horizontal-to-vertical spectral ratios (NHVSRs) or earthquake ground motion HVSRs (EHVSRs), a number of HVSR inversion methods have been proposed to identify underground velocity structures. Compared with the studies on NHVSR inversion, the research on the EHVSR-based inversion methods is relatively rare. In this paper, to make full use of the widely available and constantly accumulating strong-motion observation data, we propose an S-wave HVSR inversion method based on diffuse-field approximation. Herein, the S-wave components of earthquake ground motion recordings are considered as data source. Improvements to the objective function has been achieved in this work. An objective function with the slope term is introduced. The new objective function can mitigate the multisolution phenomenon encountered when working with HVSR curves with multipeaks. Then, a synthetic case is used to show the verification of the proposed method and this method has been applied to invert underground velocity structures for six KiK-net stations based on earthquake observations. The results show that the proposed S-wave EHVSR inversion method is effective for identifying underground velocity structures.

Aftershocks localization and shallow crustal velocity structure following the <italic>M</italic>s6.9 Mainling earthquake in Tibet, China

At 06:34 CST on November 18, 2017, an M s6.9 earthquake struck Mainling prefecture, Nyingchi City, Tibet. The earthquake occurred at the frontier zone of collision and subduction in the eastern Himalayan syntaxis, a region of persistently strong compression between India and Eurasia. To monitor aftershock activity, the Institute of Tibetan Plateau Research, Chinese Academy of Sciences, installed 28 broadband seismic stations around the epicentral area and obtained 180 days of seismic observation data. The average spacing between stations was approximately 5 km, which yielded good coverage of the source area and enriched the seismic data for the Eastern Himalayan syntaxis. In this paper, the hypodense events are refined by the hypoDD double-difference seismic location method and the local tomography software (LOTOS-12), an algorithm for seismic location and velocity structure imaging joint inversion. The hypoDD seismic localization method uses waveform cross-correlation technology to achieve positioning using the minimum residuals of the actual and theoretical travel times of two earthquakes. It is widely used for close-range earthquakes and is suitable for regional earthquake research; the LOTOS algorithm is more appropriate for velocity structure inversion in regions with significant differences in physical properties. The results show that the aftershock zone is approximately 50 km long and 30 km wide and trends NW-SE. According to the aftershock distribution, three active faults are basically consistent with the strike of the Xixingla fault zone. These active fractures are similar in shape and have a NE dip direction, and the dip angles of the active faults gradually become steeper from NW to SE. Under the continuous NE thrusting action of the Namche Barwa syntaxis, the three secondary fractures have undergone bending deformation. This deformation is associated with the adjustment mode of the eastern Himalayan syntaxis where the Indian plate continues to push Eurasia northward. At the same time, combining these results with those of the LOTOS-12 algorithm, the distribution of the Pg wave and Sg wave velocity disturbance in the shallow crust (20 km) and the wave speed ratio ( V p/ V s) are obtained. The main shock was located in an abnormal transition zone of body wave velocity, and the aftershocks mostly occurred in the negative anomaly zone for P wave and S wave velocities, which reflects the characteristics of stratigraphic fractures and faults in the area with concentrated earthquakes. We suggest that this area should be avoided in the future when construction of engineering facilities is required north of Namche Barwa Peak. Overall, with Namche Barwa Peak as the boundary, the P wave and the S wave are generally characterized by high-speed anomalies in the north and low-speed anomalies in the south. This phenomenon may be related to the geological structure and petrophysical properties in this area, as the lithologies of the Lhasa terrane are old and cold; meanwhile, the metamorphic body in the south and the Namche Barwa syntaxis are relatively broken, the ages of rock formation are relatively young, and the lithologies are relatively hot and soft. In addition, high-speed P wave anomalies in the northwestern part of Namche Barwa Peak may be related to enrichment in garnet minerals in this area. The higher wave velocity ratio on the southwest side of the epicentral area, the lower P wave velocity on the right side of profile 1–1′ and the P wave on the left side of profile 2–2′ may be related to the fragmentation of the terrain in this area and the development of valley glaciers. The area where the wave velocity ratio is relatively large may also be related to the upwelling of asthenospheric and lithospheric material in the deep mantle.

