A New Body‐Wave Amplitude Ratio‐Based Method for Imaging Shallow Crustal Structure and Its Application in the Sichuan Basin, Southwestern China

Abstract

Information on the composition and structural heterogeneity of the shallow crust provides important constraints for understanding the deformation and evolution of the outermost shell of the Earth. Seismic measurements probe the elastic properties of the Earth's subsurface, from which compositional and structural information can be inferred. Of particular note is Poisson's ratio, which can be determined uniquely from the ratio of P wave velocity (Vp) to S wave velocity (Vs). As an important complement to seismic velocity, Poisson's ratio is highly sensitive to lithology, cracks, and porosity, as well as fluid and melt contents (Mavko et al., 2009); thus, the combination of velocity and Poisson's ratio would reduce the nonuniqueness when determining the nature of rocks.Previous attempts to image the seismic velocity and Poisson's ratio structures of the shallow crust have been limited due to various difficulties with the methods. A popular method is to invert the P and S wave travel times of local earthquakes, but it works only in areas with dense earthquakes and stations because of the strict requirements in ray density (Aki & Lee, 1976;Eberhart-Phillips, 1990). With this method, uncertainties in hypocenter locations also affect the accuracy of the results (e.g., Zhang et al., 2009). Surface-wave-imaging methods utilize surface-wave dispersion and ellipticity data with longer periods than those of body

Shallow Crustal Response to Arabia‐Eurasia Convergence in Northwestern Iran: Constraints From Multifrequency P‐Wave Receiver Functions

“…Thus, we redefine the P‐RF H / V ratio (Chong et al., 2018) as frequency‐dependent (Figure 2b) as follows: $$$\mathrm{PRFHV}(T,\alpha )=\frac{{\int }_{-T}^{T}{R}_{\mathrm{PRF}}(\tau ,\alpha ){\mathrm{cos}}^{2}\left(\frac{\pi \tau }{2T}\right)\mathrm{d}\tau }{{\int }_{-T}^{T}{Z}_{\mathrm{PRF}}(\tau ,\alpha ){\mathrm{cos}}^{2}\left(\frac{\pi \tau }{2T}\right)\mathrm{d}\tau },$$$ where $${R}_{\mathrm{P}\mathrm{R}\mathrm{F}}$$ and $${Z}_{\mathrm{P}\mathrm{R}\mathrm{F}}$$ are the radial and vertical P‐RFs, calculated by deconvolving the vertical from radial components and the vertical component from itself, respectively; α is the Gaussian parameter governing the high cutoff frequency (e.g., $${f}_{\mathrm{c}}$$ in Equation 2 of Wang et al., 2019); and $$T$$ is the half‐length of a low‐pass squared‐cosine filter for integration. When time $$T$$ is exactly zero, the P‐RF H / V ratio represents the P‐wave particle motion (Svenningsen & Jacobsen, 2007), which can recover structures down to ∼10 km depth for teleseismic earthquakes (Wang et al., 2021). Unlike the zero‐point case, in this study we set the range of $$T$$ to vary from 0 to 2 s, which allows conversions and reflections from deeper interfaces to enter the P‐RF H / V ratio, providing additional constraints for shallow structural imaging.…”

“…where 𝐴𝐴 𝐴𝐴PRF and 𝐴𝐴 𝐴𝐴PRF are the radial and vertical P-RFs, calculated by deconvolving the vertical from radial components and the vertical component from itself, respectively; α is the Gaussian parameter governing the high cutoff frequency (e.g., 𝐴𝐴 𝐴𝐴c in Equation 2of Wang et al, 2019); and 𝐴𝐴 𝐴𝐴 is the half-length of a low-pass squared-cosine filter for integration. When time 𝐴𝐴 𝐴𝐴 is exactly zero, the P-RF H/V ratio represents the P-wave particle motion (Svenningsen & Jacobsen, 2007), which can recover structures down to ∼10 km depth for teleseismic earthquakes (Wang et al, 2021). Unlike the zero-point case, in this study we set the range of 𝐴𝐴 𝐴𝐴 to vary from 0 to 2 s, which allows conversions and reflections from deeper interfaces to enter the P-RF H/V ratio, providing additional constraints for shallow structural imaging.…”

“…However, previous studies, aiming at the structure of the whole crust, did not pay special attention to the direct P phase in P‐RFs as well as its waveform variation with frequency. The frequency‐dependence of the waveform, especially the amplitude of the direct P phase in P‐RFs contains a wealth of information on the P‐wave particle motion (Julià, 2007; Wang et al., 2019), which is better at resolving structures at shallow depths than surface wave dispersions (Park et al., 2019; Wang et al., 2021). Multifrequency analysis on seismic data, as already applied in travel time tomography (Dahlen et al., 2000; Hung et al., 2000) and P‐RF waveform inversion (Wu et al., 2007), can achieve a higher resolution than single‐frequency approaches.…”

E‐W and S‐N Differences in the Sedimentary Cover and Crystalline Crust in the Cratonic Ordos Basin From Receiver Function Analysis

“…The thickness ( h ) and V p/ V s ratio ( k ) of the crust are two key parameters to study the rock composition and related tectonic evolution of the crust (Christensen, 1996). In recent years, some approaches have been developed to study the thickness and V p/ V s ratio together for the sediment crystalline crust, such as the h‐k stacking method of receiver functions (RFs; Zhu & Kanamori, 2000), the wavefield downward continuation and decomposition method (Tao et al., 2014), a transfer function approach (Frederiksen et al., 2015) and the joint inversion of the horizontal‐to‐vertical ratios of P‐wave and S‐wave (X. Wang et al., 2021). For the h‐k stacking method (Zhu and Kanamori., 2000), which is widely used to obtain the crustal thickness and average V p/ V s ratio by searching the most energetic stack of the Ps phase and multiples of the Moho, Yeck et al.…”

Shallow Crustal Structure of S-Wave Velocities in the Coastal Area of South China Constrained by Receiver Function Amplitudes

“…Moreover, the amplitude of the receiver function can also be adapted to obtain the SWV of the upper crust, as well as the velocity and density contrasts of the Moho discontinuity. Referring to previous studies on receiver function amplitudes [18][19][20], Qian et al [21] first proposed this idea of data processing, and developed a method to inverse the SWV structure of the upper crust with the amplitude of the receiver function [22,23].…”