The H-κ method (Zhu & Kanamori, 2000, https://doi.org/10.1029/1999JB900322) has been widely used to estimate the crustal thickness (H) and the ratio of P to S velocities (V P /V S ratio, κ) with receiver functions. However, in regions where the crustal structure is complicated, the method may produce biased results, arising particularly from dipping Moho and/or crustal anisotropy. H-κ stacking in case of azimuthal or radial anisotropy with flat Moho has been proposed but not for cases with plunging anisotropy and dipping Moho. Here we propose a generalized H-κ method called H-κ-c, which corrects for these effects first before stacking. We consider rather general cases, including plunging anisotropy and dipping interfaces of multiple layers, and use harmonic functions to correct for arrival time variations of Ps and its crustal multiples with back azimuth (θ). Systematic synthetic tests show that the arrival time variations can be well fitted by cosθ and cos2θ functions even for very complex crustal structures. Correcting for the back azimuthal variations significantly enhances H-κ stacking. We verify the feasibility of the H-κ-c method by applying it to 40 permanent stations in various geological setting across the Mainland China. The results show clear improvement after the harmonic corrections, with clearer multiples and stronger stacking energy, as well as more reliable H-κ values. Large differences in H (up to 5.0 km) and κ (up to 0.09) between the new and traditional methods occur mostly in mountainous regions, where the crustal structure tends to be more complex. We caution in particular about systematic bias when the traditional method is used in the presence of dipping interfaces. The modified method is simple and applicable anywhere in the world.Plain Language Summary Crustal thickness and crustal V P /V S ratio are important parameters for regional geology and tectonics. We propose a method that corrects for the effect of crustal anisotropy and dipping interface on receiver functions, which results in more reliable estimation of crustal thickness and V P /V S ratio. Synthetic tests and application to real data suggest that this method is powerful and widely applicable.
We introduced a P velocity model into the traditional joint inversion of P receiver function (RF) and surface wave dispersions to reduce model ambiguity. The method was implemented using a global search-based algorithm and a flexible parameterization of a sedimentary layer and spline-based parameterization that can represent sharp discontinuities. We applied the method to a dense array in SE Tibet (longitude~97.5°E to 107°E, latitude~25.3°N). Extensive tests using synthetic and real data suggest that the method is suitable and robust for a variety of velocity structures and Moho discontinuities and can simultaneously provide the crustal V p /V s profile and better constrained Moho depth. The flexibility of the parameterization and the inclusion of the V p constraint are crucial in the improved model recovery. Artifacts may be created without including the sedimentary layer. Even when it is less perfect, a reasonable V p model is valuable in such a joint inversion. We showed that crustal multiples in RFs may bias the traditional H-k results when the crust structure is complex and should be avoided in a joint inversion before appropriate corrections can be made. The results from the joint inversion show two low-velocity zones (LVZs) reported previously and were identified as channels of crustal flow. A prominent isolated LVZ is observed in the mid-lower crust under the Xiaojiang fault area, which correlates with anomalously high V p /V s ratios, indicating possible partial melting. However, the other LVZ is imaged to be in the brittle shallow upper crust without very high V p /V s ratios, which is likely associated with crustal fault zones rather than partial melting. We observe clear low-velocity structures in the mantle beneath the two crustal LVZs, which also correlate with zones of low resistivity. The crust-mantle correlation may suggest influence of mantle processes on crustal deformation.Plain Language Summary Nonunique in seismic inversion is a classical problem. The widely used inversion of surface wave dispersions or the joint inversion of dispersions and receiver functions suffers also the problem. In this study, we introduce P velocity model into the mix and adopted a flexible parameterization and inversion scheme, which we demonstrated to be robust and highly effective. We applied the method to a dense array in SE Tibet, which deforms strongly with active seismicity. The array was studied before, and two crustal flow channels were identified. The new imaging suggests that one low-velocity zone may indeed associated with partial melting with the additional constraint from the Poisson's ratio, but the other low-velocity zone is too shallow to be associated with partial melting. We also found significant mantle low-velocity zones, which may influence the crustal deformation of the region. PUBLICATIONS Key Points: • Joint inversion of and surface wave dispersions, receiver functions, and P velocity model were developed to reduce model ambiguity • Flexible parameterization and addition of the V p constra...
Several geodynamic models have been proposed for the deformation mechanism of Tibetan Plateau (TP), but it remains controversial. Here we applied a method of joint inversion of receiver functions and surface wave dispersions with P wave velocity constraint to a dense linear array in the NE Tibet. The results show that the geological blocks, separated by major faults at the surface, are characterized by distinct features in the crust, the Moho, and the uppermost mantle. The main features include crustal low-velocity zones (LVZs) with variable strengths, anomalous Vp/Vs ratios that are correlated with LVZs, a large Moho jump, and other abrupt changes near major faults, strong mantle lithosphere anomalies, and correlation of crustal and mantle velocities. The results suggest a lithospheric-scale deformation of continuous shortening as well as localized faulting, which is affected by the strength of the lithosphere blocks. The thickened mantle lithosphere can be removed, which facilitates the formation of middle-lower crustal LVZ and flow. However, such flow is likely a consequence of the deformation rather than a driving force for the outward growth of the TP. The proposed model of TP deformation and growth can reconcile the continuous deformation within the blocks and major faults at the surface.Plain Language Summary How did the Tibetan Plateau grow to its present height and size? Models have been controversial for decades, including end-members of continuous deformation, rigid block extrusion, and channel flow in middle-lower crust. Here we used a recently developed joint inversion scheme to resolve several key seismic parameters simultaneously in a self-consistent manner for a linear array in the northeast margin of the plateau, which is ideal for testing models for the plateau growth. Our joint inversion results show distinct block-like features, which suggest a lithospheric-scale deformation of continuous shortening as well as localized faulting at the lithospheric scale. The proposed model of Tibetan Plateau deformation and growth can reconcile the continuous deformation within the blocks and major faults at the surface. Key Points: • High-resolution images of Vs and Vp/Vs along a profile in NE Tibet were obtained from a joint inversion with Vp constraints • The geological blocks are characterized by distinct seismic features with correlated crustal and mantle lithosphere structure • Deformation is both continuous and localized and occurs at lithospheric scale Supporting Information: • Supporting Information S1
A receiver function (RF) is the response of the Earth's structure below a seismometer to an incident teleseismic wave and consists of a series of P-to-S (Ps) or S-to-P (Sp) converted waves generated at structural interfaces (Langston, 1979), mainly those generated by the velocity discontinuities in the crust and upper mantle below the seismometer. Crustal thickness (H) and P-wave and S-wave velocity ratio (κ) are important parameters reflecting the crustal structure and internal material composition and can provide an important basis for regional tectonics and dynamics. Zhu and Kanamori (2000) noted that when the Moho converted phase Ps and crustal multiple waves are considered at the same time, H and κ can be estimated under the assumption of the average crustal P wave velocity. This is currently the most commonly used H-κ method. The H-κ algorithm stacks the amplitudes of RFs at predicted arrival times for Ps and crustal multiples (PpPs and PpSs + PsPs; here referred to as M1 and M2, respectively, for convenience, as in J. Li et al., 2019) for different values of H and κ in a grid search.The assumptions of the H-κ method are very simple, which leads to its limitations. It assumes that the crustal medium is isotropic, the Moho surface is horizontal, and the P-wave velocity of the crust is fixed. However, the above assumptions cannot be satisfied under complex geological settings, so the results of the H-κ method may be inaccurate. The Chinese continent is known to have highly heterogeneous crustal structure and extremely variable Moho depth, as revealed for example, by seismic tomography (
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