A study of full waveform inversion of seismic velocity structure under a rugged seafloor by using joint OBN and VSP synthetic seismic data

A numerical study of full waveform inversion (FWI) of seismic velocity structure was carried out by using synthetic ocean-bottom node (OBN) and vertical seismic profile (VSP) data. Based on an undulating seafloor modified from the commonly known Marmousi2 model, seismic forward modeling was performed to synthesize seismic P-wave data for the study. FWI experiments were performed using synthetic VSP, OBN and combined VSP + OBN data, separately, for the inversion of the velocity structure. The results show that the joint inversion of using the combined VSP + OBN synthetic data resulted in a very good velocity structure. Its accuracy is significantly higher than either using the VSP or OBN data alone. This suggests that the joint FWI using combined VSP + OBN data is a better image processing technique for the rugged seabed and complex structure below.

Seismic velocity structure across the 2013 Craig, Alaska rupture from aftershock tomography: Implications for seismogenic conditions

The 2013 Craig, Alaska MW 7.5 earthquake ruptured along ∼150 km of the Queen Charlotte Fault (QCF), a right-lateral strike-slip plate boundary fault separating the Pacific and North American plates. Regional shear wave analyses suggest that the Craig earthquake rupture propagated in the northward direction faster than the S-wave (supershear). Theoretical studies suggest that a bimaterial interface, such as that along the QCF, which separates oceanic and continental crust with differing elastic properties, can promote supershear rupture propagation. We deployed short-period ocean-bottom seismometers (OBS) as a part of a rapid-response effort less than four months after the Craig earthquake mainshock. During a 21-day period, 1,133 aftershocks were recorded by 8 OBS instruments. Aftershock spatial distribution indicates that the base of the seismogenic zone along the QCF approaches ∼25 km depth, consistent with a thermally-controlled fault rheology expected for igneous rocks at oceanic transform faults. The spatial distribution also provides supporting evidence for a previously hypothesized active strand of the QCF system within the Pacific Plate. Tomographic traveltime inversion for velocity structure indicates a low-velocity (VP and VS) zone on the Pacific side of the plate boundary at 5–20 km depths, where Neogene Pacific crust and upper mantle seismic velocities average ∼3–11% slower than the North American side, where the Paleozoic North American crust is seismically faster. Our results suggest that elastic properties along the studied portion of the QCF are different than those of a simple oceanic–continental plate boundary fault. In our study region, velocity structure across the QCF, while bimaterial, does not support faster material on the west side of the fault, which has been proposed as one possible explanation for northward supershear propagation during the Craig earthquake. Instead, we image low-velocity material on the west side of the fault. Explanations could include that part of the rupture was subshear, or that fault damage zone properties or fault smoothness are more important controls on supershear rupture than a bimaterial contrast.

Crustal velocity structure of central Gansu Province from regional seismic waveform inversion using firework algorithm

The firework algorithm (FWA) is a novel swarm intelligence-based method recently proposed for the optimization of multi-parameter, nonlinear functions. Numerical waveform inversion experiments using a synthetic model show that the FWA performs well in both solution quality and efficiency. We apply the FWA in this study to crustal velocity structure inversion using regional seismic waveform data of central Gansu on the northeastern margin of the Qinghai-Tibet plateau. Seismograms recorded from the moment magnitude (MW) 5.4 Minxian earthquake enable obtaining an average crustal velocity model for this region. We initially carried out a series of FWA robustness tests in regional waveform inversion at the same earthquake and station positions across the study region, inverting two velocity structure models, with and without a low-velocity crustal layer; the accuracy of our average inversion results and their standard deviations reveal the advantages of the FWA for the inversion of regional seismic waveforms. We applied the FWA across our study area using three component waveform data recorded by nine broadband permanent seismic stations with epicentral distances ranging between 146 and 437 km. These inversion results show that the average thickness of the crust in this region is 46.75 km, while thicknesses of the sedimentary layer, and the upper, middle, and lower crust are 3.15, 15.69, 13.08, and 14.83 km, respectively. Results also show that the P-wave velocities of these layers and the upper mantle are 4.47, 6.07, 6.12, 6.87, and 8.18 km/s, respectively.

Validation of a New Velocity Structure Inversion Method Based on Horizontal-to-Vertical (H/V) Spectral Ratios of Earthquake Motions in the Tohoku Area, Japan

The earthquake motion generated by the multiple scattering due to the complexity of the underneath soil structure can be referred to as a diffuse wave field. Under the assumption of the well‐diffused wave field it is accepted that the average autocorrelation of a single receiver is proportional to the imaginary part of the Green’s function when both source and receiver are located at the same point. In this study we focus on sites where the site effect can be described using a 1D model. Previous studies show that the imaginary part of the Green’s function at the free surface is proportional to the square of the absolute value of the corresponding transfer function for a plane, vertically incident wave with unit amplitude. It is then possible to carry out an inversion of the 1D velocity structure using the relationship between the horizontal‐to‐vertical (H/V) spectral ratio and the ratio of horizontal and vertical transfer functions. We verify that the average H/V spectral ratio computed with a sufficiently large number of earthquake data depends only on the underneath geological structure and not on the set of data used to compute it. We then carry out inversions of the velocity structures for 10 sites of the K‐NET and KiK‐net networks in the Tohoku area, Japan, following the proposed theory for earthquake H/V spectral ratios. We verify that there is a good match between the observed H/V spectral ratios and the theoretical ones corresponding to the proposed velocity structures for the 10 target sites studied in the present work. Online Material: Figures showing variations of H/V spectral ratios and results of the inversions and tables of inversion parameters.

Visco-acoustic transmission waveform inversion of velocity structure in space-frequency domain

According to the least square criterion of minimizing the misfit between modeled and observed data, this paper provides a preconditioned gradient method to invert the visco-acoustic velocity structure on the basis of using sparse matrix LU factorization technique to directly solve the visco-acoustic wave forward problem in space-frequency domain. Numerical results obtained in an inclusion model inversion and a layered homogeneous model inversion demonstrate that different scale media have their own frequency responses, and the strategy of using low-frequency inverted result as the starting model in the high-frequency inversion can greatly reduce the non-uniqueness of their solutions. It can also be observed in the experiments that the fast convergence of the algorithm can be achieved by using diagonal elements of Hessian matrix as the preconditioned operator, which fully incorporates the advantage of quadratic convergence of Gauss-Newton method.

Constraining the anisotropy structure of the crust by joint inversion of seismic reflection travel times and wave polarizations

In the context of wide-angle seismic profiling, the determination of the physical properties of the Earth crust, such as the elastic layer depth and seismic velocity, is often performed by inversion of P- and/or S-phases propagation data supplying the geometry of the medium (reflector depths) or any other structural parameter (P- or S-wave velocity, density...). Moreover, the inversion for velocity structure and interfaces is commonly performed using only seismic reflection travel times and/or crustal phase amplitudes in isotropic media. But it is very important to utilize more available information to constrain the non-uniqueness of the solution. In this paper, we present a simultaneous inversion method of seismic reflection travel times and polarizations data of transient elastic waves in stratified media to reconstruct not only layer depth and vertical P-wave velocity but also the anisotropy feature of the crust based on the estimation of the Thomsen’s parameters. We carry out a checking with synthetic data, comparing the inversion results obtained by anisotropic travel-time inversion to the results derived by joint inversion of seismic reflection travel times and polarizations data. The comparison proves that the first procedure leads to biased anisotropic models, while the second one fits nearly the real model. This makes the joint inversion method feasible. Finally, we investigate the geometry, P-wave velocity structure and anisotropy of the crust beneath Southeastern China by applying the proposed inversion method to previously acquired wide-angle seismic data. In this case, the anisotropy signature provides clear evidence that the Jiangshan-Shaoxing fault is the natural boundary between the Yangtze and Cathaysia blocks.

Analysis of the Trapped Wave Recorded in Kunlun Fault Zone

Artificial seismic experiment was deployed on the fault zone generated by the Kunlun Ms8.1 earthquake on Nov. 4, 2001 and the seismograms recorded on two profiles, one along and the other across the fault zone, were analyzed and processed for the purpose of fine structure investigation of the fault zone with the guided wave trapped in fault zone. The analysis includes amplitude spectra calculation, group velocity and dispersion curves extraction, S wave velocity structure inversion with surface wave methods and the estimation of Q factor with an amplitude-ratio method. The result of field experiment reveals that the time delay between the S wave and trapped wave train increases with the epicentral distance. The site effect related to the fault zone is also observed in seismogram recorded on the stations located outside the fault zone. The final result suggests that the fracture zone of Kunlun fault is about 250m wide, and has a layered internal velocity structure and an average Q of 15, while the average Q of wall rocks is 30. It is also found that the width of rupture zone generated by the Kunlun earthquake is the same as that of Landders Ms7.6, California, earthquake, although both the magnitude and the length of fracture zone of the former are much larger than those of the latter.

Evidence for short-scale stress field variations within intraplate central-western France

SUMMARY Refinement of the seismicity distribution (4574 events) in western and central France, has been done by synthesis of seismological bulletins. Earthquakes have then been relocated by joint hypocentre and velocity structure inversion. The new hypocentre distribution indicates that the seismicity of those regions is much less diffuse than previously thought, mainly with regard to the depth distribution. The hypocentre improvement allows us to compute 44 new focal mechanisms and to revise bibliographic focal mechanism solutions. Then, the regional stress field was determined from 119 available focal mechanisms. It is characterized by a regionally significant strike-slip regime with NW-trending σ 1. However, the refinement in location and increasing available focal mechanism solutions allow us to show that this strike-slip regime is overprinted by local extensional perturbations in three distinct areas. In the Massif Central, the Sillon Houiller, a ancient vertical shear zone, appears to be acting as a passive boundary between a western unit and an eastern unit that is uplifted by the ascension of a hot mantle plume at the base of the lithosphere. Extension is unexpectedly observed in the southern Armorican Massif (SAM), as well as in northwestern Massif Central. One can observe a good relation between the observed perturbed zones and lateral variation of Pn anisotropy in the mantle. This correlation and the scale of these areas are arguments in favour of a lithospheric process as the origin for the stress characteristics. We believe these extensional perturbations could be related to the anticlockwise rotation of the Iberian microplate and/or incipient subduction in the Bay of Biscay.

Imaging the transition between the region of mantle melt generation and the crustal magma chamber beneath the southern East Pacific Rise with short‐period Love waves

Beneath the southern East Pacific Rise resides a crustal magma chamber confined to the immediate vicinity of the rise axis, yet the mantle melt production region is broad and asymmetric. Short‐period (4–17 s) Love waves propagating along the rise provide a means of probing the transition from the asymmetric melt production region to the rise‐centered crustal magma chamber. Low velocities beneath the rise act as a waveguide for Love wave energy. We model this complex wave propagation with a Gaussian beam representation as part of our nonlinear inversion for velocity structure. Our results show that the asymmetry exists at the crust‐mantle interface, or Moho, and increases downward; the lowest velocities are located beneath the rise at the Moho; beneath the rise there is an asymmetric gap in the high‐velocity “lid” at the top of the mantle; and there are lower asthenospheric velocities beneath the Pacific plate than the Nazca plate. The transition from the broad upwelling region to the narrow crustal magmatic system occurs over a ∼15‐km‐depth interval at the top of the mantle. Our observations indicate higher temperatures and greater melt production beneath the Pacific plate. East of the rise, melting appears to abruptly shut off at shallow levels, perhaps due to a component of downward mantle flow. We suggest that melt percolates upward, increasing in concentration, until reaching a permeability barrier beneath the lithosphere, it then moves up along the lithosphere to accumulate at the Moho beneath the rise. West of the rise, higher temperatures and upward percolation of melt delay growth of the lithosphere.

Peeling Genetic Algorithm of Receiver Functions Inversion for Upper Mantle Structure and Discontinuities

AbstractTo study the depths of seismic discontinuities in the upper mantle using the observed receiver function, this paper developed an inversion method of peeling genetic algorithm: dividing the whole depth into several sections from surface and doing inversion for the velocity gradually; peeling the effects of multiple reflected and reflect‐transformed phases at the shallower discontinuities on the refract‐transformed phase at the deeper one. Floating point coding, arithmetical crossover, and non‐uniform mutation were used in the genetic algorithm of inversion in each section. The method was checked using the model ispai'91 and applied to the inversion of velocity structure beneath HIA station.

Receiver functions from regionalPwaves

SUMMARY P waves from regional-distance earthquakes are complex and reverberatory, as would be expected from a combination of head waves, post-critical crustal reflections and shallow-incident P waves from the upper mantle. Although designed for steeply-incident teleseismic P waves, receiver functions (RFs) can also retrieve information about crustal structure from regional P. Using a new computation method based on multiple-taper spectral analysis, regional-distance RFs for GSN stations RAYN and ANTO show broad agreement with teleseismic RFs. At RAYN the moveout of the Moho-converted Ps phase, relative to direct P, follows the predictions of the IASP91 earth model well. The Moho-converted Ps phase shows complexity associated with the transition-zone triplications near Δ=20° and constant delay (zero moveout) as Δ0, consistent with conversion from Pn. Similar behaviour is seen for ANTO for events that arrive from the west. For eastern backazimuths the ANTO RFs show features whose moveout is negative as Δ0. This moveout is poorly fit by reverberations in flat layers or by direct scattering from a dipping interface, but is consistent with a topographic scatterer 20–30 km eastward of the ANTO site. Regional receiver functions may therefore be useful in judging whether teleseismic RFs at a particular station are suitable candidates for a 1-D velocity structure inversion. Synthetic seismograms of regional P phases, computed with a locked-mode reflectivity approach in a 1-D structure, confirm several gross features of the RAYN and ANTO regional receiver functions.